Brake Valve By-Pass

Section 1
FUNDAMENTAL PRINCIPLES
Lesson Objectives 1. Describe the cycle of heat as it applies to automotive brakes.

2. Explain the effect of heat transfer as it relates to brake fade.
3. Describe how the coefficient of friction affects the rate of heat
transfer.
4. Relate the effect of hydraulic theory as it applies to a closed
hydraulic circuit.
5. Explain how output force in a hydraulic circuit can be tailored for
specific applications by changing the diameter of the output
piston.
6. List the requirements of brake fluid in an automotive brake
system.
Section 1
Fundamental The most important safety feature of an automobile is its brake

Principles system. The ability of a braking system to provide safe, repeatable
stopping is the key to safe motoring. A clear understanding of the
brake system is essential for anyone involved in servicing Toyota
vehicles.
The basic principle of brake operation is the conversion of energy.

Energy is the ability to do work. The most familiar forms of energy in
automotive use are; chemical, electrical and mechanical. For example
starting an engine involves several conversions. Chemical energy in
the battery is converted to electrical energy in the starter. Electrical
energy is converted to mechanical energy in the starter as it cranks the
engine.
Cycle of Heat Energy Burning hydrocarbons and oxygen in the engine creates heat energy.
Nothing can destroy energy once it is released, it can only be converted
into another form of energy. Heat energy is converted into kinetic
energy as the vehicle is put into motion. Kinetic energy is a
fundamental form of mechanical energy; it is the energy of a mass in
motion. Kinetic energy increases in direct proportion to weight increase
and increases by four times for speed increases.
Cycle of Heat
Heat energy converts
to kinetic energy which
converts back to
heat energy.
Friction is the resistance to movement between two objects in contact

with each other. It also converts energy of motion to heat. If we allow
the vehicle to coast in neutral on a level surface, eventually the kinetic
energy would be converted to heat in the wheel bearings, drivetrain
bearings, and at the tire and road surface to bring the vehicle to a
complete stop. The brake system provides the means of converting
kinetic energy through stationary brake shoes or pads which press
against a rotating surface, generating friction and heat.
The amount of friction produced is proportional to the pressure between

the two objects, composition of surface material and surface condition.
The greater the pressure applied to the objects, the more friction and
2 LEXUS Technical Training

Fundamental Principals
heat is produced. The more heat produced by friction, the sooner the
vehicle is brought to a stop which results in stopping control.
The coefficient of friction is a measurement of the friction between

two objects in contact with each other. Force is the effort required to
slide one surface across the other. It is determined by dividing the force
required to move an object by the weight of an object.
Coefficient
of Friction
Coefficient of friction varies
based on composition of
material and condition
of the surface.
The following example illustrates how the type of friction surface can
influence the coefficient of friction (COF).
100 pounds of ice pulled across a concrete floor may require 5 pounds of
force to move.
5 / 100 = 0.05
COF = 0.05
However 100 pounds of rubber pulled across a concrete floor may

require 45 pounds of force to move.
45 / 100 = 0.45
COF = 0.45
The coefficient of friction varies in the two examples above based on

the materials used. The same is true in a brake system, the coefficient
of friction varies on the type of lining used and the condition of the
drum or rotor surface.
Section 1
Basic Brake System The most widely utilized brake systems at present are the foot
operated main brake and manual type parking brake. The main brake
actuates the brake assemblies at each wheel simultaneously using
hydraulic pressure. Fluid pressure created at the master cylinder is
transmitted to each of the wheel cylinders through brake tubing. The
wheel cylinders force the shoes and pads into contact with a drum or
rotor spinning with the wheels generating friction and converting
kinetic energy to heat energy. Large amounts of heat is created
resulting in short distance stopping and vehicle control. The converted
heat is absorbed primarily by the brake drums and dissipated to the
surrounding air.
Foot Operated
Brake System
Fluid pressure is transmitted
to each of the wheel
cylinders through
brake tubing.

Brake Fade Brake drums and rotors are forced to absorb a significant amount of
heat during braking. Brake fade describes a condition where heat is
generated at a faster rate than they are capable of dissipating heat into
the surrounding air. For example, during a hard stop the temperature
of drums or rotors may increase more than 100 degrees F in just
seconds. It may take 30 seconds to cool these components to the
temperature prior to braking. During repeated hard stops, overheating
may occur and a loss of brake effectiveness or even failure may result.
There are primarily two types of brake fading caused by heat;

• Mechanical fade.
• Lining fade.
Mechanical fade occurs when the brake drum overheats and expands
away from the brake lining resulting in increased brake pedal travel.
Rapidly pumping the pedal will help to keep linings in contact with the
drum.
Brake Fade
Drums and rotors are
forced to absorb heat
during braking at a faster
rate than they are capable
of dissipating the heat.
Lining fade affects both drum and disc brakes and occurs when the
friction material overheats to the point where the coefficient of friction
drops off. When the coefficient of friction drops off, friction is reduced
and the brake assemblies ability to convert added heat is reduced.
Brake fade is the primary reason for weight limits for towing and
trailer brake requirement for vehicles above a given trailer weight. The
added kinetic energy resulting from increased vehicle mass requires
added heat conversion capacity when the brakes are applied.
Section 1
Basic Hydraulic Brake systems use hydraulic fluid in a closed system to transmit
Theory motion. The hydraulic brake system is governed by physical laws that
makes it efficient at transmitting both motion and force. Blaise Pascal
discovered the scientific laws governing the behavior of liquids under
pressure. Pascal’s Law states that pressure applied anywhere to an
enclosed body of fluid is transmitted equally to all parts of the fluid. In
other words, 100 psi generated at the master cylinder is the same at
each wheel cylinder as well as anywhere within a static system.
A feature of hydraulic theory can be seen in the illustration below

which demonstrates the pressure in the master cylinder is transmitted
equally to all wheel cylinders.
Pascal’s Law
Pressure applied anywhere
to an enclosed body of fluid
is transmitted equally to all
pans of the fluid.
Another important distinction to make is that liquids cannot be

compressed, whereas, air is compressible. A hydraulic system must be
free of air in order to function properly. Pedal travel will increase as air
in the system is compressed.
Air is Compressible
Liquids cannot be
compressed, whereas,
air is compressible.

Fluid pressure is indicated in pounds per square inch (psi). It is

determined by dividing the input force applied to a piston by the area
of the piston. (force/area = pressure in psi) If a force of 100 pounds is
applied to a master cylinder piston, an area of 2 square inches, the
resulting pressure will be 50 psi. This pressure is transmitted to all
parts of the fluid in the container equally.
force / area = psi
100 / 2 = 50 psi
In the series of examples below we are examining working force and

transfer of motion based on different working piston diameters. In each
example, piston A is the same diameter (1") and the same 100 lb. input
force is applied. When the force is applied to piston A, piston B has 100
psi of output force and travels an equal distance to piston A.
By contrast piston C will have an output force of twice that of piston A

because piston C has twice the area. In addition, piston C transfers
only half the distance of piston A.
Yet another contrast is piston D which is half the area of piston A. The
system pressure is the same as the two previous examples but since
piston D is half the area of piston A, the pressure is half the apply
pressure and the motion transfer is twice that of piston A.
Working Force and

Transfer of Motion
The braking force varies,
depending on the diameter
of the wheel cylinders.
Hydraulic brakes deliver equal braking force to all wheels with a

minimum of transmission loss. Hydraulic brakes have a wide design
flexibility because braking force can be changed merely by changing
the diameter of the master cylinder and wheel cylinders.
Section 1
Brake Fluid Brake fluid is specifically designed to be compatible with its

environment of high heat, high pressure and moving parts. Standards
for brake fluid have been established by the Society of Automotive
Engineers (SAE) and the Department of Transportation (DOT).
Requirements of a fluid used in automotive brake applications must
include the following:
• remain viscous.
• have a high boiling point.
• act as lubricant for moving parts.
The Federal Motor Vehicle Safety Standard (FMVSS) states that by

law, brake fluid must be compatible regardless of manufacturer. Fluids
are not necessarily identical however, any DOT approved brake fluid
can be mixed with any other approved brake fluid without damaging
chemical reactions. Although the fluid may not always blend together
into a single solution, it does not effect the properties of liquid under
pressure.
Brake Fluid Types Two types of brake fluid are used in automotive brake applications,
each having specific attributes and drawbacks. Polyglycol is clear to
amber in color and is the most common brake fluid used in the
industry. It is a solvent and will immediately begin to dissolve paint.
Flush the area with water if brake fluid is spilled on paint.
One of the negative characteristics of polyglycol is that it is

hygroscopic, that is, it has a propensity to attract water. Water can be
absorbed through rubber hoses and past seals and past the vent in the
master cylinder reservoir cap. Moisture in the hydraulic circuit reduces
the boiling point of the fluid and causes it to vaporize. In addition,
moisture causes metal parts to corrode resulting in leakage and /or
frozen wheel cylinder pistons.
Extra caution should be taken with containers of brake fluid because it

absorbs moisture from the air when the container is opened. Do not
leave the container uncapped and close it tightly.
Silicone is purple in color. It is not hygroscopic and therefore has

virtually no rust and corrosion problems. It has a high boiling point
and can be used in higher heat applications. It will not harm paint
when it comes in contact with it.
Silicone has a greater affinity for air than polyglycol. Because the air
remains suspended in the fluid it is more difficult to bleed air from the
hydraulic system.

DOT Grades There are three grades of brake fluid which are determined by Federal
Motor Vehicle Safety Standard 116. Fluid grades are rated by the
minimum boiling point for both pure fluid (dry) and water
contaminated fluid (wet):
• DOT 3 − Polyglycol
• minimum boiling point − 401°F dry, 284 °F wet
• blends with DOT 4
• DOT 4 − Polyglycol
• minimum boiling point − 446 °F dry, 311 °F wet
• blends with DOT3
• DOT 5 − Silicone
• minimum boiling point − 500 °F dry, 356 °F wet
• compatible by law with DOT 3 and 4 but will not blend
with them
Toyota recommends the exclusive use of Polyglycol DOT 3 brake fluid

in all its products.
Section 2
MASTER CYLINDER
Lesson Objectives 1. Explain the difference between conventional and diagonal split
piping system and their application.
2. Describe the function of the compensating port of the master
cylinder.
3. Explain the operation of the residual check valve on the drum
brake circuit of the master cylinder.
4. Explain the safety advantage of having two hydraulic circuits in
the master cylinder.
5. Describe the difference between the Portless and Lockheed master
cylinders.

Master Cylinder
Master Cylinder The master cylinder converts the motion of the brake pedal into hydraulic
pressure. It consists of the reservoir tank, which contains the brake fluid;
and the piston and cylinder which generate the hydraulic pressure.
The reservoir tank is made mainly of synthetic resin, while the

cylinders are made of cast iron or an aluminum alloy.
Master Cylinder
Stores brake fluid and
converts the motion of
the brake pedal into
hydraulic pressure.
Tandem Master The tandem master cylinder has two separate hydraulic chambers.
Cylinder This creates in effect two separate hydraulic braking circuits. If one of
these circuits becomes inoperative, the other circuit can still function to
stop the vehicle. Stopping distance is increased significantly, however,
when operating on only one braking circuit. This is one of the vehicles’
most important safety features.
Conventional On front−engine rear−wheel−drive vehicles, one of the chambers

Piping provides hydraulic pressure for the front brakes while the other
provides pressure for the rear.
Section 2
Conventional Piping
for Front Engine
Rear Drive
When one circuit fails the
other remains intact to
stop the vehicle.
Diagonal Split Piping On front−engine front−wheel−drive vehicles, however, extra braking load
is shifted to the front brakes due to reduced weight in the rear. To
compensate for hydraulic failure in the front brake circuit with the
lighter rear axle weight, a diagonal brake line system is used. This
consists of one brake system for the right front and left rear wheels,
and a separate system for the left front and right rear wheels. Braking
efficiency remains equal on both sides of the vehicle (but with only half
the normal braking power) even if one of the two separate systems
should have a problem.
Diagonal Piping for

Front Engine
Front Drive
Improves braking efficiency
if one circuit fails by having
one front wheel and one
rear wheel braking.

Master Cylinder
Construction The Master Cylinder has a single bore separated into two separate
chambers by the Primary and Secondary Pistons. On the front of the
master cylinder Primary Piston is a rubber Piston Cup, which seals the
Primary Circuit of the cylinder. Another Piston Cup is also fitted at the
rear of the Primary Piston to prevent the brake fluid from leaking out
of the rear of the cylinder.
At the front of the Secondary Piston is a Piston Cup which seals the
Secondary Circuit. At the rear of the Secondary Piston the other Piston
Cup seals the Secondary Cylinder from the Primary Cylinder. The
Primary Piston is linked to the brake pedal via a pushrod.
Master Cylinder
Components
The Master Cylinder has a
single bore separated into
two separate chambers
by the Primary and
Secondary Pistons.
Normal Operation When the brakes are not applied, the piston cups of the Primary and
Secondary Pistons are positioned between the Inlet Port and the
Compensating Port. This provides a passage between the cylinder and
the reservoir tank.
The Secondary Piston is pushed to the right by the force of Secondary

Return Spring, but prevented from going any further by a stopper bolt.
When the brake pedal is depressed, the Primary Piston moves to the
left. The piston cup seals the Compensating Port blocking the passage
between the Primary Pressure Chamber and the Reservoir Tank. As
the piston is pushed farther, it builds hydraulic pressure inside the
cylinder and is applied or transmitted to the wheel cylinders in that
circuit. The same hydraulic pressure is also applied to the Secondary
Section 2
Piston. Hydraulic pressure in the Primary Chamber moves the

Secondary Piston to the left also. After the Compensating Port of the
Secondary Chamber is closed, fluid pressure builds and is transmitted
to the secondary circuit.
Brake Application
As the piston cup
passes the compensating
Port pressure begins
to increase in the
hydraulic circuit.
When the brake pedal is released, the pistons are returned to their
original position by hydraulic pressure and the force of the return
springs. However, because the brake fluid does not return to the
master cylinder immediately, the hydraulic pressure inside the cylinder
drops momentarily. As a result, the brake fluid inside the reservoir
tank flows into the cylinder via the inlet port, through small holes
provided at the front of the piston, and around the piston cup. This
design prevents vacuum from developing and allowing air to enter at
the wheel cylinders.
Brake Release
Brake fluid inside the
reservoir tank flows into the
cylinder via the inlet port,
through small holes
provided at the front of the
piston, and around the
piston cup.

Master Cylinder
After the piston has returned to its original position, fluid returns from
the wheel cylinder circuit to the reservoir through the Compensating Port.
Fluid Return
Fluid returns to the
reservoir tank through the
compensating port.
Fluid Leakage In When fluid leakage occurs in the primary side of the master cylinder, the
One of the Primary Piston moves to the left but does not create hydraulic pressure in
Hydraulic Circuits the primary pressure chamber. The Primary Piston therefore compresses
the Primary Return Spring, contacting the Secondary Piston and directly
moving the Secondary Piston. The Secondary Piston then increases
hydraulic pressure in the Secondary Circuit end of the master cylinder,
which allows two of the brakes to operate.
Leakage In
Primary Circuit
The primary piston
compresses the return
spring, contacts the
secondary piston, and
manually moves it.
Section 2
When fluid leakage occurs on the secondary side of the master cylinder,
hydraulic pressure in the Primary Chamber easily forces the
Secondary Piston to the left compressing the return spring. The
Secondary Piston advances until it reaches the far end of the cylinder.
Leakage in the
Secondary Circuit
Pressure is not generated
in the secondary side
of the cylinder. The
secondary piston
advances until it touches
the wall at the end
of the cylinder.
When the Primary Piston is pushed farther to the left, hydraulic

pressure increases in the rear (primary) circuit or pressure chamber of
the master cylinder. This allows one half of the brake system to operate
from the rear Primary Pressure Chamber of the master cylinder.
Separated The master cylinder we have been covering so far has only two piston
Reservoir Tank cups on the Secondary Piston and a single fluid reservoir. A third
piston cup is added to the Secondary Piston of master cylinders having
separate fluid reservoirs for the primary and secondary chambers.
Dual Reservoir
Master Cylinder
An additional piston
cup is added to the
secondary piston to seal
the secondary cylinder from
the primary cylinder.

Master Cylinder
The third piston cup is located between the front and rear piston cup of
the secondary piston and seals the Secondary Chamber from the Primary
Chamber. When the brakes are released after brake application, the
master cylinder pistons return faster than the fluid can, momentarily
creating low pressure (vacuum) in the Primary Chamber. It is the job of
the third piston cup to prevent fluid passage between the Secondary
Chamber and the Primary Chamber. If the piston cup were missing or
worn, fluid passing the third piston cup would fill the Primary Reservoir
and deplete the Secondary Reservoir. If left unchecked, the Secondary
Reservoir would empty allowing air into the secondary hydraulic circuit.
Role of the Second

Piston Cup of the
Secondary Piston
Prevents transfer of fluid
from the front tank
to the rear tank.
Residual Check Valve The Residual Check Valve is located in the master cylinder outlet to
the rear drum brakes. Its purpose is to maintain about 6 to 8 psi in the
hydraulic circuit. When the brakes are released the brake shoe return
springs force the wheel cylinder pistons back into the bore. Without the
Residual Valve the inertia of fluid returning to the master cylinder may
cause a vacuum and allow air to enter the system. In addition to
preventing a vacuum, the residual pressure pushes the wheel cylinder
cup into contact with the cylinder wall.
Master Cylinder
Residual Check Valve
Maintains about 6 to 8 psi in
the hydraulic circuit to
prevent air from entering.
Master Cylinder
Reservoir Tank The amount of the brake fluid inside the Reservoir Tank changes
during brake operation as Disc Brake Pads wear. A small hole in the
reservoir cap connects the reservoir to the atmosphere and prevents
pressure fluctuation, which could result in air being drawn into the
hydraulic circuit.
A tandem master cylinder having a single reservoir tank has a separator

inside that divides the tank into front and rear as shown below. The
two−part design of the reservoir ensures that if one circuit fails due to
fluid leakage, the other circuit will still be available to stop the vehicle.
Single Fluid
Reservoir Tank
A separator inside divides
the tank into front and rear
parts to ensure that if
one circuit fails the other
will still have fluid.
Brake Tubing Brake hydraulic components are connected by a network of seamless

steel tubes and hoses. Brake tubing is made of copper plated steel
sheets rolled at least two times and brazed into a single piece and
plated with tin and zinc for corrosion resistance. It is produced in
different lengths and pre−bent for the specific model applications. Each
end is custom flared in a two step process and fitted with a flare nut.
Double Flare Tubing

The tapered seats and
double flare tube provide a
compression fitting to
seal the connection.
Section 2
Portless Master The master cylinder design discussed up to this point has been the
Cylinder conventional compensating port and inlet port type used on most brake
systems. A new style master cylinder is used on late model vehicles
equipped with ABS and ABS/TRAC (Traction Control).
Initially introduced on the 1991 MR2 and Supra, which were rear wheel
drive vehicles, the front piston has a port−less design. The single passage
from the reservoir to the secondary piston is non−restrictive. The
secondary piston provides a machined passage to the secondary circuit
which is controlled with a valve. The valve is spring loaded to seal the
piston passage however, a stem attached to the valve holds it from contact
with the piston in the at rest" position. When the brakes are applied the
valve closes, sealing the passage and pressure is built in the secondary
circuit. The front piston controls pressure to the rear brake calipers.
The master cylinder on the 1997 Camry and Avalon incorporates

another master cylinder portless design. In this design a spring loaded
valve seals the passage in the piston however, in the at rest" position,
a stem attached to the valve contacts the piston retaining pin and
unseats the valve.
Three types of master cylinders are available on the 1997 Camry and
Avalon depending on the brake system options.
1. Non ABS Brake System − Conventional primary and secondary
master cylinder.
2. ABS Brake System − Portless secondary and conventional
primary master cylinder.
3. ABS and TRAC Brake System − Portless secondary and Portless
primary master cylinder.
Portless Master
Cylinder
The single passage from the
reservoir to the secondary
piston is non restrictive.
The secondary piston
provides a machined
passage to the secondary
circuit which is controlled
with a valve.

Section 2
Brake Fluid Level The brake fluid level warning switch is located on the reservoir cap or
Warning Light in some models, is wired within the reservoir body. It normally remains
Switch off when there is an appropriate amount of fluid. When the fluid level
falls below the minimum level, a magnetic float moves down and
causes the switch to close. This activates the red brake warning lamp
to warn the driver.
Brake Fluid Level

Warning Switch
If fluid level falls below
the minimum level, a
magnetic float moves down
and turns the switch on.
A typical brake warning lamp electrical circuit is shown below. It also

turns ON when the parking brake is applied.
Brake Warning Light

Electrical Circuit
Low brake fluid level or
parking brake light turn on.

Section 3
DRUM BRAKES
Lesson Objectives 1. Identify the components of the drum brake system.

2. Explain the operation of the drum brake system during brake
application.
3. Explain brake fluid flow return from the wheel cylinder to the
master cylinder.
4. Describe the function and operation of the self adjuster
mechanism.
5. Demonstrate the operation of adjusting the brake shoe clearance
using a vernier caliper or drum caliper.
Section 3
Drum Brakes The drum brake has been more widely used than any other brake
design. Braking power is obtained when the brake shoes are pushed
against the inner surface of the drum which rotates together with the
axle.
Drum brakes are used mainly for the rear wheels of passenger cars and
trucks while disc brakes are used exclusively for front brakes because
of their greater directional stability.
The backing plate is a pressed steel plate, bolted to the rear axle
housing. Since the brake shoes are fitted to the backing plate, all of the
braking force acts on the backing plate.
Drum Brake
Assembly
Drum Brakes are now
used mainly for the rear
wheels of passenger
cars and trucks.
Wheel Cylinder The wheel cylinder consists of a number of components as illustrated

on the next page. One wheel cylinder is used for each wheel. Two
pistons operate the shoes, one at each end of the wheel cylinder. When
hydraulic pressure from the master cylinder acts upon the piston cup,
the pistons are pushed toward the shoes, forcing them against the drum.
When the brakes are not being applied, the piston is returned to its
original position by the force of the brake shoe return springs.

Drum Brakes
Wheel Cylinder
Hydraulic pressure acting
upon the piston cup,
forces the pistons
outward toward the shoes.
Brake Shoes Brake shoes are made of two pieces of sheet steel welded together. The
friction material is attached to the lining table either by adhesive
bonding or riveting. The crescent shaped piece is called the web and
contains holes and slots in different shapes for return springs,
hold−down hardware, parking brake linkage and self adjusting
components. All the application force of the wheel cylinder is applied
through the web to the lining table and brake lining. The edge of the
lining table generally has three V" shaped notches or tabs on each side
called nibs. The nibs rest against the support pads of the backing plate
to which the shoes are installed.
Each brake assembly has two shoes, a primary and secondary. The
primary shoe is located toward the front of the vehicle and has the
lining positioned differently than the secondary shoe. Quite often the
two shoes are interchangeable, so close inspection for any variation is
important.
Linings must be resistant against heat and wear and have a high
friction coefficient. This coefficient must be as unaffected as possible by
fluctuations in temperature and humidity. Materials which make up
the brake shoe include friction modifiers, powdered metal, binders,
fillers and curing agents. Friction modifiers such as graphite and
cashew nut shells, alter the friction coefficient. Powdered metals
such as lead, zinc, brass, aluminum and other metals increase a
material’s resistance to heat fade. Binders are the glues that hold the
friction material together. Fillers are added to friction material in
small quantities to accomplish specific purposes, such as rubber chips
to reduce brake noise.
Section 3
Brake Shoes
and Lining
The friction material
is attached to the lining
table. The crescent shaped
web contains holes and
slots in different shapes for
return springs, hold-down
hardware, parking brake
linkage and self
adjusting components.
Brake Drum The brake drum is generally made of a special type of cast iron. It is
positioned very close to the brake shoe without actually touching it, and
rotates with the wheel and axle. As the lining is pushed against the inner
surface of the drum, friction heat can reach as high as 600 degrees F.
The brake drum must be:

1. Accurately balanced.
2. Sufficiently rigid.
3. Resistant against wear.
4. Highly heat−conductive.
5. Lightweight.

Drum Brakes
Drum Type Brake It is very important that the specified drum−to−lining clearance be
Adjustment accurately maintained at all times. In some types of brake systems, this is
done automatically. In others, this clearance must be periodically adjusted.
An excessively large clearance between the brake drum and lining will
cause a low pedal and a delay in braking. If the drum to lining
clearance is too small the brakes will drag, expand with increased heat,
and seizure between the drum and brake lining may occur.
Furthermore, if the clearance is not equal the rear−end of the vehicle
may fishtail (oscillate from side to side) as one brake assembly locks−up.
Automatic Brake Shoe Automatic clearance adjusting devices may be divided into two types:
Clearance Adjustment
• Reverse Travel Adjuster.
• Parking Brake Adjuster.
Reverse Travel Adjustment effected by braking effort during reverse travel is used
Adjuster with duo−servo type brakes. Duo−servo brake shoes have a single
anchor located above the wheel cylinder. When the leading shoe
contacts the drum it transfers force to the trailing shoe which is
wedged against the anchor. This system uses an:
• adjusting cable assembly.
• adjusting lever.
• shoe adjusting setscrew (star wheel).
• cable guide.
• lever return spring.
The adjusting cable is fixed at one end to the anchor pin, while the
other end is hooked to the adjusting lever via a spring.
The adjusting lever is fitted to the lower end of No. 2 brake shoe, and
engages with the shoe adjusting setscrew.
Reverse Travel
Brake Shoe
Adjustment
The adjusting cable
is fixed to the anchor pin,
the other end is hooked to
the adjusting lever and
engages with the shoe
adjusting set screw.
Section 3
When the brake pedal is depressed while the vehicle is moving

backward, the brake shoes expand and contact the drum. The shoes are
forced by the drum to begin rotating; however, the upper end of No. 1
shoe is wedged against the anchor pin. Since No. 2 shoe is moving away
from the anchor pin, it causes the adjusting lever to pivot and turn the
shoe adjusting screw and reduce the clearance. If clearance is proper,
the adjusting lever will not engage the tooth of the adjusting screw.
The shoe adjusting screw consists of a bolt and two nuts as shown
below. The bolt end is marked with a R" or L" to indicate which side
of the vehicle it is mounted on.
Shoe Adjusting
Set Screw
Each end of the screw is in
contact with a brake shoe.
Clearance decreases as
the screw turns.
Since each end of the adjusting screw is in contact with a brake shoe,
the brake shoe clearance decreases as the screw turns.
Adjusting Lever
Action
As the No. 2 shoe moves
away from the anchor pin,
the adjusting lever pivots
causing the adjusting
screw to turn.

Drum Brakes
Parking Brake The second type of automatic clearance adjustment operates by

Automatic Adjuster applying the parking brake. The adjusting lever is attached, together
with the parking brake lever, to the shoe. The lower end of the
adjusting lever is held to the brake shoe via a spring, and the other end
of the lever engages the adjusting screw pulling it downward.
When the parking brake is released, the brake lever is pushed to the
right. At the same time, the adjusting lever pivots, turning the
adjusting screw.
Parking Brake
Shoe Adjustment
The adjusting lever is
attached with the parking
brake lever to the shoe. The
lever engages the adjusting
screw pulling it downward.
When brake shoe clearance is greater than standard and the parking
brake lever is pulled, the adjusting lever moves over to the next tooth
of the adjusting screw.
When the parking brake lever is released, the adjusting lever spring
pulls the lever down. This causes the adjusting screw to rotate,
reducing the brake shoe clearance.
Section 3
Adjusting Lever
Rotates Adjusting
Screw
When the parking brake
lever is pulled the adjusting
lever engages the next tooth
on the adjusting screw.
When the parking brake
lever is released, the
adjusting lever rotates the
adjusting screw.
When the brake shoe clearance is normal and the parking brake lever
is pulled, the adjusting lever moves only a small distance. The
adjusting lever does not move to the next tooth of the adjusting screw.
Brake shoe clearance remains unchanged as a result.
Normal Brake
Shoe Clearance
With proper clearance the
adjusting lever does not
engage the next
tooth of the screw.
The adjusting lever is arranged in such a way as to engage with one

adjusting screw tooth. Therefore, one operation of the parking brake
lever only advances the adjusting screw by one tooth, reducing brake
shoe clearance by approximately 0.012" (0.03mm), even when there is a
large amount of brake shoe clearance.

Drum Brakes
Lining that is eccentrically ground, that is having clearance at the heel

and toe when held against the drum face, can tolerate a closer drum to
shoe clearance. As the brakes are applied, the center of the lining
contacts the drum first. As hydraulic pressure increases, the shoe will
stretch slightly and allow additional lining contact and ensures
consistent pressure over a larger area of lining. As the shoes wear−in
they will fit the contour of the drum more closely.
Place the lining inside the drum and press it against the contour of the
drum to ensure heel and toe clearance. If the heel and toe have heavy
contact it is likely that the brakes will grab and cause the wheels to
lockup.
Eccentrically Ground
Brake Lining
The center of the lining
contacts the drum first. As
pressure increases the shoe
will stretch slightly and allow
additional lining contact and
ensures consistent pressure
over a larger area of lining.
Initial Brake Shoe Initial clearance between the shoe and the drum must be set when new
Adjustment brake shoes are installed. A specific clearance of 0.60 mm, (0.024") is
stated in the Repair Manual for most models.
Use the following procedure to set the initial adjustment:

• Shoes must be centered on the backing plate.
• Measure the inside diameter of the drum with a vernier caliper.
Section 3
• Reduce the measurement by 0.024" or (0.60 mm).

• Turn the adjuster until the distance between the shoes at the center
of the arc just contacts the vernier caliper.
• When installing the drum, there should be no heavy drag of the
drum and shoes as the drum is turned. Apply the parking brake
several times to center the shoes and check for drag. Back−off
adjustment if brakes continue to drag.
Setting the
Brake Shoe
Initial Adjustment
Measure the inside diameter
of the drum with a vernier
caliper. Reduce the
measurement by 0.024”.
Turn the adjuster until the
distance between the shoes
at the center of the arc just
contacts the vernier caliper.

Drum Brakes
Brake Adjustment A special gauge shown below is available from domestic tool sources
Caliper which provides a built−in 0.030" clearance.
Using the narrow end of the gauge, place it in the drum and extend it
the full diameter. Use the thumb screw to lock the position. Use the
wide end of the gauge to set the brake shoe position. The shoe to drum
clearance is preset in the tool design.
Brake Adjustment
Caliper
Adjusting the caliper
to the inside diameter
of the drum establishes the
correct shoe to
drum clearance.
Section 4
DISC BRAKES
Lesson Objectives 1. Identify the components of the disc brake system.

2. List the advantages of a disc brake system over a drum brake
system.
3. Describe the self−adjustment of the brake caliper piston.
4. Explain the function of anti−squeal shims and support plates for
brake noise reduction.
5. List the advantages of multiple pistons on a fixed caliper design.

Disc Brakes
Components A disc brake assembly consists of a:

and Operation • cast−iron disc (disc rotor) that rotates with the wheel.
of Disc Brakes • caliper assembly attached to the steering knuckle.
• friction materials (disc pads) that are mounted to the caliper
assembly.
When hydraulic pressure is applied to the caliper piston, it forces the

inside pad to contact the disc. As pressure increases the caliper moves
to the right and causes the outside pad to contact the disc. Braking
force is generated by friction between the disc pads as they are
squeezed against the disc rotor. Since disc brakes do not use friction
between the lining and rotor to increase braking power as drum brakes
do, they are less likely to cause a pull.
The friction surface is constantly exposed to the air, ensuring good heat
dissipation, minimizing brake fade. It also allows for self−cleaning as
dust and water are thrown off, reducing friction differences.
Unlike drum brakes, disc brakes have limited self−energizing action

making it necessary to apply greater hydraulic pressure to obtain
sufficient braking force. This is accomplished by increasing the size of
the caliper piston. The simple design facilitates easy maintenance and
pad replacement.
Disc Brake
Assembly
Disc rotor, caliper and
disc pads are the
major components.
Section 4
Disc Rotor Generally, the disc rotor is made of gray cast iron, and is either solid or
ventilated. The ventilated type disc rotor consists of a wider disc with
cooling fins cast through the middle to ensure good cooling. Proper cooling
prevents fading and ensures longer pad life. Some Ventilated rotors have
spiral fins which creates more air flow and better cooling. Spiral finned
rotors are directional and are mounted on a specific side of the vehicle.
Ventilated rotors are used on the front of all late model Toyotas.
The solid type disc rotor is found on the rear of four wheel disc brake
systems and on the front of earlier model vehicles.
A third style rotor can be either the ventilated or solid type which
incorporates a brake drum for an internal parking brake assembly.
Disc Rotor Types

The type of rotor is
determined by the
vehicles intended use.

Disc Brakes
Caliper The caliper, also called the cylinder body, houses one to four pistons,
(Cylinder Body) and is mounted to the torque plate and steering knuckle or wheel
carrier. It is found in floating caliper designs or fixed caliper designs on
Toyotas.
Floating Caliper Type The floating caliper design is not only more economical and lighter
weight but also requires fewer parts than it’s fixed caliper counterpart.
Depending on the application, the floating caliper has either one or two
pistons.
The piston is located in one side of the caliper only. Hydraulic pressure
from the master cylinder is applied to piston (A) and thus presses the
inner pad against the disc rotor. At the same time, an equal hydraulic
pressure (reaction force B) acts on the bottom of the cylinder. This
causes the caliper to move to the right, and presses the outer pad
located opposite the piston against the disc rotor.
Floating Caliper
The piston exerts pressure
on the inside pad as well
as
moving the caliper body to
engage the outside pad.
Section 4
Fixed Caliper Type The fixed caliper design has pistons located on both sides of the caliper
providing equal force to each pad. The caliper configuration can
incorporate one or two pistons on each side. The ability to include
multiple pistons provides for greater braking force and a compact
design. Because these assemblies are larger and heavier than the
floating caliper, they absorb and dissipate more heat. This design is
able to withstand a greater number of repeated hard stops without
brake fade.
This design is found on models which include larger engine

displacement such as the V−6 Camry and Avalon as well as the Supra
and four−wheel−drive Truck, T100 and Tacoma.
Fixed Caliper
The ability to include
multiple pistons provides for
greater braking force and
a compact design.

Disc Brakes
Brake Pad Different brake design applications require different kinds of friction
materials. Several considerations are weighed in development of brake
pads; the coefficient of friction must remain constant over a wide range
of temperatures, the brake pads must not wear out rapidly nor should
they wear the disc rotors, should withstand the highest temperatures
without fading and it should be able to do all this without any noise.
Therefore, the material should maximize the good points and minimize
the negative points.
Materials which make up the brake pad include friction modifiers,

powdered metal, binders, fillers and curing agents. Friction
modifiers such as graphite and cashew nut shells, alter the friction
coefficient. Powdered metals such as lead, zinc, brass, aluminum and
other metals increase a material’s resistance to heat fade. Binders are
the glues that hold the friction material together. Phenolic resin is the
most common binder in current use. Fillers are added to friction
materials in small quantities to accomplish specific purposes such as
rubber chips to reduce brake noise.
Brake Pad Assembly

Multiple plates called anti-
squeal shims, are provided
on the piston side of the pad
to minimize brake squeal.
Various springs and clips are
used to reduce rattle as well
as reduce brake noise.
The brake pad material is bonded to a stamped steel backing plate

with a high temperature adhesive to which heat and pressure are
applied to cure the assembly. A slit is provided on the face of the pad to
indicate the allowable limit of pad wear and provide a path for brake
dust and gas to escape.
A metal plate, or in some applications multiple plates called anti−squeal

shims, are provided on the piston side of the pad to minimize brake
squeal. Various springs and clips are used to reduce rattle as well as
reduce brake noise. Shims and plates should be inspected for wear and
rust and can be re−used when replacing pads. Fresh approved grease
should be applied to the shims prior to installation.
Section 4
Pad Wear Indicator A pad wear indicator has been adopted on some models that produces a
high screeching noise when the pad is worn down to a predetermined
thickness. The purpose of the indicator is to warn the driver and
prevent damage to the rotor should the brake pad wear further. The
indicator contacts the rotor while the wheel turns and the brakes are
not applied. A customer may comment that the noise stops when the
brakes are applied.
Be sure to install the wear indicators when new pads are installed.
Pad Wear Indicator

Produces a high
screeching noise when the
pad is worn down to a
predetermined thickness.

Disc Brakes
Automatic Disc brakes also have the advantage of being self adjusting. The pads
Adjustment of are always right next to the spinning rotor. This adjustment is
maintained in all models by a square cut piston seal which is seated in
Rotor-to-Pad a machined groove in the cylinder bore. Any wear of the lining is
Clearance automatically compensated for by the action of the brake caliper.
When the brakes are applied, the caliper piston moves out toward the
rotor until the brake pad contacts it. The piston seal twists or deforms
elastically as shown below. When the brake pedal is released and
hydraulic pressure is reduced, the piston seal returns to its original
shape, pulling the piston back. As the brake pads wear, the piston
continually moves outward through the seal to maintain proper pad to
rotor clearance.
Self Adjusting
Mechanism of the
Disc Caliper
Piston seal deforms as the
piston moves outward.
It returns to its original
shape, pulling the piston
back when the
brakes are released.
Section 4

Section 5
BRAKE BOOSTER
Lesson Objectives 1. Explain the function of engine vacuum in providing brake assist to
2. Perform the following booster tests using the brake pedal:
− operating test
− air tightness check
− air tightness under load
3. Using a brake booster push rod gauge SST, measure booster push
rod clearance and determine needed adjustment.
4. List the symptoms of an improperly adjusted booster push rod.
Section 5
Brake Booster The brake booster is designed to create a greater braking force from a
minimum pedal effort, using a difference in atmospheric pressure and
the engine’s manifold vacuum. It increases the pedal force 2 to 4 times
depending on the size of the diaphragm. The brake booster is located
between the brake pedal and the master cylinder.
When pressure is applied to the brake pedal, pressure is exerted on the

booster air valve. With pressure created by the booster the master
cylinder is applied. Should the booster malfunction, the normal
mechanical braking force of the master cylinder is maintained.
Construction The brake booster consists of the body, booster piston, piston return
spring, reaction mechanism, and control valve mechanism.
The body is divided into a constant pressure chamber and a variable

pressure chamber. The chambers are separated from each other by a
diaphragm. The control valve mechanism regulates the pressure inside
the variable pressure chamber.
Single Diaphragm
Booster
The body is divided into a
constant pressure chamber
and a variable pressure
chamber separated from
each other by a diaphragm.

Brake Booster
Basic Booster The basic principle of the brake booster is pressure differential. When
Operation vacuum is applied to both sides of the piston, the piston is pushed to
the right by the spring and remains there.
Control Valve Closed

When vacuum is applied
to both sides of the piston,
the piston is pushed to
the right by the spring.
When atmospheric air is allowed into chamber B the piston starts to

compress the spring, due to the difference in pressure, and moves to
the left. This causes the piston rod to move the piston of the master
cylinder, generating hydraulic pressure.
Control Valve Open

When atmospheric air is
allowed into chamber (A),
the piston starts to compress
the spring due to the
difference in pressure.
Booster Air Valve In the OFF position, the Air Valve (connected to the Valve Operating
Operation Rod) is pulled to the right by the Air Valve Return Spring. The Control
Valve is pushed to the left by the Control Valve Spring. This causes the
Air Valve to contact the Control Valve. Therefore, the atmospheric air
that passes through the air cleaner element is prevented from entering
the Variable Pressure Chamber.
Section 5
The piston’s Vacuum Valve is separated from the Control Valve in this
position, providing an opening between passage A and passage B. Since
there is always vacuum in the Constant Pressure Chamber, the
opening allows vacuum into the Variable Pressure Chamber. As a
result, the piston is pushed to the right by the piston return spring.
Booster Air Valve

Brakes Not Applied
The Vacuum Valve is open
allowing vacuum on both
sides of the booster piston.
In the ON position, when the brake pedal is depressed, the Valve

Operating Rod pushes the Air Valve to the left. The Control Valve
which is pushed against the Air Valve by the Control Valve Spring,
moves to the left until it touches the Vacuum Valve. This blocks off the
opening between passage A and passage B (Constant Pressure
Chamber (A) and Variable Pressure Chamber (B)).
Booster Air Valve

Brakes Applied
The vacuum valve is closed,
cutting off the vacuum
source to the variable
pressure chamber.

Brake Booster
As the Air Valve moves further to the left, it moves away from the
Control Valve. This allows atmospheric pressure to enter the Variable
Pressure Chamber through passage B. The pressure difference between
the Constant Pressure Chamber and the Variable Pressure Chamber
causes the piston to move to the left. This, in turn, causes the Reaction
Disc to move the Booster Push Rod to the left and exert braking force.
Booster Air Valve

Brakes Applied
Air Valve opens allowing
atmospheric air to enter the
variable pressure chamber.
Released When the brake pedal is released, the Valve Operating Rod and the Air
Position Valve are moved to the right by the Air Valve Return Spring and
reaction force of the master cylinder. This movement causes the Air
Valve to contact the Control Valve, blocking atmospheric pressure from
the Variable Pressure Chamber. At the same time, the Air Valve also
retracts the Control Valve Spring. The Control Valve moves away from
the Vacuum Valve, connecting passage A with passage B.
This allows atmospheric pressure from the Variable Pressure Chamber

to flow into the Constant Pressure Chamber. The pressure difference is
eliminated between the two chambers and the piston is pushed back to
the right by the Diaphragm/Piston Return Spring. The booster returns
to the released position.
Section 5
Booster Air Valve

Released Position
Pressure equalizes in the
two chambers and the air
valve is closed.
Lack Of Vacuum If vacuum fails to act on the brake booster, for any reason, there will be
no difference in pressure between the Constant Pressure Chamber and
the Variable Pressure Chamber. When the brake is in the OFF"
position, the piston is returned to the right by the Piston Return
Spring.
When the brake pedal is depressed, the Valve Operating Rod advances
to the left and pushes the Air Valve, Reaction Disc, and Booster Push
Rod. This movement causes the master cylinder piston to apply braking
force to the brake system, maintaining brake system operation.
Booster Air Valve

No Vacuum
Although the booster loses
self-energizing force when
vacuum is lost, it
still generates hydraulic
pressure mechanically
and can maintain brake
system operation.

Brake Booster
Tandem Brake The tandem type brake booster is a compact and extremely powerful
Booster unit having two Constant Pressure Chambers and two Variable
Pressure Chambers. A Piston separates each variable and constant
pressure chamber. With two pistons incorporated into this design, a
large surface area provides additional boost while taking up less space.
When the brakes are not applied the Air Valve and Valve Operating
Rod are pushed to the right by the tension of the Air Valve Return
Spring, and stop when they contact the Valve Stopper Key. Since the
Air Valve pushes the Control Valve back toward the right, the passage
through which atmospheric air from the air cleaner element enters the
booster, is closed. Since the Vacuum Valve and the Control Valve are
not in contact with each other, pressure is equalized between the two
chambers through passage (A) and passage (B).
Therefore, vacuum is applied to both the Constant Pressure Chambers

and the Variable Pressure Chambers; so, there is no difference in
pressure between the chambers on both sides of the piston.
Tandem Brake
Booster
The tandem type
brake booster is a
compact and extremely
powerful unit having two
vacuum chambers.
Section 5
Brakes Applied When the brake pedal is depressed, both the Valve Operating Rod and Air
Valve are pushed to the left together. As a result, the Control Valve and
Vacuum Valve come into contact with each other, closing passages (A) and
(B) (the constant pressure chamber and variable pressure chamber).
Next, the Air Valve moves away from the Control Valve, and
atmospheric air from the air cleaner element passes through passage
(B) and enters the Variable Pressure Chamber. This generates a
pressure difference between the Variable Pressure Chamber and the
Constant Pressure Chamber, and the pistons move to the left.
The forces applied by the pistons, which occur due to the pressure
difference, are transmitted to the Reaction Disc via the Valve Body. They
are further transmitted to the Booster Push Rod, becoming the booster
output force. The combined surface area of pistons No. 1 and No. 2,
multiplied by the pressure difference between the Constant Pressure
Chamber and Variable Pressure Chamber, equals the booster output force.
Tandem Brake
Booster - Brakes
Applied
The operation of the air
valve and booster is the
same as the single
diaphragm booster.

Brake Booster
Booster Diagnosis The following steps are taken to diagnose the brake booster.
Operating Check With the engine stopped, depress the brake pedal normally, several
times. The brake pedal must be depressed before the engine is started
in order to remove vacuum from the booster.
With the brake pedal depressed start the engine. When the engine is
started, vacuum is created and operates the booster. This causes the
brake pedal to go down.
If the brake pedal goes down slightly, the booster is operating normally.
If the brake pedal does not move, the booster is not receiving manifold
vacuum, or is malfunctioning.
Booster Operating
Check
The brake pedal should sink
when the engine starts.
Section 5
Air Tightness Check Start the engine and let it run for one or two minutes, then shut it off.
Now step on the brake pedal several times, applying normal pressure.
Be sure to wait about five seconds between each depression of the
pedal. If the brake pedal reserve distance increases every time the
pedal is depressed, the booster has good air tightness.
Booster Air
Tightness Check
Pedal reserve distance
increases with successive
pedal depressions.
The brake pedal reserve distance changes every time the pedal is
depressed, because the vacuum that is stored in the booster is reduced
every time the brake pedal is depressed.
The brake pedal reserve distance will not change if the Check Valve is
defective. The check valve is located on the vacuum booster body or
between the booster body and the source of engine vacuum. It’s purpose
is to act as a one−way valve and seal vacuum in the booster to provide at
least two power assist stops should the engine stop running. To check
the Check Valve and vacuum hose piping use the following procedure:
• Remove the vacuum hose and valve from the booster.
• Block the valve with a finger and start the engine.
• A strong vacuum should be felt if the piping and valve are
operating.
• The vacuum must remain unchanged for approximately one minute
after the engine is stopped.
Lack of vacuum indicates a malfunction in the check valve or the

vacuum hose piping. If the vacuum appears normal, there may be a
problem in the booster itself.

Brake Booster
Air Tightness Test Depress the brake pedal when the engine is running, then stop the
Under Load engine and wait for about 30 seconds. If the brake pedal position does
not change, the brake booster is functioning normally. It is defective if
the brake pedal moves up.
The brake pedal reserve distance remains unchanged because vacuum

is maintained in the Constant Pressure Chamber.
Booster Air Tightness

Under Load
Stop the engine with the
brake pedal depressed, the
brake pedal should maintain
the same height for more
than 30 seconds.
Section 5
Booster The Booster Push Rod projects from the front of the Brake Booster and
Push Rod activates the master cylinder. The push rod is adjustable and the
clearance must be checked any time the master cylinder or booster is
Adjustment replaced. This is required to ensure the correct gap between the master
cylinder piston and the booster push rod.
Problems can occur if the push rod is improperly adjusted:

• If the gap is too small, it may cause brake drag and premature
brake wear.
• If the gap is too large, it may cause brake delay and reduced pedal
reserve distance.
Prior to making the adjustment:

• Check the brake pedal freeplay to ensure the booster is not
partially applied.
• Make the adjustment with the engine running to ensure the booster
has vacuum. The booster body will change shape when a vacuum is
applied and may reduce the clearance.
Booster Push
Rod Gauge
The push rod is adjustable
and the clearance must be
checked any time the master
cylinder is replaced.
Adjusting Procedure:
1. Place a new gasket on the flange of the master cylinder. Set the
push rod gauge over the end of the master cylinder with the
rounded end of the tool plunger toward the piston.
2. Push the plunger down until it just touches the bottom of the
piston bore.
3. Turn the gauge over and set the flat plunger end of the gauge on
the booster and over the push rod. There should be no clearance
between the booster push rod and the plunger.
4. Adjust the booster push rod if necessary. (If the brake pedal is
depressed to expose the adjustment nut, be sure to start the engine
before checking the adjusted clearance.)

Brake Booster
Alternate Method for The preferred method of adjustment is the Booster Push Rod Gauge
Booster Adjustment procedure just described. If the special service tool is not available the
measurement procedure described here can be used to ensure a
calculated clearance prior to installation of the master cylinder.
In this procedure, measure the distance between the bottom of the bore
in the master cylinder primary piston to the top of the flange gasket
using a depth micrometer or vernier caliper.
1. Measure from the rim of the cylinder bore to the new gasket on the
flange, (measurement A")
2. Measure from the rim of the cylinder bore to the bottom of the bore
in the primary piston, (measurement B")
3. Subtract A from B will give the depth of the piston bore from the
master cylinder flange gasket, (measurement C")
Alternate
Measurement Method
If the Booster Push Rod
Gauge is not available,
use a vernier
caliper to establish
proper clearance.
Next, measure the height of the booster push rod.

1. Place a precision straight edge across the face of the booster body
adjacent to the push rod.
2. Measure from the top of the straight edge to the top of the push rod.
(measurement D")
3. Measure the width of the straight edge, (measurement E")
4. Subtract measurement D" from E" will give the height of the push
rod. (measurement F")
5. Clearance is determined by subtracting F" from C".
6. Adjust the push rod to obtain approximately 0.1 mm to 0.5 mm
clearance.
Section 5
WORKSHEET 5-1 (ON-CAR)

Brake Pedal Measurement
Vehicle Year/Prod. Date Engine Transmission
In this Worksheet you will practice the procedure for measuring pedal height, pedal free play and pedal reserve
distance.
Tools and Equipment:

• Measuring tape.
• Assortment of open-end wrenches.
• Feeler gauge.
• Trim removal tool.
Pedal Height:
1. Pull the carpet down from the bulkhead to the foot well to reveal the asphalt melt sheet, (remove sill plate or
trim as needed)
2. Using the measuring tape, measure at a right angle from the brake pedal pad to the melt sheet.
3. Record your measurement in the box below.
Measured Brake Pedal

Specification Pass/Fail
Height
1. Is the brake pedal height adjustable? If yes, explain how.
2. What effect would a low pedal height have on the brake system? Explain your answer.

Brake Booster
Brake Pedal Freeplay:

1. Stop the engine and depress the brake pedal several times until there is no vacuum in the booster.
2. Depress the pedal by hand until the beginning of resistance is felt. Record this measurement below.

Freeplay
1. Why is the vacuum booster depleted before checking brake pedal freeplay?
2. If brake pedal freeplay is less than specification, what possible adjustment should be checked?
Brake Pedal Reserve Distance:

1. Release the parking brake.
2. With the engine running, depress the pedal with approximately 110 pounds of force.
3. Measure the pedal reserve distance at a right angle from the pedal pad to the melt sheet.

Reserve Distance
1. If the brake pedal height is within specification but pedal reserve distance is insufficient, list several
possible causes?
2. Is brake pedal reserve distance adjustable? If yes, explain.

Section 5

Booster Push Rod Adjustment
Worksheet Objectives
In this Worksheet you will practice the procedure for measuring booster push rod to master cylinder clearance.

• Depth Micrometer.
• Straight Edge.
• Push Rod Gauge. (SST 09737-00010)
• 10mm combination wrench.
• Tubing Wrench set.
• Plugs for master cylinder ports.
Preparation:
• With the engine off, pump the brake pedal several times to reduce vacuum in the booster.
• Loosen and remove the brake tubes from the master cylinder.
• Remove the master cylinder from the brake booster.
Measurement: (Using the special service tool)

1. Place a new gasket on the master cylinder.
2. Centering the Push Rod Gauge pin over the master cylinder piston and position the gauge on the gasket of
3. Lower the pin into the piston until it lightly touches the bottom of the bore.
4. Start the engine and turn the opposite end of the gauge and center the head of the pin over the booster push rod.
5. Adjust the push rod as needed to ensure no gap between the push rod and the head of the pin.
6. Turn the engine OFF, deplete the vacuum in the booster by depressing the brake pedal several times.
7. Place the gauge over the booster push rod and push the pin toward the push rod. Did it move? Why?

Brake Booster
Measurement: (Using the depth gauge)

• Place a new gasket on the master cylinder.
• Using a depth micrometer measure from the rim flange of the cylinder bore to the new gasket. This is
measurement A.
• Using a depth micrometer measure from the rim flange of the cylinder bore to the bottom of the piston bore.
This is measurement B.
• Place a straight edge over the brake booster gasket mating surface.
• Measure from the top side of the straight edge to the top of the booster push rod. This is measurement C.
• Subtract measurement C from the width of the straight edge to get measurement F. (push rod height)
Summary:
1. Using the measurements below, calculate the push rod clearance.
A =13.76 mm
B = 20.8 mm
D = 28.5 mm
E = 35.5 mm
2. List two occasions when this adjustment should be done.
3. What difference is there between performing the adjustment with the engine running and not running?
4. If the push rod was too long, what is the most likely result?
5. If the push rod is too short (the clearance between the push rod and master cylinder piston is too great),
what is the most likely result?
Section 6
PARKING BRAKE
Lesson Objectives 1. Describe the purpose of the parking brake system.

2. Explain the clutch spring and sleeve nut function in maintaining
the adjustment of the caliper parking brake.
3. Describe the procedure to reseat the piston of the caliper parking
brake during pad replacement.
4. Explain the operation of the exclusive parking brake assembly.

Parking Brake
Parking Brake The parking brake system is a secondary braking system used to hold a
Mechanisms parked car in position. They are applied independently of the service
brakes. Since there is no inertia to overcome, less braking power is
required to hold the vehicle stationary and less force is required to
apply. The application of only two of the four brake assemblies are
required to hold the vehicle.
There are three styles of rear parking brake systems. Two types use
the service brake and the other is an exclusive parking brake design.
The service type parking brake uses part of the ordinary service brake
mechanism and operates the shoe or piston mechanically.
The parking brake lever is located near the driver’s seat. Pulling the
parking brake lever by hand or pressing the pedal with the foot,
operates the brake via a cable connected to the parking brake lever of
the brake assembly.
There are a number of different types of parking brake levers, as

shown below. Application depends upon the design of the driver’s seat
and the desired operating effort.
The parking brake lever is provided with a ratchet locking mechanism to

maintain the lever at the position to which it was set, until released. Some
parking levers have an adjusting screw near the brake lever so the amount
of brake lever travel can be easily adjusted. Travel is determined by the
number of clicks of the ratchet mechanism found in the Repair Manual.
Parking Brake
Levers
A ratchet locking
mechanism holds the
lever in position when set.
Section 6
Parking Brake The parking brake cable transmits the lever movement through a
Linkage typical series of components, as shown below, to the brake drum
subassembly. The Intermediate Lever multiplies the operating force to
the Equalizer. The Equalizer divides the lever operating force to brake
assemblies at both wheels. The two major parts may vary in design
however, their function remains the same.
Linkage Components
The intermediate lever
multiplies the operating
force to the equalizer.
The equalizer divides the
force to brake assemblies
at both wheels.
Drum Parking Brake On all models using drum brakes on the rear, the cable pulls the
parking brake lever. The lever is attached to the secondary shoe at the
top and transfers the lever action to the primary shoe through the shoe
strut. When released, the brake shoe springs return the shoes to their
retracted position.
Drum Type Parking

Brake Components
The cable pulls the parking
brake lever and transfers the
lever action to the primary
shoe through the shoe strut.

Parking Brake
Disc Parking Brakes There are two types of rear wheel parking brake systems for disc
brakes. The first uses the brake caliper assembly to mechanically apply
pressure to the disc. The second type is an exclusive drum brake
assembly that applies pressure to an inside drum, which is an integral
part of the disc rotor.
Caliper Parking Brake The parking brake is built into the caliper housing and is provided
with an automatic adjusting mechanism to compensate for piston
movement as the brake pads wear.
Caliper Parking
Brake Assembly
The piston is mechanically
forced to engage the
pads to the rotor.
Automatic Adjusting The automatic adjusting mechanism maintains the operating clearance
Mechanism between the pads and the rotor as the pads wear down with use. The
primary assembly which makes this possible is the Sleeve Nut and
Adjusting Bolt. The Sleeve Nut is held by the Clutch Spring which
allows it to turn in one direction only. The diameter of the Clutch
Spring is slightly smaller than the diameter of the sleeve nut and
allows it to turn in the unwind direction only. The clutch spring is held
stationary with one end attached to the piston.
When the brake pedal is depressed, hydraulic pressure forces the

piston to move to the left. The movement of the Piston exerts pressure
on the Thrust Plate and Thrust Bearing against the Sleeve Nut
causing it to be screwed out from the stationary Adjusting Bolt. The
Sleeve Nut can be easily screwed out because the Clutch Spring
unwinds and therefore does not prevent the Sleeve Nut from rotating.
The distance that the Sleeve Nut screws out from the Adjusting Bolt is
equal to the amount of pad wear.
Section 6
The piston head is provided with two recesses, one of which engages
with a pin that protrudes from the backing plate of the brake pad. This
pin prevents the piston from being rotated by the automatic adjuster.
The adjusting bolt stopper prevents the adjusting bolt from rotating.
The only part allowed to turn is the sleeve nut.
Piston Head Design

The pin on the brake pad
is indexed to the recess of
the piston head to prevent
the piston from rotating.
When brake pads are replaced, the piston with the sleeve nut must be
forcibly rotated into the cylinder with the Special Service Tool shown
below (SST 09719−00020).
Rear Caliper Pad

Replacement
The piston must be
forcibly screwed into the
cylinder with the SST.
After pad replacement, the parking brake lever travel adjustment

should be performed. Depress the brake pedal several times to activate
the automatic adjustment within the caliper. Then adjust the cable for
the proper number of clicks of the hand brake lever.

Parking Brake
Parking Brake When the parking brake is applied, the cable attached to the parking
Operation brake lever rotates the crank lever counterclockwise. The crank pin
then pushes the strut to the left. The strut moves the adjusting bolt,
sleeve nut, and piston toward the left. As the strut moves to the left, it
also compresses the adjusting bolt return spring. The assembly moves
until it presses the pads against the disc rotor.
Parking Brake
Operation
The crank pin pushes the
strut and adjusting bolt,
sleeve nut and piston
toward the disc rotor.
When the parking brake lever is released, the compressed Return

Spring pushes the Adjusting Bolt and Piston back to their previous
positions. As a result, the parking brake is released.
During this operation, the Clutch Spring prevents the rotation of the
Sleeve Nut so that the force of the parking brake lever is transferred to
the Piston via the Adjusting Bolt.
Section 6
Exclusive Parking The exclusive parking brake is found on the LandCruiser, Supra,
Brake Celica, Previa, Avalon and Camry. As illustrated below, a drum brake
is cast into the disc rotor. The shoes and other components are similar
to a conventional dual−servo drum brake system but smaller and with
no wheel cylinder.
Exclusive Parking
Brake Assembly
A pair of brake shoes are
mounted to the backing
plate and a brake drum is
cast into the disc rotor.
Activating the parking brake is similar to applying the parking brake

on conventional drum brakes. Adjustment to the exclusive parking
brake is done manually at the Shoe Adjusting Screw Set (Star Wheel)
and must be done periodically.
Exclusive Parking
Brake Components
Manual adjustment is made
at the Shoe Adjusting
Screw Set.

Section 7
BRAKE DIAGNOSIS
Lesson Objectives 1. Convey the importance of verifying the customers complaint.

2. Describe road test procedures used to determine the cause of
brake vibration.
3. Using the brake pedal, establish pedal height, pedal freeplay and
reserve distance for initial brake system diagnosis.
4. Determine the serviceability of a drum or rotor using a measuring
device and specification table.
5. Turn a rotor using the on−car brake lathe.
6. Measure hub runout and rotor runout using a dial indicator.
7. Using a dial indicator, phase−match a rotor and hub for minimum
rotor runout
8. Explain the cause of rotor parallelism.
9. Using a micrometer or vernier caliper, measure rotor parallelism.
10. Using a brake drum micrometer, determine brake drum diameter
and serviceability.
Section 7
Diagnosing Diagnosing a problem in the brake system is similar to diagnosing

Brake Problems problems in any other system in the vehicle. The plan of action should
include the following:
• verify the customers complaint.
• identify the symptoms.
• isolate the cause.
• repair the problem.
• check for proper operation.
Verify Customers Begin by determining the symptoms based on the customer’s complaint
Complaint recorded on the RO. If your information is incomplete and you proceed
to fix what you find to be a problem, other than what the customer
complained about, you both lose. The customer has to bring the car
back and you may not get paid for the work you did.
You’re the expert, the customer brings the vehicle in because they
perceive a problem. When you service the vehicle and don’t take care of
the problem, you look bad and so does the dealer. Worst of all, the
customer may not return. So get more information when in doubt.
If you can’t verify the customer’s complaint, it may be necessary to go

for a test drive with the customer so that he can point out the
symptom. The problem may not be brakes at all. It could be something
as simple as a bowling ball in the trunk that makes a banging noise
when the brakes are hit hard, or it could be a loose suspension
component or worn bushings.
Attempt to find the source of the customer’s complaint: then any

additional things you find to be improper can be brought to the
customer’s attention for their approval to repair.
Chances are you have done a previous repair for a similar symptom.
That gives you an advantage in your diagnosis.
Before you take the customer’s car out for a test drive, make sure it’s
safe. If the pedal goes to the floor sometimes but seems fine now, do
some preliminary checks before you take it out on the road.

Brake Diagnosis
Preliminary Checks Preliminary checks should establish that the essentials are intact and
operational. Check fluid level in the master cylinder. Even though it is
full, check for leaks, the reservoir may have been topped off prior to
bringing it in for service. Check the following for signs of leakage:
• brake backing plate.
• flexible brake hoses.
• connections.
• brake tubing.
• auxiliary valves.
• master cylinder.
Brake System
Inspection
Check for leakage at the
brake backing plate, flexible
brake hoses, connections,
brake tubing, auxiliary valves
and the master cylinder.
Pedal Height When checking the brake pedal travel, start with the pedal height. It
should be measured from the asphalt sheet, below the carpet, to the top
of the pedal pad. Pedal height is adjusted using the push rod to
establish the pedal position.
Section 7
Brake Pedal Height

Measured from the asphalt
sheet, below the carpet,
to the top of the pedal
pad. Adjusted using the
push rod and clevis.
Pedal Freeplay Make sure that the freeplay is at least 0.040" to 0.120" (1 − 3 mm).
Turn the engine off and apply the brakes several times to reduce the
vacuum in the booster. If freeplay is less than specified, the brakes may
be lightly applied at all times, overheating the brakes and causing
premature wear. If there is too little free play, check the Stop Light
Switch for proper clearance as shown below.
Brake Pedal Freeplay

Turn the engine off
and apply the brakes
several times to reduce the
vacuum in the booster.

Brake Diagnosis
Pedal Reserve Measure the distance from the melt sheet to the top of the brake pedal
Distance while applying the brakes with the engine running. Insufficient reserve
distance may be caused by:
• rotor run−out or loose wheel bearings. In either case the rotor
pushes the caliper piston further into the cylinder requiring
additional pedal travel to move the brake pads into contact with the
rotor.
• inoperative automatic adjusters reduced reserve distance as the
shoes must travel further to contact the drum.
• air in the line will also cause the pedal to go further to the floor as
air in the system compresses. When air is in the lines, the pedal
will also feel spongy. You may be able to verify air in the system by
pumping the pedal several times compressing the air. Remove the
reservoir cover and observe the brake fluid as the brake pedal is
released. The compressed air will cause the returning fluid to shoot
above the side of the reservoir.
Brake Pedal
Reserve Distance
With the engine running
apply the brakes and the
distance from the top of the
pedal pad to the melt sheet
should be as specified in
the Repair Manual.
If the pedal is hard and the braking inefficient, suspect the booster or
its vacuum source. Go through the booster diagnostic steps under the
booster section of this text.
Section 7
Brake Pad Inspection Worn pads or shoes may be quite obvious, but when you look closely
and compare the wear side to side it may give you a clue as to their
operation. If pads on one side are worn more severely than the opposite
side of the vehicle, the piston may be stuck in the cylinder of the
opposite caliper. If the inside pad is worn more severely than the
outside pad of the same brake assembly, the caliper may not be free to
float on the torque plate.
Suspension Inspection Areas not directly related to the brake system should be inspected as
they may indirectly cause noise or pull when the brakes are applied.
Tire condition and inflation pressure should be considered. The
pressure and tire size should be equal from side to side on the same
axle. Tire condition may indicate front suspension problems.
When excessive wheel and tire run−out is present a vibration may be

felt on brake application. Check the vehicle Repair Manual for the
specific run−out specification. Vibration may also be felt at cruise
speeds. For example, it may come in at 55 mph and be gone at 60 mph.
Worn suspension bushings or ball joints change front−end geometry

and may cause a pull or drift when the brakes are applied.
Road Test - Identify A road test should be completed in order to verify the customer’s
the Symptoms complaint. Because the customer perceives the problem when the
brakes are applied, he naturally assumes the problem to be in the
brakes. However, the brake system may be indirectly related to the
complaint. It is important to determine the correct cause of the
customer concern.
A series of decelerations from 50 to 20 mph, while noting the vehicle

speed, intensity and location of any vibration will aid in further
diagnosis of the system.
The first check is done merely by allowing the vehicle to coast down
without applying the brakes to determine if the problem lies outside
the brake system.
This will help to determine if driveshaft balance or angle is causing a

vibration. If the vibration occurs at speeds between 40 to 25 mph, the
problem is likely that the driveshaft angle varies at each end of the
driveshaft. This can be confirmed by noting any vibration on moderate
to heavy acceleration.
The second check involves applying light to moderate brake pressure.

Information found here is used in conjunction with parking brake
application to determine the area of vibration.
If the symptom occurs when the service brake is applied and not when
the parking brake is applied, the problem lies in the front brake or
wheel assemblies.

Brake Diagnosis
When vibration is most noticeable in the steering wheel, check the

front brakes, wheel and tire condition. When vibration is most
noticeable in the brake pedal, check disc rotor for parallelism. Measure
the disc thickness at eight points around the circumference, about 10
mm from the outside edge.
The third check is done by using the parking brake. This check can
only be done with those vehicles which share the service and parking
brake assemblies. It distinguishes vibration caused by front and rear
brake assemblies.
Should vibration occur while the parking brake is applied at the

suspected vehicle speed, check the drum for deformation. Using an inside
micrometer or brake drum micrometer, measure the inside diameter at
several places to determine the amount of out−of−round. If the difference
between the smallest and the largest measurement is more than 0.006",
the drum should be machined on an off−car lathe. Precautions regarding
off−car lathes mentioned on page 85 should be observed.
Drum Measurement
Using an inside micrometer
or brake drum micrometer,
measure the inside diameter
at several places to
determine the amount
of out-of-round.
Isolate the Cause If the symptom is pulling when the brakes are applied, determine if the
pull is erratic or whether it is consistent. The crown of the road will
contribute to these symptoms, so perform several brake applications on
roads with different crown surfaces.
When a pull is erratic, it will cause the vehicle to pull either left or right
with no consistency. When this occurs, check the wheel alignment and
suspension bushings. Excessive wear of the bushings and ball joints will
change the suspension geometry while braking. Wear in the strut bar
bushing allows the lower control arm to move rearward when the brakes
Section 7
are applied, inducing caster change which causes a pull. Caster will
cause a vehicle to pull to the side of the least positive caster.
A steering gear that is out of adjustment or loose in its mounting may

also contribute to an erratic pull.
When the vehicle pulls consistently in one direction, the problem is

usually in the brake system. When the pull is to the left for example,
check both the left and right brake assemblies. The cause of a greater
amount of braking on one side may be a condition on either brake
assembly. Inspection of the brake system should start with the
condition of the brake drum or disc surface. The surface condition
should be the same on both rotors.
Lining that is soaked with brake fluid or gear oil will cause a pull and
should be replaced as an axle set after repairing the source of the leak.
The brake assembly creating the greatest heat conversion will do the
greatest braking. So when a caliper is frozen on the torque plate or a
piston is frozen in the caliper, the vehicle will pull to the opposite side.
Brake Noise Brake noise is caused by friction between the pads and drum or rotor
when the brakes are applied. Occasional squeal is normal, and not a
functional problem and does not indicate loss of braking effectiveness.
When the brake noise occurs all the time, check the lining condition.
Lining that is glazed should be replaced or cleaned using emery cloth.
When sanding the lining to remove the glazed surface, be sure to cover
the entire surface evenly. (See precautions under asbestos in the
reference section of this book). Also check the drum or disc for a glazed
condition and cleanup with emery cloth or turn on a brake lathe if the
drum diameter or disc thickness permits.
Squeal can also be caused by a weak or broken hold down spring or

return spring as well as missing, damaged or improperly installed
anti−squeal shims. To inspect return springs, check for space between
the spring coils and nicks in the spring wire diameter. Nicks caused by
tools during installation and removal may cause springs to break.
Damaged springs should be replaced as a set.
Anti−squeal shims help to dampen the vibration which occurs when the
pads contact the disc. Make sure that the appropriate shims are in
place. Anti−rattle springs are used to position and hold the pad as
rigidly as possible to reduce pad movement in the caliper assembly and
thereby reduce noise caused by vibration. Make sure that they are
positioned properly so they are most effective.

Brake Diagnosis
When assembling brake pads to the caliper assembly inspect the shims
and fitting components. Anti−squeal springs and support plates may be
reused if in good condition. Inspect them for proper rebound,
deformation, burrs, cracks, wear, or rust. Clean the shims as necessary
and lubricate all sides except the side contacting the caliper piston
with a thin layer of shim grease. Shim grease can be ordered
separately under part number 08887−80409. In addition, remove any
rust from the caliper grooves into which the ears of the brake pad rest
and coat with a thin layer of shim grease.
Shim Inspection
and Lubrication
Clean the shims as necessary
and lubricate all sides except
the side contacting the caliper
piston with a thin layer of
shim grease.
When brake noise occurs on only the first few brake applications, check
for corrosion on the disc rotors. Clean the rotors with emery cloth or turn
the rotors if it falls within the specified requirement of rotor thickness.
When brake noise occurs just before the vehicle stops, check for glazed
lining, damaged anti−squeal shims or fluid soaked lining. Correct the
conditions as covered previously.
Groan or creep−groan, which occurs when the brake pedal is released

slowly while the engine idles in forward gears, is caused by the pad
allowing the disc to slip. Slightly increasing or decreasing the pedal
effort will eliminate the noise. It does not adversely affect the braking
system or braking performance.
Section 7
Brake Vibration Brake vibration is a symptom which occurs during braking and is not
Isolation accompanied by sound. With brakes applied at high speeds, the
vibration is transmitted to the suspension system, the steering wheel,
instrument panel and brake pedal. In advanced stages, vibration may
also occur at lower speeds.
If the vibration causes the steering wheel to oscillate side to side, the
cause is likely the front brake assemblies. The rear parking brake can
be used to isolate the vibration by applying the parking brake at the
speed at which the vibration occurs. If the vibration does not occur, it is
likely that the front brakes are the cause. (This procedure will not
work if the parking brake is an exclusive design found on rear disc
brakes with a drum type parking brake.)
Isolating Brake
Vibration
With brakes applied at
high speeds. The vibration
is transmitted to the
suspension system, the
steering wheel, instrument
panel and brake pedal.

Brake Diagnosis
If the vibration cannot be isolated to either the front or rear brakes,

measure the front disc rotors for parallelism. Using a micrometer,
measure the rotor at eight different places around the diameter of the
rotor about 10 mm from the outer edge. Thickness variation is
determined by subtracting the smallest thickness measurement from
the largest thickness. If the ; thickness variation is greater than 0.0008"
(0.02 mm) the rotor is the cause and should be resurfaced or replaced.
Rotor Measurement
Using a micrometer measure
the rotor at eight different
places around the diameter
of the rotor about 10 mm
from the outer edge.
Thickness Variation Thickness variation causes the thickest part of the rotor to push the
piston back into the caliper cylinder each time it rotates past the brake
pads. This increase of hydraulic pressure is transferred via the brake
line tubing to the master cylinder and translated through the booster
to the brake pedal.
Section 7
Two conditions that cause thickness variation are:

• Rotor run−out.
• Excessive rust or corrosion on rotor surface.
Rotor Run-out Lateral run−out of the rotor is the most significant cause of rotor
thickness variation and eliminating run−out is the only way to solve a
pedal pulsation complaint for good. When rotor run−out is excessive, a
portion of the rotor comes into contact with the brake pad with each
rotor revolution. Over time the rotor will wear at the contact point
causing thickness variation. Poor mating of the disc rotor and axle hub
can cause excessive, run−out. The rotor is mounted to the axle hub and
each is manufactured with a tolerance for allowable run−out. When the
tolerances are stacked one on the other, the total run−out may exceed
0.004" (0.10 mm) and cause the situation described here.
Rotor Run-out
When rotor run-out is
excessive a portion of the
rotor comes into contact
with the brake pad with
each rotor revolution.
The rotor will wear at the
contact point causing
thickness variation.
Excess Rust and In areas where salt is applied to road surfaces during winter conditions,
Corrosion vehicles parked for an extended time have rust and corrosion build−up
on areas of the disc surface not covered by the brake pads. When the
vehicle is driven, the rusted areas wear at a different rate than the
non−corroded areas, resulting in excessive thickness variation.

Brake Diagnosis
Repair the Problem Resurfacing of rotors with an on−car brake lathe is the recommended
operation to correct rotor run−out and thickness variation. The on−car
lathe is installed in the same position as the caliper, ensuring that the
rotors will be machined absolutely parallel to the brake pad and caliper
assembly. An on−car lathe is designed to take into account all of the
variations in the bearings, hub and rotor assembly and provides the
greatest accuracy by eliminating virtually all run−out.
On-Car Brake Lathe

The on-car lathe is
installed in the same
position as the caliper,
ensuring that the rotors
will be machined absolutely
parallel to the brake pad and
caliper assembly.
Section 7
If an on−car lathe is not available and the rotor is machined on an

off−car lathe:
• check the lathe arbor for excessive run−out and correct or replace as
needed.
• check the condition of the rotor mounting surfaces to ensure proper
mounting.
• make certain the adapters and cones are free of burs and particles
that might prevent the proper mounting of the rotor.
• check the rotor run−out with a dial indicator and correct mounting
as needed.
• measure the rotor thickness to ensure it is greater than the
minimum thickness.
Typically when a rotor is turned on a lathe, the opposite rotor should

also be turned so that they are both within 0.010" (0.25 mm) of each
other. Also, if one rotor has a rougher surface finish than the other, the
difference in friction may cause a pull.
Phase Match Rotor When mounting machined rotors or replacing new rotors, check the
to Hub condition of the axle bearing and be sure to seat the rotor on the axle
using the lug nuts and tighten them evenly with a torque wrench.
Using a dial indicator, measure the lateral run−out. Run−out should not
exceed 0.002" (0.05 mm). If run−out is excessive, index the rotor one lug
and measure the run−out again. Repeat the procedure, indexing the
rotor one lug at a time until the minimum run−out is achieved.
Measuring Rotor
Run-out
If run-out is excessive,
index the rotor one lug at a
time and measure the
runout again until minimum
run-out position is found.

Brake Diagnosis
Of equal importance is the necessity of tightening all wheel lug nuts to

the specified torque using a star sequence when installing the wheels.
All the care and attention in machining and measurement for run−out
can all be destroyed by using an impact wrench to tighten lug nuts.
Improper torque or uneven torque can distort the rotor and create a
comeback.
Wheel Tightening
Procedure
All the care and attention
in machining and
measurement for run-out
can all be destroyed by
using an impact wrench to
tighten lug nuts.
Check for Proper Following the repair, road test the vehicle to ensure that the customers
Operation concern has been eliminated. This will help to ensure customer
satisfaction with your service.
Section 7

Brake Rotor Run-Out and Rotor Phase Matching
In this Worksheet you will practice the procedure for checking for bearing wear, measuring Rotor Run-Out and
Phase Match a rotor.

• Dial indicator and base.
• Abrasive cloth.
• Torque wrench.
• Permanent Marker.
Preparation:
• Mount the stem adapter to the indicator plunger.
• Mount the indicator to the base.
• Raise vehicle on a lift or support with jack stands.
• Mark the wheel stud nearest the valve stem to ensure wheels balance and remove the wheel.
• Mark the disc rotor and wheel stud, then remove the disc rotor and inspect for corrosion build-up and clean
as needed with abrasive cloth.
• Reinstall the disc rotor and all lug nuts to the wheel studs to hold the rotor firmly in place.
Measurement:
1. Place the indicator plunger in line with the axle near the center of the axle hub. Compress the indicator
plunger stem slightly to preload the dial indicator plunger.
2. Push inward on the hub to seat the wheel bearing and zero the indicator. Pull the hub outward and note the
amount of wheel bearing backlash or axial play in the box below.
Measured Wheel Bearing

Specification Result
Free Play

Brake Diagnosis
3. Place the indicator plunger perpendicular to the rotor braking surface, 10 mm from the outer diameter.
Compress the indicator plunger stem slightly to preload the plunger.
4. Rotate the rotor slowly to locate the lowest point of indicator travel and zero the indicator.
5. Rotate the rotor several times and verify the indicator returns to zero.
6. Continue to rotate the hub noting the maximum indicated runout and compare your measured value to
specification.
Measured Rotor Run-Out Specification Pass/Fail
Brake Rotor Phase Matching

Procedure:
1. Mark a stud and its position on the rotor for index purposes.
2. Place lugnuts flat side toward the rotor and torque evenly.
3. Measure rotor runout and record in the chart below.
4. Reposition rotor in clockwise direction to the next lug and measure runout.
5. Repeat for each lug and record.
Lug Number Measurement

Section 7
Summary:
1. How could excessive wheel bearing free play influence runout measured at the brake rotor?
2. How could excessive wheel bearing free play affect brake pedal travel?
3. What affect would a rotor with excess runout have on brake operation in the first 500 miles?
4. What happened to the rotor in question 3 if after several thousand miles of wear, the brake pedal pulsates?
5. What is the proper service procedure to correct the condition identified in question 4?
6. Referring to the measurements made for Phase Matching, what is the difference between the highest runout
and the lowest runout? (Show your math calculations)

Brake Diagnosis
Section 7

Brake Rotor Parallelism Measurement
In this Worksheet you will practice the procedure for measuring brake rotor parallelism for thickness variation.

• 1” Micrometer.
• Permanent Marker Pen.
• Repair Manual.
• Lug Nut Wrench.
• Straight Edge.
Preparation:
• Raise vehicle on a lift or support with jack stands.
• Mark the wheel stud nearest the valve stem and remove the wheel.
• Install all lug nuts to the wheel studs to hold the rotor firmly in place.
• Using a marker pen and straight edge, divide the rotor into eight equal parts and mark a line 10mm from the
outside circumference.
Measurement:
1. Measure the rotor thickness at the intersection of the eight lines and the circular mark.
2. Record the readings in the chart below.
Measurement
Measured Rotor Thickness
Point
1.
2.
3.
4.
5.
6.
7.
8.

Brake Diagnosis
Measurement (Cont’d):
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
3. Record the largest and smallest thickness measurement in the chart and record the thickness variation.
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
Largest
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
Smallest
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
Thickness Variation
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
Rotor Minimum
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
Thickness
Summary:
1. What is the specification for maximum allowable thickness variation?
2. What symptoms would be experienced with excessive thickness variation?
3. List two causes of rotor thickness variation?
4. What is the correct repair procedure for thickness variation?

Section 7

On-Car Brake Lathe
In this Worksheet you will practice the procedure for setup and machining a rotor using the On-Car Brake Lathe.
• Accu-Tum Brake Lathe with Accessories.
• Extension Cord.
• Assorted Hand Tools.
• Micrometer and Dial Indicator and Stand.
• Brake Clean.
Thickness Measurement
Preparation:
• Raise vehicle on a lift.
• Check the wheel bearings for excessive play or roughness. (Adjust or replace as necessary)
• Mark the stud nearest the valve stem and remove the wheel.
• Remove the Caliper from the steering knuckle and secure it with mechanic’s wire or the “S” hook provided.
• Measure the rotor thickness to determine its serviceability. Check the minimum thickness cast into the rotor.
Thickness Measurement Minimum Thickness Spec. Allowable Cut
• Install the drive yoke and secure the rotor to the hub using the flat side of the vehicles lug nuts (concave
receiving washers are provided for closed end lugs). Securely tighten the lug nuts. (Yoke crossbar
should be centered to axle)
• Install the silencer band on the rotor.
• Remove rust and debris from the steering knuckle caliper mounting surface.
• Mount the dial indicator and measure rotor runout.
Rotor Runout Measurement
Lathe Mounting:
• Attach lathe to the steering knuckle caliper mounting surface using the plate specified for the vehicle.
• Use spacers to center the lathe to the rotor as needed. (See figures on set-up sheet)

Brake Diagnosis
Auxiliary Drive Installation:

• Rotate the rotor so that the yoke crossbar is horizontal.
• Adjust the auxiliary drive so that the shaft and nylon coupler are centered to the axle.
• Attach the nylon coupler to the yoke crossbar leaving a 1/4” gap in the rear of the slot.
• Lock the rear caster on the drive stand.
• Turn the drive unit “ON” in the direction of the arrow located on the lathe body.
• Any shaking or oscillation will smooth out with a slight height adjustment of the drive unit.
Machining the Rotor:

• Be sure to note the directional arrow on the side of the lathe to ensure that the direction of rotation is
observed. (Failing to do so will result in damage to the cutting bits.)
• While the drive unit is rotating, turn the hand wheel to position the tool holders over the friction area.
• Adjust the left and right tool holders until each bit makes surface contact and record the reading on the
adjusting knobs and record it in the chart below.
• Adjust the left and right tool holders until each bit just makes contact with the rotor and record the reading of
the adjusting knob below. Adjust the left and right tool holders until each bit makes contact for a full 360
degrees, and record the readings of the adjusting knobs.
Inside Reading Outside Reading
Initial Contact
360 Contact
• Retract the tool holders several turns and position them past the inner friction surface near the hat section
of the rotor.
• Remove any ridge at the outer edge or near the hat area if necessary, being certain not to take a cut deeper
than the amount indicated at the 360 degree contact made earlier.
Section 7
• Turn the adjusting knobs back to the original reading plus an additional one-half mark (0.002”).
• Push the power feed switch to the rear position.
• Rotate the clutch knob on the handwheel clockwise to engage the feed.
• After machining is complete, measure the rotor thickness to ensure it is above minimum thickness.
Rotor Thickness Measurement
• Mount the dial indicator and measure rotor runout.
Final Rotor Runout Measurement
Summary:
1. Why should wheel bearings be checked for excessive play or roughness before the rotor is turned?
2. Why is it important to torque the lug nuts evenly when securing the brake rotor to the hub before machining
the rotor.
3. What advantage would there be in providing a non-directional finish on the rotor?
4. When a cut of .002” is made on each side of a rotor, the overall thickness of the rotor is reduced by:
5. What is the difference between the initial rotor thickness measurement and the final thickness measurement?

Brake Diagnosis
Section 8
HYDRAULIC CONTROL
Lesson Objectives 1. Describe the purpose of a proportioning valve.

2. Describe the purpose of a load sensing proportioning valve.
3. Describe the function of the bypass valve in a LSPBV.
4. Measure front and rear brake pressure using LSPV pressure
gauge set.
5. Adjust LSPBV rear brake pressure.

Hydraulic Control
Hydraulic Hydraulic control valves regulate hydraulic pressure to the rear brakes
Control Valves to ensure efficient braking. Types of control valves used on Toyotas:
• Proportioning Valve.
• Proportioning and Bypass Valve.
• Double Proportioning Valve.
• Load Sensing Proportioning Valve.
• Load Sensing Proportioning and Bypass Valve.
Control of rear brake pressure is necessary because inertia, created

when the brakes are applied, shifts vehicle weight toward the front
wheels. Because the rear wheels have less weight during braking, they
can lockup, causing the vehicle to lose traction and skid out of control.
The front of a front−engine vehicle is heavier than the rear, so when the
brakes are applied, the vehicle’s center of gravity tends to move
forward because of inertia. This adds to the front load, and the rear
load decreases as a result. With greater braking force, the center of
gravity moves further forward and the rear load decreases even more.
Shifting Center of
Gravity
When the brakes are
applied, the vehicle’s center
of gravity tends to move
forward reducing load at the
rear wheels.
Assuming that the front and rear wheels exert an identical braking
force in the above condition, the rear tires, which are subject to a
smaller load, tend to lockup early. This will cause the rear tires to lose
traction or skid.
When the tires skid, the friction between the tires and the road
becomes extremely small, and the tires will fail to remain in sufficient
contact with the road. Unless the vehicle is moving straight ahead, it
will fishtail", which can be very dangerous.
Section8
The braking force of the rear tires must be reduced below that of front
tires in order to prevent early lock−up. This is achieved by the
proportioning valve (P. valve). It is designed to automatically reduce
the hydraulic pressure for the rear wheel cylinders in proportion to
hydraulic pressure from the master cylinder.
Proportioning Valve
Location
The braking force of the rear
tires must be reduced below
that of front tires in order to
prevent early lock-up.
Hydraulic Pressure The graph below shows an ideal hydraulic pressure curve for the front
Curve and rear wheels (actual values vary from one vehicle model to another).
The proportioning valve is designed to bring actual pressure curves as
close to the ideal as technically possible.
Hydraulic Pressure
Curve
The P-valve is designed to
bring actual pressure curves
as close to the ideal as
technically possible.

Hydraulic Control
Proportioning Valve The spring in the Proportioning valve holds the valve in the open
Operation position. During normal braking the brake fluid flows through the
valve without any proportioning action. However, when heavier
braking occurs, pressure on the wheel cylinder side of the
proportioning valve pushes the valve against spring tension and closes
the valve. This in effect reduces pressure to the rear brakes. As
pressure increases on the master cylinder side, it lifts the valve,
increasing pressure to the wheel cylinder side of the valve. As pressure
increases on the wheel cylinder side of the valve, it seats again. This
occurs in rapid succession as long as pressure from the master cylinder
increases.
Proportioning Valve
Pressure on the rear cylinder
side pushes the valve
against spring tension and
closes the valve.
Section8
In order to release the pressure between the proportioning valve and

the rear wheel cylinders, the valve seat floats as shown in the
illustration. When the pressure from the master cylinder is released,
the pressure difference on the valve seat causes it to be pushed away
from its seated position on the valve body. This allows fluid to pass the
valve seat and the brakes to release.
Proportioning Valve
Brake Released
When the pressure from the
master cylinder is released,
the valve seat pushed away
from its seated position
allows fluid to pass.

Hydraulic Control
Proportioning and The proportioning function of this valve is the same as that described
Bypass Valve on the previous pages however, a Bypass Valve is incorporated into
Operation the valve body. It ensures maximum braking pressure to the rear
brakes when there is a loss of brake pressure in the front brake circuit.
The hydraulic circuit from the master cylinder to the front brakes flows
through part of the proportioning valve housing where the Bypass
Valve
monitors front brake pressure. The spring pushes the bypass valve to
the left and pushes the proportioning valve to the right, providing the
proper spring tension for proportioning valve operation.
Rear brake hydraulic pressure pushes the bypass valve to the right while
front brake pressure pushes the valve to the left. The overall hydraulic
effect on the valve is neutral and the spring holds it to the left.
Bypass Valve
Operation
Spring loaded to the left, the
bypass valve establishes the
spring position for normal
proportioning operation.
Section8
Should the hydraulic circuit to the front brakes fail, rear brake
pressure will move the bypass valve to the right, forcing the
proportioning valve to the right, which allows unregulated pressure to
apply the rear brakes.
Bypass Valve
Operation
The bypass valve moves
right when front brake
pressure drops, increasing
spring tension of the
proportioning valve thereby
ensuring maximum pressure.
Double Proportioning The diagonal split brake system incorporated on all FWD vehicles uses
Valve a double proportioning valve in which two valves are arranged parallel
to one another in the same valve housing. One valve controls pressure
to the right rear brakes and the other valve controls pressure to the left
rear brakes.
Proportioning Valve
on Diagonal Split
Brake System
All FWD vehicles use a
double proportioning valve
to control one front brake
and one rear brake on the
opposite side.

Hydraulic Control
The movement of both valves is controlled by the tension of one spring.

With a single spring, a balanced pressure is applied to each valve
through the retainer.
Double Proportioning
Valve
Both valves are controlled
by the tension of one spring.
Pressure Loss in The real advantage to one spring is seen when one hydraulic circuit
One Circuit loses pressure. In this case only one valve counteracts the spring
tension which requires additional hydraulic pressure to compress it.
This results in higher pressure to the rear brake, providing a greater
degree of vehicle control.
Rear Wheel Cylinder

Pressure
When one circuit fails,
pressure to the rear
cylinder increases higher in
proportion to master
cylinder pressure because
only one piston
compresses the spring.
Section8
Load Sensing The LSPV is used on Toyota models such as Truck, Van and Station
Proportioning Wagons which may be used to carry a variety of loads. The heavier the
Valve load, the greater the portion of braking is required of the rear brakes.
The LSPV allows higher pressure to the rear brakes to accomplish this.
The LSPV is attached to the body or frame above the left rear control
arm or axle housing. Load sensing is accomplished by suspending the
sensing spring between the vehicle body and the rear axle housing. The
load sensing spring movement caused by vehicle height changes due to
load, is transmitted to the proportioning valve.
Load Sensing
Proportioning Valve
Load sensing is
accomplished by
suspending the sensing
spring between the
vehicle body and the
rear axle housing.

Hydraulic Control
LSPV Operation As a vehicle is loaded, the leaf springs are compressed as the vehicle
body lowers. The load sensing spring provides a variable force pushing
the proportioning piston up as the vehicle is loaded. As the piston is
lifted, a higher brake hydraulic pressure is required to force the piston
down resulting in higher pressure at the rear wheels.
LSPV Lever Balance

The load sensing spring
provides additional pressure
to the proportioning valve
based on vehicle load.
Rear wheel cylinder pressure is adjusted according to increases or

decreases in vehicle load. The pressure change for one rear wheel is
shown below.
LSPV Rear Wheel

Cylinder Pressure
Pressure to the rear brakes
is regulated at a higher
pressure when the
vehicle is loaded.
Section8
Unloaded Vehicle When unloaded, a vehicle body rises to normal vehicle height and no
force from the load sensing spring is applied to the piston. The rear
wheel cylinder is regulated at a lower pressure as shown by the line O
− A − B in the chart on the previous page.
When the fluid pressure from the master cylinder is low the piston is
pushed upward by the force of the piston spring. Fluid pressure is
transmitted from chamber A through the passage into chamber B and
to the rear wheel cylinder.
When the master cylinder pressure rises and pressure on the valve top
(A2) becomes greater than piston spring tension, the piston is pushed
downward and the valve is closed. Hydraulic pressure to the rear
cylinders at this time will be as indicated by the point of deflection A"
in the graph. The upward force of the piston spring is equal to the
downward force of hydraulic pressure when the valve is in the closed
position.
As the master cylinder fluid pressure increases with further brake

application, the piston is pushed upward again and the valve opens.
Pressure to the rear wheel cylinders increases when the valve opens,
but the piston is pushed down before wheel cylinder pressure becomes
equal to master cylinder pressure, and the valve closes.
Proportioning Valve in
Unloaded Position
The proportioning valve is
normally open. When the
master cylinder pressure
rises and pressure on the
valve top (A2) becomes
greater than piston spring
tension, the piston is
pushed downward and
the valve is closed.

Hydraulic Control
Loaded Vehicle As the load in the vehicle is increased, the vehicle body moves down, and
the LSPV piston is pushed up by the lever causing the rear wheel cylinder
to be regulated at a higher pressure as shown in the graph (O − C − D).
When the fluid pressure from the master cylinder is low, the hydraulic
pressure going to the rear wheel cylinder is not controlled. As master
cylinder pressure rises and becomes greater than the combined spring
tension, the piston is pushed downward and the valve is closed
regulating pressure to the rear brake cylinder.
Proportioning Valve in
the Loaded Position
The LSPV piston is pushed
up by the lever causing the
rear wheel cylinder
pressure to increase.
Section8
Load Sensing The LSPBV is used on Previa’s, Trucks, Tacoma’s, T−100’s, Cab and
Proportioning Chassis 2WD and 4WD models. The LSPBV is a LSPV to which a
and Bypass Valve bypass circuit has been added. The operation of the bypass valve is
similar to the Proportioning and Bypass Valve.
When the front brake circuit is operating normally, the LSPBV varies
the pressure transmitted from the master cylinder to the rear wheels
based on vehicle load, in the same way as the LSPV. However, if the
front brake circuit fails, hydraulic pressure is transmitted directly to
the rear wheel cylinders, bypassing the proportioning part of the valve
so that enough braking force can be applied.
The hydraulic sensing circuit which links the front brake hydraulic
circuit to the LSPBV, is part of the front hydraulic circuit. When
bleeding the front brake system, be sure to bleed air from the LSPBV
as well, or the pedal may feel spongy with diminished brake
performance.
Load Sensing
Proportioning and
Bypass Valve

Hydraulic Control
Bypass Valve When the front brake circuit is operating normally, pressure from the
Operation master cylinder front and master cylinder rear are equal. The bypass
piston is pushed and held down by the spring.
If pressure from the front brakes falls to zero, a difference will exist
between the hydraulic pressure pushing the bypass valve up and the
pressure pushing the valve down. This causes the bypass valve to be
pushed upward, pushing the piston upward, and opening the passage
at the top of the valve. The hydraulic pressure from the master
cylinder is not controlled. Full pressure from the master cylinder is
transmitted to the rear wheel cylinder.
Fail-safe Operation
When pressure from the
front brakes (Pf) is lost,
Piston No. 2 rises
compressing the spring
and opening the valve.
Section8
LSPV Adjustment Adjustment of the LSPV is accomplished by changing the length of (A)
in the illustrations below. The distance has an initial length which can
be found in the Repair Manual.
Adjustment Length
If distance (A) is too short, the hydraulic pressure breaking point will
decrease. Hydraulic pressure to the rear wheel cylinders will be lower
than normal, reducing braking performance.
When distance (A) is too long, the hydraulic pressure breaking point
will rise. Hydraulic pressure to the rear wheel cylinder will be higher
than normal, increasing the braking force of the rear wheels.
To adjust the valve properly and ensure efficient braking, the LSPV
gauge (SST 09709−29017−01) must be used to measure the front and
rear brake pressure.
Pressure Gauge SST

To adjust the valve properly
and ensure efficient braking,
the LSPV gauge (SST
09709-29017-01) must be
used to measure the front
and rear brake pressure.

Hydraulic Control
The gauges are provided in the SST Kit. Install one gauge at the front
wheel cylinder. The other gauge is installed at the rear wheel cylinder.
Having opened the system, the air must be bled from the system before
accurate system pressures can be read. Bleed screws are located on the
hose end of the gauge.
Follow the procedure outlined in the Repair Manual to determine the

following:
1. Rear axle load (based on vehicle model).
2. Front brake pressure specifications.
3. Rear brake pressure specifications.
The weight of the vehicle measured at the rear axle must be

determined and additional weight added to meet the Repair Manual
specification. This will establish the proper relationship of the
proportioning valve and the rear axle housing.
Hydraulic Pressure
Measurement
Hydraulic pressure at the
front brake should be
compared with the
pressure of the rear brake
in two stages.
Next, the hydraulic pressure at the front brake should be compared

with the pressure of the rear brake in two or three stages as specified
in the Repair Manual.
• First, the front pressure is brought to a specified pressure
(example: 1,138 psi) and the rear pressure should be within a
specific pressure range (example: 583 psi to 768 psi).
• Second, without releasing the brake pedal, the front pressure is
increased (example: 1,422 psi) and the rear brake pressure should
increase (example: between 688 psi to 873 psi).
Rear pressure readings should be taken within two seconds of

NOTE obtaining the specified front pressure.
Section8
If the rear pressures do not fall within the stated specification, adjust
distance (A):
• Lengthening (A) if the pressure is low.
• Shortening (A) if the pressure is high.
LSPV Adjustment
If the rear pressures do
not fall within the
stated specification,
adjust distance A.
If adjustment of the springs does not bring the rear pressure into
specification, adjust the valve body:
• If pressure is low, lower the valve body.
• If the pressure is high, raise the valve body.

Hydraulic Control

LSPV Adjustment
In this Worksheet you will practice the procedure for measuring and adjusting the LSPV.

• LSPV Pressure Gauge SST. (09709-29017-01)
• Weight Scale Printout.
• Hand Tool Set.
• Tape Measure or Ruler.
• Brake Fluid.
• Repair Manual.
Preparation:
• Raise the vehicle on a lift and install the LSPV gauges.
• Bleed air from the brake lines.
• Place weight scales under the wheels and lower the vehicle onto the scales.
Measurement:
1. Record the specified weight from Repair Manual, subtract the rear vehicle weight to find amount of
additional weight required. Additional weight should be placed above the rear axle.
Specified Weight
Rear Axle Weight
Added Weight
Section8
Measurement (cont’d):
2. Raise front brake pressure and check rear brake pressure.
Model Front Pressure Spec. Rear Axle Spec. Rear Axle Measured
3. How does this compare to the specified pressure?
LSPV Adjustment:
1. Record the initial length of the No.2 shackle.
2. Rotate the adjusting nut two (2) complete turns and record the change in rear pressure.
3. Recheck the pressures, has the pressure at the rear brakes increased or decreased?
If so, by how much?
Summary:
1. Why is the weighting of the rear of the vehicle important?
2. What effect would mis-adjustment have on the brake system?
3. Refer to the Repair Manual for Previa and Camry and record the pressure change for each rotation of the
No.2 shackle adjusting nut.
4. To increase the pressure at the rear wheels, would the No.2 shackle be shortened or lengthened?

Section 9
ANTI-LOCK BRAKES
INPUT PROCESS OUTPUT
Speed Actuator
Sensors Solenoids
Actuator
Pump Motor
ABS
Battery ECU
Data Link
Connector
Stop Light ABS Warning

Switch Light
Lesson Objectives 1. Identify and describe the function of components in the ABS system.
2. Relate the basic operation strategy of the ABS system.
3. Explain the control of the solenoid and pump relays.
4. Describe the signal generation of a speed sensor.
5. Describe the operation of the two−position solenoid actuator for
controlling wheel lock−up.
Section 9
Fundamental Toyota Antilock Brake Systems (ABS) are integrated with the
ABS Systems conventional braking system. They use a computer controlled actuator
unit, between the brake master cylinder and the wheel cylinders to
control brake system hydraulic pressure.
Antilock Brake Systems address two conditions related to brake

application; wheel lockup and vehicle directional control. The brakes
slow the rotation of the wheels, but it is actually the friction between
the tire and road surface that stops the vehicle. Without ABS when
brakes are applied with enough force to lock the wheels, the vehicle
slides uncontrollably because there is no traction between the tires and
the road surface. While the wheels are skidding, steering control is lost
as well.
An antilock brake system provides a high level of safety to the driver by

preventing the wheels from locking, which maintains directional
stability. A professional driver may be capable of maintaining control
during braking by pumping the brake pedal which allows a locked wheel
to turn momentarily. Whereas a professional driver may be capable of
modulating the brakes approximately once per second, ABS is capable of
modulating the brake pressure at a given wheel up to fifteen times per
second. An ABS system does something else that no driver can do, it
controls each front brake separately and the rear brakes as a pair
whenever one of the wheels starts to lock. ABS helps stop a car in the
shortest possible distance without wheel lockup while maintaining
directional control on most types of road surface or conditions. If a
Toyota ABS system malfunctions, normal braking will not be affected.
ABS System Diagram

ABS is combined with the
conventional braking system
and located between the
master cylinder and the
wheel cylinders.
Legend:
Electrical
Hydraulic

Anti-lock Brakes
Tire Traction The chart below shows the slip tolerance band (shaded area) in which the
and ABS most efficient braking occurs. From a slip ratio of zero (0), at which the
wheel speed and the vehicle speed are equal, to a slip ratio of 10, braking
is mild to moderate and good traction between the tire and the road
surface is maintained. Between slip ratios of 10 to 30 the most efficient
braking occurs. This is where the tires are at a point where they may
begin to lose traction with the road surface. This is also the band in which
ABS operation occurs. Beyond a slip ratio of 30%, braking efficiency is
reduced, stopping distance is increased and directional control is lost.
The amount of braking force on the left vertical line will vary based on
the driver’s pressure on the brake pedal and on the road surface; less
braking force may be applied on wet asphalt than on dry concrete
before lockup occurs, therefore the stopping distance is increased.
Braking Force Chart

Maximum braking force
occurs between 10 to 30%
slip ratio. Wheels spinning
freely is 0% slip ratio.
100% slip ratio reflects
a wheel completely
locked up.
Basic Four Wheel ABS Systems use a speed sensor at each front wheel and
Operation either a single speed sensor for both rear wheels or individual speed
sensors at each rear wheel. The speed sensors are monitored by a
dedicated ECU. The system controls the front brakes individually and
rear brakes as a pair.
In a panic braking situation, the wheel speed sensors detect any

sudden changes in wheel speed. The ABS ECU calculates the
rotational speed of the wheels and the change in their speed, then
calculates the vehicle speed. The ECU then judges the slip ratio of each
wheel and instructs the actuator to provide the optimum braking
pressure to each wheel. For example, the pressure to the brakes will be
less on slippery pavement to reduce brake lockup. As a result, braking
distance may increase but directional control will be maintained. It is
also important to understand that ABS is not active during all stops.
Section 9
The hydraulic brake actuator operates on signals from the ABS ECU to
hold, reduce or increase the brake fluid pressure as necessary, to
maintain the optimum slip ratio of 10 to 30% and avoid wheel lockup.
Typical ABS
Control System
The ECU monitors the four
wheel sensors, processes
the data and controls the
actuator solenoids and
pump motor through
the ABS Relay.
Speed Actuator
Sensors Solenoids
Actuator
Pump Motor
ABS
Battery ECU
Data Link
Connector
Stop Light ABS Warning

Switch Light

Anti-lock Brakes
Types of There are four types of ABS systems used in current Toyota models
Toyota ABS distinguished by the actuator. The four actuator types include:
• 2−position solenoid valves.
• 3−position solenoid valves with mechanical valve (Bosch).
• 3−position solenoid valves (Nippondenso).
• 2−position solenoid controlling power steering hydraulic pressure
which controls brake hydraulic pressure.
2-Position Solenoid 2−position solenoid actuators come in configurations of six or eight

solenoids. The eight solenoid configuration uses two solenoids per
brake assembly. The six solenoid configuration uses two solenoids to
control the rear brake assemblies while the front brake assemblies are
controlled independently by two solenoids each.
2-Position Solenoid
Types
Controls pressure to four
brake assemblies in three
stages: pressure holding,
increase and reduction.
Section 9
3-Position Solenoid This actuator uses three, 3−position solenoid valves. Two solenoids
and Mechanical Valve control the front wheels independently while the third solenoid controls
the right rear and the mechanical valve translates controls to the left
rear.
3-Position Solenoid
With Mechanical Valve
The third solenoid controls
the right rear and the
mechanical valve translates
controls to the left rear.

Anti-lock Brakes
3-Position Solenoids The 3−position solenoid valve actuator comes in three solenoid or four
solenoid configurations. The four−solenoid system controls hydraulic
pressure to all four wheels. In the 3−solenoid system, each front wheel
is controlled independently while the rear wheels are controlled in
tandem.
3-Position Solenoids
Controls pressure to four
brake assemblies in three
stages: pressure holding,
pressure increase and
pressure reduction.
Power Steering The last actuator type uses power steering pressure to regulate brake
Control pressure using a single 2−position solenoid, a cut valve and bypass
valve. Brake system pressure is controlled for the rear brakes only.
Power Steering
Hydraulic Pressure
Controls Brake
Hydraulic Pressure
A single 2-position solenoid
regulates power steering
pressure which controls
brake hydraulic pressure to
the two rear wheels only.
Section 9
System Each ABS type shares common components which provide information
Components to the ECU. This section will examine each of these components and
then describe each of the actuator types and their operation.
The components identified below are typical of most Toyota ABS systems.
• Speed Sensors monitor wheel speed.
• G−Sensor monitors rate of deceleration or lateral acceleration.
• ABS Actuators control brake system pressure.
• Control Relay controls the Actuator Pump Motor and Solenoids.
• ABS ECU monitors sensor inputs and controls the Actuator.
• ABS Warning Lamp alerts the driver to system conditions.
The location of components may vary by model and year, therefore, for
accurate location of components, consult your EWD or Repair Manual.
Typical Component
Layout
Component location is
typical for most models:
speed sensors at each
wheel, actuator in engine
compartment. The ECU
however may require an
EWD or Repair Manual to
pinpoint its location.

Anti-lock Brakes
Wheel Speed Sensor A wheel speed sensor is mounted at each wheel and sends a wheel
rotation signal to the ABS ECU. The front and rear wheel speed
sensors consist of a permanent magnet attached to a soft iron core
(yoke) and a wire wound coil. The front wheel speed sensors are
mounted to the steering knuckle, and the rear speed sensors are
mounted to the rear axle carrier. Serrated rotors are mounted to the
drive axle shaft or brake rotor, and rotate as a unit.
Wheel Speed Sensors

Speed sensors monitor
individual wheel speed.
Early Supras including the 1993 model year and Cressidas equipped
with ABS, used a single rear speed sensor mounted on the
transmission extension housing to monitor rear wheel speed.
Section 9
Operation Speed sensor operation is similar to the magnetic pick−up in a distributor.

When the teeth of the Sensor Rotor pass the iron core, the magnetic lines
of force cut through the coil windings causing a voltage to be induced into
the coil. As the tooth approaches the iron core, the magnetic field
contracts causing a positive voltage to be induced in the coil. When the
tooth is centered on the iron core the magnetic field does not move and
zero volts are induced in the coil. As the tooth moves away from the iron
core the magnetic field expands, resulting in a negative voltage. As the
rotation of the sensor rotor increases, the voltage and the frequency of
this signal increase, indicating to the ECU a higher wheel speed.
Speed Sensor
Operation
Voltage is induced into
the coil when the magnetic
field changes each time
the sensor rotor teeth
pass the iron core.
Deceleration Sensor The deceleration sensor is used on some systems to provide input to the
ABS ECU about the vehicle’s rate of deceleration to improve braking
performance. In a typical ABS system, the ECU compares individual
speed sensors to determine the speed of the vehicle and rate of wheel
deceleration. The deceleration sensor is used on all full−time 4WD
vehicles equipped with ABS to determine deceleration, as the front and
rear axles are connected through the transfer case and present unique
braking characteristics. Models equipped with only rear−wheel ABS
have a single speed sensor and no means of determining the actual
vehicle speed or rate of deceleration.
The deceleration sensor is composed of two pairs of LEDs (light

emitting diodes) and phototransistors, a slit plate, and a signal
conversion circuit. The deceleration sensor senses the vehicle’s rate of
deceleration and sends signals to the ABS ECU. The ECU compares
the rate of deceleration and vehicle speed to determine the precise road
surface conditions and takes appropriate control measures.

Anti-lock Brakes
Deceleration Sensor
Components
The slit plate swings
between the LED’s and
Phototransistors.
Operation Both LED’s are located on one side of the slit plate and both photo
transistors are located on the opposite side. The LED’s are ON when
the ignition switch is in the ON position. When the vehicle’s rate of
deceleration changes, the slit plate swings in the vehicle’s rear−to−front
direction. The slits in the slit plate act to expose the light from the
LEDs to the phototransistors. This movement of the slit plate switches
the phototransistors ON and OFF.
Deceleration Sensor
Operation
As the slit plate swings
forward the slits expose the
phototransistor to the LED.
Lateral Acceleration The lateral acceleration sensor has similar construction to the
Sensor deceleration sensor described above. Rather than having the slit plate
swing rear−to−front, the sensor is mounted in such a way that the slit
plate swings from side to side. This sensor is found only on the 1993 1/2
and later model Supra to detect lateral forces while braking.
Section 9
Deceleration Rate The combinations formed by these phototransistors switching ON and

OFF distinguish the rate of deceleration into four levels, which are sent
as signals to the ABS ECU. The chart below indicates the rate of
deceleration based on input from the two phototransistors. For
example: when the No. 1 and No. 2 photo transistors are both blocked
and turned OFF, the deceleration rate is medium.
Deceleration Rate
Level
Deceleration Rate Level

Rate of
Low-1 Low-2 Medium High
deceleration
No. 1 Photo
ON OFF OFF ON
Transistor
No. 2 Photo
ON ON OFF OFF
Transistor
Position of
Slit Plate
No. 1 Photo No. 1 Photo (OFF) (ON) (OFF) (OFF) (ON) (OFF)
Transistor Transistor
(ON) (ON)
Semiconductor A new style deceleration sensor was introduced in the 1996 4WD RAV4
Deceleration Sensor only. The sensor consisted of two semiconductor sensors. They are
mounted at 90° to one another and installed so that each has an angle
of 45° to the centerline of the vehicle. Each semiconductor sensor is
provided with a mass which exerts pressure based on the deceleration
force applied to the vehicle. The sensor converts the force into
electronic signals, and outputs the signals to the ABS ECU.
Semiconductor
Deceleration Sensor

Anti-lock Brakes
Actuator The actuator controls hydraulic brake pressure to each disc brake
caliper or wheel cylinder based on input from the system sensors,
thereby controlling wheel speed. These solenoids provide three
operating modes during ABS operation:
• Pressure Holding.
• Pressure Reduction.
• Pressure Increase.
2-Position The two position solenoid actuator was first used on the 1993 Corolla
Solenoid Type and subsequently on all Toyota models by 1997 except Land Cruiser.
Consult the ABS Comparison Chart on page 225 of this publication for
the specific model application.
The actuator consists of six or eight 2−position solenoid valves, a pump and
reservoir. Each hydraulic circuit is controlled by a single set of solenoids:
• pressure holding solenoid.
• pressure reduction solenoid.
Aside from the 2−position solenoid valves, the basic construction and
operation of this system is the same as the 3−position solenoid system:
• four speed sensors provide input to the ECU which controls the
operation of the solenoids and prevent wheel lock−up.
• the two front wheels are controlled independently and the two rear
wheels are controlled simultaneously for three channel control.
• Supra has four channel control where the two rear wheels are
controlled independently just like the front wheels.
2-Position Solenoid
Hydraulic Circuit
The actuator consists of six
or eight 2-position
solenoids. Two solenoids
are used to control each
wheel hydraulic circuit.
Section 9
Pressure Holding Valve The pressure holding valve controls (opens and closes) the circuit
between the brake master cylinder and the wheel cylinder. The valve is
spring loaded to the open position (normally open). When current flows
in the coil the valve closes. A spring loaded check valve provides an
additional release passage when pressure from the master cylinder
drops.
Pressure Holding
Valve
Controls the circuit between
the brake master cylinder
and the wheel cylinder.
Pressure The pressure reduction valve controls (opens and closes) the circuit
Reduction Valve between the wheel cylinder and the actuator reservoir. The valve is
spring loaded in the closed position (normally closed). When current
flows through the coil, the valve compresses the spring and opens the
valve.
Pressure Reduction
Valve
Controls the circuit
between
the wheel cylinder and the
actuator reservoir.

Anti-lock Brakes
Operation During During normal braking the solenoids are not energized so the pressure
Normal Braking holding valve remains open and the pressure reduction valve remains
(ABS Not Activated) closed.
When the brake pedal is depressed, the master cylinder fluid passes
through the pressure holding valve to the wheel cylinder. The pressure
reduction valve prevents fluid pressure from going to the reservoir. As
a result normal braking occurs.
Normal Braking Mode

During normal braking the
solenoids are not energized
so the pressure holding
valve remains open and the
pressure reduction valve
remains closed.
Section 9
Pressure When any wheel begins to lock, the ABS ECU initially goes to hold
Holding Mode mode to prevent any additional increase in pressure. The ECU turns
OFF the Pressure Reduction Valve and turns the Pressure Holding
Valve ON. The pressure reduction valve closes, preventing hydraulic
fluid from going to the reservoir. The pressure holding valve remains
closed so no additional fluid pressure can reach the wheel cylinder.
Pressure
Holding Mode
The pressure reduction valve
closes, preventing hydraulic
fluid from going to
the reservoir.

Anti-lock Brakes
Pressure Reduction After the initial hold mode operation, the ABS ECU energizes both the
Mode holding valve and the reduction valve. The pressure holding valve
closes and blocks pressure from the master cylinder. The open
reduction valve allows hydraulic pressure from the wheel cylinder
circuit into the reservoir, reducing brake pressure. The pump is also
energized to direct hydraulic fluid back to the master cylinder. This
causes brake pedal feedback and alerts the driver to ABS operation.
Pressure
Reduction Mode
When the slip ratio of any
wheel exceeds 30%, the
ABS ECU energizes both the
holding valve and the
reduction valve.
Section 9
Pressure Increase As pressure inside the wheel cylinder is reduced and the speed sensor
Mode sends a signal indicating that the speed is above the target level, the
ECU turns OFF both the Pressure Reduction Valve and the Pressure
Holding Valve. The pressure reduction valve closes, preventing
hydraulic fluid from going to the reservoir. The pressure holding valve
opens so additional pressure enters the wheel cylinder if the driver
maintains pedal pressure. The operation is the same as Normal Mode
except the pump is on.
Pressure
Increase Mode
The ECU turns OFF both
the Pressure Reduction
Valve and the Pressure
Holding Valve.

Anti-lock Brakes
ABS ECU The ABS ECU senses the rotational speed of the wheels as well as the
vehicle speed based on signals from the wheel speed sensors. During
braking, the deceleration rate will vary depending on pedal pressure,
the vehicle speed during braking, and the road surface conditions. For
example, the deceleration rate is much greater on dry asphalt,
compared to a wet or icy surface.
The ECU judges the slip condition between the wheels and the road
surface by monitoring the change in the wheel’s rotational speed
during braking. The ECU controls the ABS actuator to deliver the
optimum hydraulic pressure to the brake cylinder to precisely control
the speed of the wheels, maintaining maximum brake force with a 10
to 30% slip ratio.
ABS Wiring Diagram

The ECU controls the ABS
actuator to deliver the
optimum hydraulic pressure
to the disc brake cylinder to
precisely control the speed
of the wheels.
Section 9
Solenoid Relay The Solenoid Relay supplies power to the solenoids. The ECU turns the
Control Solenoid Relay ON when the following conditions are met:
• Ignition switch ON.
• The Initial−Check Function is completed properly.
The ECU turns the solenoid relay OFF if any of the above conditions
are not met.
ABS Control Relay

The motor relay switches
voltage to the pump motor.
The solenoid relay
switches voltage to the
actuator solenoids.
Pump Motor Relay The Pump Motor Relay supplies power to the ABS pump motor located
Control in the Actuator. The ECU turns the relay ON when the following
conditions are met:
• During ABS operation or during the Initial Check.
• When the Solenoid Relay is ON .
The ECU turns the pump motor relay OFF when any of the above
conditions are not met.

Anti-lock Brakes
Wheel Speed Control The ECU continuously receives wheel speed signals from the speed
sensors and deceleration sensor. By calculating the speed and
deceleration of each wheel, the ECU estimates the vehicle speed. When
the brake pedal is depressed, the hydraulic pressure in each disc brake
cylinder begins to increase and the wheel speed begins to decrease. If
any of the wheels are near a lock−up condition the ECU goes into
pressure hold mode to stop the increase of hydraulic pressure in the
disc brake cylinder of that wheel.
SECTION A
The ECU sets the solenoid valves to the pressure reduction mode based
on wheel speed, thus reducing the hydraulic pressure in the disc brake
cylinder.
After the pressure drops, the ECU switches the solenoid valves to the
Holding Mode then monitors the change in wheel speed.
If the ECU judges that the hydraulic pressure needs to be reduced

further, it will return to reduction mode.
SECTION B
When the hydraulic pressure inside the disc brake cylinder decreases
(section A), the hydraulic pressure applied to the wheel falls. This
allows the wheel that was locking up to speed up. However, if the
hydraulic pressure is held down, the braking force acting on the wheel
will become too low. To prevent this, the ECU sets the solenoid valves
to the pressure increase mode and holding mode alternately as the
wheel which was locking up, recovers speed.
SECTION C
As the hydraulic pressure is gradually increased in the brake cylinder

by the ECU actuator (section B), the wheel tends to lock up again. In
response, the ECU again switches the solenoid valves to the pressure
reduction mode to reduce the hydraulic pressure inside the disc brake
cylinder.
SECTION D
Since the hydraulic pressure in the brake cylinder is decreased again

(section C), the ECU starts to increase the pressure again as in section B.
The cycle of Hold, Reduce and Increase is repeated many times until
the wheels are no longer outside the 30% slip ratio.
Section 9
Wheel Speed
Control Chart

Anti-lock Brakes
Section 10
ABS DIAGNOSIS
Lesson Objectives 1. Using the actuator tester and the appropriate Repair Manual
and/or TSB, select the proper SSTs to use in diagnosing an ABS
system.
2. Using the On−Board Diagnosis (OBD) system and a Repair
Manual, perform a dynamic diagnosis of speed sensors and
deceleration sensor.
3. Using a Repair Manual perform the self−diagnosis to access
trouble codes to determine malfunctions within the ABS and/or
TRAC systems.

ABS Diagnosis
Diagnosis and The ABS ECU has a self−diagnostic system which monitors the input
Troubleshooting and output circuits. The ABS ECU operates the solenoid valves and the
pump motor in sequence in order to check each respective electrical
system. This function operates only once each time the ignition switch
is turned ON. On some earlier models it operates when the vehicle is
traveling at a speed greater than 4 mph with the stop light (brake
light) switch OFF. During this check the operation of the actuator can
be heard, however this is normal and does not indicate a malfunction.
Initial Check Function

The check function may
vary depending on the
number of solenoids. Front RH Pressure Holding Valve
Front RH Pressure Reduction Valve
Front LH Pressure Holding Valve
Front LH Pressure Reduction Valve
Rear RH Pressure Holding Valve
Rear RH Pressure Reduction Valve
Rear LH Pressure Holding Valve
Rear LH Pressure Reduction Valve
Pump Motor
Diagnostic Function When a problem is detected in any of the signal systems, the ECU
turns on the ABS warning light in the combination meter to alert the
driver that a malfunction has occurred. The code is stored in memory
for access at a later time. Diagnostic trouble codes can be read from the
Warning light. The ABS ECU will also store the diagnostic trouble
codes for any ABS malfunction.
ABS Warning Light

The light warns the driver
of an ABS malfunction.
Section 10
Trouble Code Check To access the diagnostic trouble codes stored in the ECU, locate the Data
Link Connector (DLC1) or (DLC2). Consult the Repair Manual or the
ABS Reference Card to determine whether the ABS Check Connector is
physically disconnected or the short pin for Wa and Wb is removed.
To access diagnostic codes:

• Disconnect the check connector or remove the short pin in DLC1.
• Jumper terminals Tc and E1 of the Data Link Connector (DLC1 or
DLC2).
• Turn the ignition switch ON and read the trouble code from the
ABS warning light on the Combination Meter.
Camry and Avalon w/Bosch ABS, no Short Pin. Jumper Tc and El.
NOTE
Trouble Code Check

To access diagnostic
codes, remove the short
pin, jumper Tc and E1
in DLC1 and turn the
ignition switch ON.
Diagnostic Codes If the computer has not detected a malfunction, the lamp will blink two
times per second after a two second pause. When a malfunction has
been detected there will be a 4 second pause, then the first digit will
begin. The number of times the lamp blinks before a one and a half
second pause is the first digit of the code. Next, the number of blinks
before the second pause is the second digit of the code. In the example
below, the first code is Code 11 and the second code is 21.
If there is more than one trouble code, the code with the smallest number
will appear first, followed by a pause for 2.5 seconds, then the next code
Diagnostic Codes
A normal code (steady
flashing trouble light)
is output when there is no
fault found. If more than
one fault is detected,
each code is displayed.

ABS Diagnosis
will appear in the same manner as described earlier. Finally, the entire
procedure will be repeated after a four second pause.
The chart below identifies each code and reveals the circuit or
component which requires further diagnosis. The total number of
diagnostic codes may vary between vehicles so it is important to refer
to the Repair Manual for the specific vehicle you are diagnosing.
Trouble Code Chart

The code identifies the
component or circuit which
requires further diagnosis.
Code ABS Warning Light ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Diagnosis
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
Blinking Pattern
ON
11 Open circuit in ABS control (solenoid) relay circuit
OFF BE3831
12
ON
OFF ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
BE3831
Short circuit in ABS control (solenoid) relay circuit
ON
13 Open circuit in ABS control (motor) relay circuit
OFF
BE3831
ON
14 Short circuit in ABS control (motor) relay circuit
OFF BE3831
21
ON
Open or short circuit in 3-position solenoid circuit for
BE3832 right front wheel
ON Open or short circuit in 3-position solenoid circuit for
22
OFF BE3832 left front wheel
23
ON
BE3832
Open or short circuit in 3-position solenoid circuit for
right rear wheel
ON Open or short circuit in 3-position solenoid circuit for
24
OFF BE3832 left rear wheel
ON
31 Right front wheel speed sensor signal malfunction
OFF BE3833
32
ON
BE3833
Left front wheel speed sensor signal malfunction
ON
33 Right rear wheel speed sensor signal malfunction
OFF BE3833
ON
34 Left rear wheel speed sensor signal malfunction
OFF BE3832
35
ON
Open circuit in left front or right rear speed sensor
BE3833 circuit
ON Open circuit in right front or left rear speed sensor
36
OFF BE3833 circuit
37
ON
BE3833
Faulty rear speed sensor rotor
41
ON
Low battery positive voltage or abnormally high
OFF BE3834 battery positive voltage
ON Pump motor is locked
51
OFF BE3836 Open in pump motor ground
Always
on
ON
Malfunction in ECU
Section 10
Circuit Inspection The Repair Manual takes the diagnosis several steps further in
providing a circuit inspection and inspection procedure for each
diagnostic code. It provides a circuit description as well as the
parameters under which the code was set for each stored code. A
wiring diagram schematic of the electrical circuit is also provided for
ready reference.
Circuit Description
and Wiring Diagram
A circuit description
includes the parameters
for setting the code.

ABS Diagnosis
An inspection procedure follows the circuit inspection with

components, connectors, pin locations and measurement values to
diagnose the circuit. Each step is outlined in sequence, leading to a
repair.
Inspection Procedure
The inspection procedure
includes components,
connectors and pin
locations as well as
measurement values to
diagnose the circuit.
Section 10
Diagnostic Trouble Following diagnosis and repair, clear the trouble codes stored in the
Code Clearance ECU. The procedure will vary depending on the model and year. Either
refer to the ABS Reference Card or Repair Manual for specifics. The
essential difference is in disconnecting the actuator check connector on
earlier models as compared to the removal of the short pin connector in
the DLC1 or DLC2 connector. A typical procedure is outlined below:
• Jumper terminals Tc and E1 of the DLC2 or DLC1 and remove the
short pin from DLC1.
• Turn ignition switch ON.
• Depress the brake pedal 8 or more times within 3 seconds.
• Check that the warning light shows the normal code.
• Remove the jumper wire and reinstall the short pin.
To ensure that the brake light switch opens and closes each time, allow
the brake pedal to return to the full up position each time when clearing
codes. If the code does not clear, the ignition switch must be cycled OFF
then ON before depressing the brake pedal 8 times in 3 seconds.
Clearing Diagnostic
Trouble Codes

ABS Diagnosis
Speed Sensor Eight additional diagnostic codes (71 through 78) are available to
Signal Check trouble−shoot the speed sensors and rotors. They determine whether
the signal to the ECU is a low output voltage or an abnormal change in
output voltage. When using the signal check, make sure that the
vehicle is driven straight ahead.
The ECU is placed in signal check mode differently based on model and
year, so again it is important to have the appropriate repair manual
available. In some early models, the parking brake in conjunction with
the service brake were used to enter this mode.
In most cases, connect terminals Tc and E1 at DLC1 prior to driving

the vehicle. To read the code:
• Connect terminals Ts and E1 (Tc and E1 remain connected).
• In earlier models the actuator check connector was disconnected.
Diagnostic Trouble
Codes for
Speed Sensor
Check Function
Consult the Repair Manual
for specific instructions to
place the ECU into the
diagnostic mode and to
read the codes.
ÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁ
Code No.
ÁÁÁÁÁÁÁÁÁÁÁ Diagnosis Trouble Area
ÁÁÁÁ
71
ÁÁÁÁÁÁÁÁÁÁÁ
Low output voltage of right front speed sensor
• Right front speed sensor
• Sensor installation
ÁÁÁÁ
72
Low output voltage of left front speed sensor
• Left front speed sensor
ÁÁÁÁ
73
Low output voltage of right rear speed sensor
• Right rear speed sensor
ÁÁÁÁ
74
Low output voltage of left rear speed sensor
• Left rear speed sensor
ÁÁÁÁ
75
Abnormal change in output voltage of right front speed sensor • Right front speed sensor rotor
ÁÁÁÁ
76
Abnormal change in output voltage of left front speed sensor • Left front speed sensor rotor
ÁÁÁÁ
77
Abnormal change in output voltage of right rear speed sensor • Right rear speed sensor rotor
ÁÁÁÁ
78
Abnormal change in output voltage of left rear speed sensor • Left rear speed sensor rotor
Section 10
Deceleration The deceleration sensor can be checked both statically and

Sensor Check dynamically. Jumpering E1 and Ts at DLC1 and observing the ABS
light are the essential steps.
Static Testing With Ts and E1 jumpered and the engine running, raise the rear of the
vehicle slowly to the specified height as described in the Repair
Manual, then observe the ABS light. The light should blink 4 times per
second. If the light remains ON, inspect the sensor installation. If its
properly installed, replace the deceleration sensor.
Lower the vehicle slowly and then raise the front slowly to the specified
height and observe the light as in the procedure described above.
Static Testing
Raising the front and rear
separately to a specific
height and note the
condition of the ABS light.
Dynamic Testing For most vehicles except 1996 RAV4, jumper terminals Ts and E1 in
DLC1, drive the vehicle straight forward at about 12 mph:
• Lightly depress the brake pedal and the light should remain
flashing 4 times per second.
• Bring the speed up to 12 mph or more and depress the brake pedal
moderately hard and the light should come on while braking.
• Bring the speed up to 12 mph or more and depress the brake pedal
strongly and again the light should come on while braking. If the
light does not operate as specified, inspect the sensor installation. If
the installation is OK, replace the deceleration sensor.

ABS Diagnosis
RAV4 Static Test The RAV4 deceleration sensor is tested after removal from the vehicle.
• assemble three 1.5 volt dry cell batteries in series.
• connect the positive side of the battery to terminal VGS and the
negative side to the GGND.
• check the output of GL1 and GL2 terminals with a voltmeter
comparing your readings with the chart below.
Do not turn the sensor upside down when removed from the vehicle. If
CAUTION dropped, it should be replaced.
RAV4 Deceleration
Sensor Test
Applies to 1996 RAV4 only.
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁ
Symbols Condition Standard Value
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁ
GL1 Horizontal about 2.3 V
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ
GL1
ÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
GL1
ÁÁÁÁÁÁÁÁÁ
Lean forward
Lean rearward
0 - about 2.3 V
about 2.3 V - 4.5 V
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
GL2
ÁÁÁÁÁÁÁÁÁ
Horizontal about 2.3 V
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
GL2
ÁÁÁÁÁÁÁÁÁ Lean forward about 2.3 V - 4.5 V
ÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
GL2
ÁÁÁÁÁÁÁÁÁ Lean rearward 0 - about 2.3 V
ABS Actuator The actuator operation can be checked using a Special Service Tool
Checker called an ABS Actuator Checker and related subharness and overlay
sheet where needed. This special service tool can check the operation of
the solenoid valves and the pump motor. The actuator is disconnected
from the vehicle harness, taking the ECU out of the loop and operated
independently by the special service tool.
ABS Actuator Checker

This special service tool can
check the operation of the
solenoid valves, the by-pass
valve and the pump motor.
Section 10
The ABS actuator checker statically checks the operation of the

actuator which includes: the pump, solenoids, and relays. If a normal
code is displayed, but symptoms still occur, refer to the Problem
Symptoms Chart in the Repair Manual. The chart indicates when the
checker is to be used.
When to Use
Actuator Checker
If a normal code is displayed
but symptoms still occur,
the Repair Manual chart
identifies circuits to
check and use of the
Actuator Checker.
ÁÁ
ÁÁÁÁÁÁÁ
ÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ ÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ÁÁ
ÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
Symptoms Inspection Circuit See page
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ABS does not Only when 1. ~ 4. are all normal and the problem is still occurring,
operate. replace the ABS ECU.
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
BR-50
1. Check the DTC, reconfirming that the normal code is output.
BR-70
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
2. IG power source circuit.
BR-66
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
3. Speed sensor circuit.
BR-37
4. Check the ABS actuator with a checker. If abnormal, check the
ÁÁÁÁÁÁÁ
ÁÁ
ABS does not ÁÁ
ÁÁÁÁÁÁÁ
hydraulic circuit for leakage (see page BR-79).
Only when 1. ~ 4. are all normal and the problem is still occurring,
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
operate efficiently. replace the ABS ECU.
BR-50
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
1. Check the DTC, reconfirming that the normal code is output.
BR-66
2. Speed sensor circuit.
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
BR-72
3. Stop light switch circuit.
BR-37
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
4. Check the ABS actuator with a checker. If abnormal, check the
ÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
hydraulic circuit for leakage (see page BR-79).
ÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ABS warning light 1. ABS warning light circuit. 2. ABS ECU.
BR-74
abnormal.
ÁÁ
ÁÁÁÁÁÁÁ
DTC check cannot
ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ
ÁÁÁÁÁ
Only when 1. and 2. are all normal and the problem is still occurring,
ÁÁ ÁÁ
be done. replace the ABS ECU.
ÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
1. ABS warning light circuit. BR-74
2. Tc terminal circuit. BR-76
ÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
ÁÁ ÁÁ
Speed sensor signal 1. Ts terminal circuit. BR-78
check cannot be 2. ABS ECU.
ÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ
done.
ÁÁ
ÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁ

ABS Diagnosis
Refer to the Repair Manual for adapter harnesses required and the
illustration depicting proper connections. The illustration below shows
the connections for Supra models with or without TRAC.
ABS Actuator
Checker Installation
Follow the references in the
Repair Manual for the proper
harnesses and connector
configuration. This Supra
example shows different
adapter harnesses based
on it being equipped with
Traction Control.
Pay particular attention to the operating procedure as you perform the

diagnosis. Note that the illustrations of the checker buttons are
darkened to emphasize which ones are pressed for a given step. When
the procedure indicates that the pedal goes UP or DOWN it means that
the pedal will move a short distance in that direction.
Actuator Checker
Overlay
The illustrations of the
Actuator Checker buttons in
the Repair Manual are
darkened to emphasize
which ones are pressed
for a given step.
Section 10
The Actuator cannot be disassembled for service. If a malfunction

develops with the solenoid valves or pump motor, the entire ABS
Actuator assembly must be replaced.
Bleeding the ABS hydraulic system does not differ from the bleeding
procedure of a conventional brake system except for rear wheel ABS.
As fluid is bled, it flows through the solenoids to the wheels. The part
of the actuator hydraulic circuit going from the solenoids through the
No. 1 check valve is sealed to prevent air entry when the ABS is not
activated.
If a malfunction occurs in the electrical system to the ECU, current to

the actuator from the ECU is turned off. As a result, the brake system
operates the same as if the antilock brake system is not operating and
normal braking function is assured.
Toyota Diagnostic Toyota models equipped with a DLC2 connector located under the
Tester instrument panel have the capability to read diagnostic codes using the
Diagnostic Tester. In addition, ECU pin voltage values on all ABS
ECU’s can be read on the Tester screen using the Vehicle Break−out
Box feature. The Diagnostic Tester has a number of components and
harnesses which vary, based on the vehicle and ECU being tested. An
Operators Manual is provided with the Tester which describes the test
functions and tool set−up. The Vehicle Break−out Box is connected
between the vehicle harness and the ECU connectors.
Remember that while diagnosing any electrical system, disconnecting

and reconnecting electrical connectors may eliminate a system fault.
Diagnostic Tester
ABS and TRAC diagnostic
codes and speed sensor
codes can be read using
the tester.

ABS Diagnosis

ABS Actuator Checker
In this Worksheet you will practice the use of the ABS Actuator Checker (Not to be used with TMM Camry/Avalon).

• ABS Actuator Checker (09990-00150).
• Vehicle Specific Harness Adapters.
• Vehicle Repair Manual.
• DVOM.
• Jumper Wire or SST 09843-18020.
Preparation:
• Disconnect the actuator/control relay electrical connectors.
• Connect the actuator checker to the actuator, control relay and body side harness. Place the cover sheet on
to the checker if needed. Refer to the appropriate Repair Manual for the proper adapter harnesses.
Required Adapter Required Checker

Harnesses Cover sheet
Section 10
Actuator Testing:
1. Inspect battery voltage.
Measured Battery Battery Voltage

Pass/Fail
Voltage Specification
2. Connect the sub-wire harness battery connectors to the battery.
3. Start the engine and run at idle.
4. Turn the selector switch to “FRONT RH” position.
5. Push and hold the MOTOR switch for a few seconds.
6. Depress and hold the brake pedal and push the POWER Switch (do not depress switch for more than 10
seconds).
a. What happened to the brake pedal?
7. Release the switch and notice the brake pedal action.
a. What happened to the brake pedal?
b. What component(s) were checked?
8. While pressing the brake pedal, press the MOTOR Switch for a few seconds.
a. What happened to the position of the brake pedal?
b. What component(s) were checked?
9. Depress and hold the brake pedal for 15 seconds. While holding the brake pedal press the MOTOR Switch.
a. Does the brake pedal pulsate?
10. Turn the main selector switch to the other three wheel positions and repeat the actuator tests above.
11. Disconnect and remove ABS Checker and harnesses.
12. Clear diagnostic codes (if any) from memory.

ABS Diagnosis

Speed Sensor Signal Check
In this Worksheet you will practice the procedure for checking the speed sensor signal.

• Jumper wire (SST 09843-18020) or equivalent jumper.
Procedure:
1. Turn the ignition switch OFF.
2. Jumper terminals Ts and E1 at DLC1.
3. Start engine.
4. Check the ABS warning light and record your observation.
a. If the warning light does not blink, what should be checked first?
5. Drive the vehicle faster than 28 mph for several seconds.
6. Stop the vehicle and jumper terminals Tc and E1 of DLC1.
7. Record the codes as output by the ABS Warning Light.
8. Disconnect terminals Ts and E1 and Tc and E1 at DLC1 and turn ignition switch OFF.
Section 10
Summary:
1. Answer the following questions for Speed Sensor Code 72.
a. Which sensor is at fault?
b. What is the cause of the fault?
2. Answer the following questions for Speed Sensor Code 78.
a. Which sensor is at fault?
b. What could cause this fault?

ABS Diagnosis

Toyota Diagnostic Tester
In this Worksheet you will practice the use of the Toyota Diagnostic Tester
to access ABS diagnostic codes.
Tools and Equipment

• Diagnostic Tester (TOY220036).
Procedure:
1. Attach the DLC2 Cable, Vehicle Interface Module (VIM) and the DLC Cable to the Tester.
2. Attach the DC Power Cable to the DLC Cable and plug into the auxiliary power source.
3. Connect the DLC2 Cable to the vehicle.
4. Power up the unit and select ENTER.
5. At the Main Menu select OBD and press ENTER.
6. Follow the screen prompts for the vehicle you are working on.
7. At the OBD MENU select CODES (ALL).
8. Record the codes and components/circuits that appear on the tester screen.
9. Clear the diagnostic codes.

a. Jumper TC and E1 and remove the short pin in DLC1.
b. Turn ignition switch ON.
c. Depress the brake pedal 8 or more times within 3 seconds.
d. Is the Normal Code given at the warning light?
e. Remove jumper wire and install short pin.
Section 10

ABS Diagnosis

Toyota Diagnostic Tester and Vehicle Break-out Box
In this Worksheet you will practice the use of the Toyota Diagnostic Tester and Break-out Box to:
• access ABS ECU terminal signals
• access speed sensor oscilloscope patterns
Tools and Equipment

• Diagnostic Tester (TOY220036).
• Program Card.
• V-BOB Interface Card.
• V-BOB.
• Misc. V-BOB Harnesses.
Procedure:
1. Insert the Program Card in the Tester.
2. Connect the I/P Cable to the I/P connector on the bottom of the tester and the I/P connector on the
Breakout Box.
3. Connect both the Tester and Break-out Box to a power source.
4. Power up the unit and select ENTER.
5. At the Main Menu select Breakout Box and press ENTER.
6. Follow the screen prompts for the vehicle you are working on.
7. At the Vehicle Confirmation Screen verify the information is correct for the vehicle and select YES.
8. When the vehicle and system have been selected, the Tester displays which ECU Interface Box, harness,
and connectors are required to attach the V-BOB.
9. With the ignition switch OFF, disconnect the ECU harness connector.
10. Connect the vehicle harness to the ECU interface Box.
11. Connect the 50-pin and 80-pin Data Cables to the Break-out Box and ECU Interface Box.
12. Select DATA LIST from the Break-out Box Menu.
13. Turn ON the ignition switch and record the values for the items listed below:
IG1_________ MT_________ MR_________ SR_________ AST_________
14. Refer to the Repair Manual or EWD and identify the circuit of each of the terminals above.
Section 10
IG1
MT
MR
SR
AST
Oscilloscope Function:
1. From the Break-out Box Menu select oscilloscope and press ENTER.
2. Select FR+ (for front-wheel-drive) or RR (for rear-wheel-drive) and press ENTER.
3. Drive the vehicle at 15 mph and note the oscilloscope pattern height and frequency.
a. Pause screen
b. Print Screen or copy the oscilloscope pattern in the space below (SEND)
4. Drive the vehicle at 30 mph and note the oscilloscope pattern.
a. Pause screen
b. Print Screen or copy the oscilloscope pattern in the space below
3b. 4b.
5. What is the difference between the oscilloscope patterns in 3 and 4 above?
6. Describe the A\C wave form for a speed sensor with a missing tooth.

Section 11
OTHER ABS ACTUATORS
Lesson Objectives 1. Describe how the solenoid controlled power steering fluid controls
brake pressure in the rear wheel ABS system.
2. Describe the function of the mechanical valve in the 3−position
solenoid and mechanical valve actuator.
3. Describe the three control positions of the 3−position solenoid in
maintaining ABS operation.
Section 11
Other Actuator Toyota uses several types of ABS actuators, each differs in how the
Designs modulation of pressure is accomplished. The function of sensors and
ECU control already discussed in Section 8, do not differ.
3-Position The 3−position solenoid valve uses a 3−position valve, electrical coil and
Solenoid Type check valve. As current flows through the solenoid windings, it creates
a magnetic field around the 3−position valve causing it to move toward
the center of the coil compressing the return spring. Current from the
ABS ECU is switched in three steps; 0 amps, 2 amps and 5 amps in
order to control the strength of the magnetic force in the coil.
There are four 3−position solenoid valves in the ABS actuator described
here; those for the front wheels control the left and right wheels
independently, while those for the rear wheels control both the left and
right wheels simultaneously. The system is therefore known as a
three−channel system.
3-Position
Solenoid Actuator
The 3-position solenoid
valves for the front wheels
control the left and right
wheels independently, those
for the rear wheels control
both left and right wheels
simultaneously.

Actuator Types
Operation During During normal braking ABS is not activated and the 3−position valve is
Normal Braking pushed down by a return spring. The solenoid inlet, port A", remains
(ABS Not Activated) open while the outlet to the reservoir, port B" remains closed.
When the brake pedal is depressed, brake fluid passes from port A" to
port C", to the disc brake cylinder. Brake fluid is prevented from
flowing into the pump by the No. 1 Check Valve located in the pump
circuit.
When the brake pedal is released, the brake fluid returns from the disc
brake cylinder to the brake master cylinder through port C" to port
A" and the No. 3 Check Valve in the 3−position solenoid valve.
Normal Braking
With zero amps applied to
the solenoid, port A is open,
pressure is applied to the
brake cylinder.
Section 11
“Holding” Mode When the ECU determines that a wheel is about to lockup, it switches
to the holding mode to stop the increase in hydraulic pressure. As the
pressure inside the disc brake cylinder is reduced or increased, and the
speed sensor indicates that the speed is at the target level, the ECU
supplies a 2 ampere signal to the solenoid coil to hold the pressure in
the disc brake cylinder at that level.
When the current supplied to the solenoid coil is reduced from 5

amperes (in the pressure reduction mode) to 2 amperes (in holding
mode), the magnetic force generated in the solenoid coil also decreases.
The 3−position solenoid valve then moves down to the middle position
by the force of the return spring, closing port B".
With the ECU holding Port A closed, and pedal pressure closing check
valves #1 & #3, brake caliper pressure holds steady, and cannot be
increased.
“Holding” Mode
Two amps applied to the
solenoid, Port A and Port B
are closed, pressure remains
constant.

Actuator Types
“Pressure Reduction” When the wheels are about to lock, the ECU supplies a 5 ampere signal
Mode to the solenoid to close port A" and open port B" in the 3−position
solenoid. As a result, brake fluid from the disc brake cylinder passes
through port C" to port B" in the 3−position solenoid and flows to the
reservoir. At the same time, a computer signal causes the pump to
operate. Fluid stored in the reservoir is pumped to the master cylinder.
The forced return of pressure from the brake caliper circuit to the
master cylinder forces the pedal up slightly. This causes the driver to
feel the ABS system operation.
“Pressure Reduction”
Mode
Five amps applied to
solenoid, port A dosed, port
B open, allowing pressure
from wheel cylinder to flow
to the reservoir.
Section 11
“Pressure Increase” When the pressure in the disc brake cylinder needs to be increased to
Mode apply more braking force, the ECU stops sending current to the
solenoid coil. This opens port A" of the 3−position valve and closes port
B". This allows the fluid in the master cylinder to pass from port C"
in the three−position solenoid valve to the disc brake cylinder. The
hydraulic pressure increase rate is controlled by the repetition of the
pressure increase and holding modes.
Brake caliper pressure will increase as long as the driver continues to

apply pedal pressure.
“Pressure Increase”
Mode
Zero amps applied to
solenoid, Port A
open allowing
pressure application
at wheel cylinder.

Actuator Types
3-Position Solenoid This actuator is fundamentally the same as the 3−position solenoid type
and Mechanical just discussed. It consists of three, 3−position solenoid valves, a
Valve mechanical valve, pump and reservoir. The solenoid that controls
pressure to the right rear wheels also uses a mechanical valve that
controls pressure to the left rear wheel. This actuator was first
introduced on the 1994 Camry produced by Toyota Motor
Manufacturing (TMM) in Georgetown, Kentucky and later in the
Avalon.
3-Position Solenoid
and Mechanical
Valve Actuator
The solenoid which controls
pressure to the rear wheels
also uses a mechanical valve
which controls pressure to
the left rear wheel.
Section 11
Mechanical Valve The mechanical valve consists of two sets of cylinders and pistons and
Construction a plunger to link their movement. Piston A monitors the pressure from
the master cylinder on its left side and monitors pressure to the right
rear brake circuit from the 3−position solenoid valve on its right side.
Piston A moves based on differences in pressure since piston surface
areas are equal.
Any movement of piston A is traced by piston B through the plunger. In

addition a spring loaded valve opens and closes the master cylinder
passage to the left rear brake. Piston B is spring loaded to the left
which leaves the valve unseated and the fluid path open.
Mechanical Valve
This valve controls pressure
to the left rear brake
eliminating the need for
another solenoid.
Pressure Increase Mode The pressure increase mode is also the normal braking position for
brake operation without ABS, as shown in the illustration above.
Pressure from the master cylinder is equal to the pressure coming
through the 3−position solenoid to the right rear brake cylinder. Piston
A does not move and is held to the left by the spring acting on piston B.
Pressure increase comes from the master cylinder through the brake
pedal pressure.

Actuator Types
Pressure Hold Mode When the 3−position solenoid goes into the hold position, it blocks
hydraulic fluid and pressure in the circuit between the solenoid, the
right rear brake cylinder and the right side of piston A. With pressure
in the right rear brake held, pressure from the master cylinder
increases, causing piston A to move right. The piston forces the plunger
to move piston B to the right, blocking fluid from the left rear brake
cylinder. This action mirrors the pressure hold on the right rear
cylinder.
Pressure Hold Mode

The Mechanical Valve
causes pressure at the
left rear brake to mirror
the pressure to the
right rear brake.
Section 11
Pressure Reduction The 3−position solenoid goes into the reduction position venting
Mode hydraulic fluid and pressure in the circuit between the solenoid, the
right rear brake cylinder and the right side of piston A. Pressure from
the master cylinder increases causing piston A to move right forcing
the plunger to move piston B to the right compressing the spring. The
movement of piston B increases the area volume in the left rear brake
cylinder hydraulic circuit. This action mirrors the pressure reduction in
the right rear cylinder.
Pressure Reduction
Mode
Pressure reduction is
accomplished by the
movement of piston B
creating an increase in
volume to the circuit to
the left rear brake.

Actuator Types
Power Steering This system was an option on ’90 − ’94 Trucks, 4Runners and T100’s. It
Pressure Controls represents the greatest departure from the systems we have studied to
Brake Pressure this point. The following summary highlights the unique features of
this system:
• Controls the rear wheels only.
• Single 2−position solenoid in the actuator.
• Uses a deceleration sensor and a single speed sensor.
• Power steering pressure is used to control brake pressure.
Rear Wheel ABS

Diagram
The ECU uses input from the
single speed sensor and
deceleration sensor
to control rear wheel
brake pressure.
A single 2−position solenoid controls the hydraulic pressure from the

power steering to control brake pressure. This system uses only one
speed sensor, mounted on top of the ring gear on the differential case.
For this reason, a deceleration (G") sensor is necessary for the ECU to
sense the deceleration rate to help determine the slip rate of the rear
wheels.
Section 11
The actuator modulates power steering fluid pressure to control brake

hydraulic pressure when the rear wheels slip while the brakes are
applied. Modulation is accomplished by the ECU controlled
two−position solenoid based on input from the deceleration sensor and
speed sensor.
The actuator is made up of several components each having a specific

function:
• Solenoid valve is controlled by the ABS ECU and modulates the
power steering pressure on the pressure reduction piston.
• Pressure regulator valve regulates the power steering pressure
in relation to brake pressure. Brake pressure acting on the right
side of the valve compressing the spring would raise power steering
pressure. (Located in the Actuator)
• Relief valve relieves the power steering pressure if it gets too high
in the actuator.
• By−pass piston opens and closes the by−pass valve according to
power steering pressure. It constantly monitors power steering
pressure to keep the by−pass valve closed. If the power steering
pressure is lost, the by−pass valve opens to allow brake pressure to
by−pass actuator control.
• Pressure reduction piston opens and closes the cut and by−pass
valve to direct the brake fluid. Changes the brake pressure to the
rear wheels by increasing or decreasing the volume in its bore.
• Cut valve and By−pass valve controls the path of brake fluid
depending on mode of operation.
Actuator Hydraulic
Circuit
The actuator modulates
power steering fluid
pressure to control brake
hydraulic pressure when
the rear wheels slip.

Actuator Types
Control of the brake fluid pressure acting on the rear brake cylinders is
carried out in three modes:
• pressure holding.
• pressure reduction.
• pressure increase.
Operation During The rear−wheel anti−lock brake system is not activated during normal
Normal Braking braking. In this mode the power steering fluid pressure acts on
chambers C" and D" pushing both the by−pass valve and cut valve
toward the right. This causes the cut valve to open and the normal port
on the left side of the by−pass valve to open.
When the brake pedal is depressed, the master cylinder fluid pressure
rises. The brake fluid passes from the cut valve to the normal port in
the by−pass valve, and is sent to the rear brake wheel cylinders.
Normal Braking
Power steering fluid
pressure acts on chambers
“C” and “D” pushing both
the by-pass valve and cut
valve toward the right.
Section 11
Holding Mode This system does not have a specific holding mode: instead, rear brake
cylinder pressure hold mode is maintained by the ECU quickly pulsing
the solenoid valve ON and OFF between pressure increase and
pressure reduction modes. The ECU maintains the pressure to the rear
wheel cylinders within a narrow range as it continues to monitor wheel
speed.
Holding Mode
The holding mode is
maintained by the ECU
quickly pulsing the solenoid
valve ON and OFF between
pressure increase and
reduction to maintain the
brake pressure to the
rear brakes.

Actuator Types
Pressure Reduction When the rear wheel begins to lock−up, the ECU energizes the solenoid
Mode coil, generating a magnetic force. The plunger moves upward and port
A" closes while port B" opens. As a result, the power steering fluid
acting on chamber D" returns to the power steering reservoir. This
causes the pressure reduction piston to move to the left, first closing
the cut valve, then causing the brake fluid pressure, acting on the rear
brake wheel cylinders, to accumulate in chamber E".
As a result, the pressure level inside the rear brake wheel cylinders
decreases to prevent wheel locking.
Pressure Reduction
Mode
The solenoid plunger moves
upward and port “A” closes
while port “B” opens. As a
result, the power steering
fluid acting on chamber “D”
returns to the reservoir.
Section 11
Pressure Increase When the fluid pressure in the rear brake wheel cylinders needs to be
Mode increased to apply more braking force, the ECU changes the ratio in
which the solenoid valve is turned on and off. In the pressure increase
mode, the brake fluid pressure in the rear wheel brake cylinders is
increased while the solenoid valve is switched on and off repeatedly. By
extending the amount of time the solenoid is switched off in the on/off
cycle, the amount of time port A" is open and port B" is closed is
extended and this causes the pressure in chamber E" to rise gradually.
The cut valve remains closed during the operation. The pressure
reduction piston is moved gradually to the right and increases the
brake fluid pressure acting on the rear brake wheel cylinders.
Pressure Increase
Mode
By extending the amount of
time the solenoid is switched
off in the on/off cycle, the
amount of time port “A” is
open and port “B” is closed
is extended and this causes
the pressure in chamber “E”
to rise gradually.

Actuator Types
Fail-safe Mode In the event power steering fluid pressure is insufficient, the By−pass
Piston and Pressure Reduction Piston move to the left by brake
hydraulic pressure. This causes the Cut Valve and the Normal Port of
the Bypass Valve to close. With the Normal Port closed, the By−pass
Port is open allowing the master cylinder to apply pressure to the
wheel cylinder.
In the event of a malfunction in the signal system to the ABS ECU, the
solenoid relay is shut OFF. The spring loaded Solenoid Valve allows
power steering pressure to move the Pressure Reduction piston and
By−pass Piston to the right opening the Cut Valve. The brake system
operates as a normal brake system without ABS.
Fail-safe Mode
Allows brake fluid to by-pass
the actuator control in the
event of power steering
pressure loss.
Section 11
Bleeding the Rear Rear wheel ABS requires a special bleeding procedure when a
Wheel Antilock component of the steering system or the actuator is replaced. A typical
Brake System procedure is outlined here however, check the appropriate Repair
Manual as procedures may vary:
• Bleed the power steering system using the conventional method.
• Bleed the brake system with the engine running.
• Bleed the brake system with the engine OFF.
• Bleed the power steering system using the brake actuator checker.
The conventional method of bleeding the power steering system

requires that the reservoir be full.
• Run the engine at 1000 rpm or less.
• Turn the steering wheel from lock to lock three or four times.
• The fluid in the reservoir should not be foamy or cloudy indicating
presence of air.
After the brakes are bled connect the ABS actuator checker.
• Run the engine at idle.
• Turn the selector switch on the actuator checker to AIR BLEED".
• Strongly depress the brake pedal and hold it.
• Push the ON/OFF switch five times for three seconds each time
while holding the brake pedal down.
Do not press the ON/OFF switch before depressing the brake

CAUTION pedal and do not release the brake pedal while the ON/OFF
switch is ON or damage to the master cylinder piston cups may
occur.
Rear Wheel
ABS Bleeding
Bleeding the ABS requires
bleeding both the power
steering system as well as
the brake system.

Actuator Types
Section 12
TRACTION CONTROL SYSTEM (TRAC)
Actuator
Solenoids
Wheel
Speed
Sensors Actuator
Pump Motor
Battery Slip Indicator

ABS/TRAC ECU Light
TRAC OFF TRAC OFF

Switch Light
Stop
Light ABS Warning
Switch Light
Throttle Injectors
Position
Sensor ECM
Shift Solenoid
Valves
Lesson Objectives 1. Identify the main components of the TRAC systems.

2. Describe the function of components in the TRAC system.
3. Describe the engine torque control system used on the Camry and
Avalon.
4. Describe the fail−safe function of the Camry and Avalon actuator.
5. Describe the primary differences between Camry /Avalon and the
Supra TRAC systems.

Traction Control Systems (TRAC)
Traction Control Traction Control was first introduced on the 1994 Turbo Supra and
System expanded to include the six cylinder Camry and Avalon models in 1997.
The purpose of the Traction Control System is to prevent wheel spin

from occurring due to acceleration. The maximum torque that can be
transmitted to the wheels is determined by the coefficient of friction
generated between the road and the tires. If torque exceeds that level,
the wheels are likely to spin. Conditions for TRAC operation may
include: loose gravel, slippery road surfaces, acceleration while
cornering and hard acceleration.
Once activated, the TRAC System reduces engine torque and drive wheel
speed as necessary to bring the vehicle under control which improves
vehicle stability when starting, accelerating or turning on slippery roads.
Although the Supra and Camry/Avalon TRAC system both control

engine torque and drive wheel braking, how that is accomplished varies
and therefore the two systems are covered separately in this section.
Camry TRAC
Component Locations
The TRAC Systems share
some ABS components
to control traction
control functions.
Section 12
Camry/Avalon TRAC The ABS/TRAC ECU and ECM work together to provide traction
control. The ABS/TRAC/ECU monitor signals from the four speed
sensors to determine the speed of each wheel and vehicle speed. When
slippage is determined:
• The ABS/TRAC ECU activates the actuator solenoids and pump
motor which applies hydraulic pressure to the brakes at the drive
wheels.
• The ECM monitors the throttle position sensor and denies fuel
injection on up to five cylinders to limit engine torque.
• The ECM prohibits shifting of the automatic transaxle.
• The slip indicator light is turned ON to notify the driver of TRAC
operation and a signal is sent to the ECM.
Typical ABS/TRAC
Control System
Actuator
Solenoids
Wheel
Speed
Sensors Actuator
Pump Motor
Battery Slip Indicator

ABS/TRAC ECU Light
TRAC OFF TRAC OFF

Switch Light
Stop
Light ABS Warning
Switch Light
Throttle Injectors
Position
Sensor ECM
Shift Solenoid
Valves

Wheel Speed Control Wheel speed control is accomplished by two means:

• Brake application through the TRAC & ABS actuator.
• Reduce engine torque by temporarily disabling from one to five
injectors.
Fuel injector control is performed through the ECM. In the example

below, speed of the front wheel exceeds the Control Starting Speed. The
ECM initially shuts off five injectors and turns some injectors back ON
as wheel speed decreases. As wheel speed approaches Target Control
Speed, an additional injector is shut off temporarily to prevent
over−shooting the Control Starting Speed.
Wheel Speed Control

Brake pressure and
control of injectors
reduce wheel speed
when slippage occurs.
Operation of The TRAC OFF switch located on the instrument panel allows the
Components driver to activate or deactivate the TRAC system. The system defaults
to ON when the ignition switch is cycled.
The TRAC OFF Indicator Light goes on when the TRAC system is
turned OFF. Additionally, it blinks when a malfunction has occurred in
the engine or the TRAC system.
The Slip Indicator Light blinks when the TRAC system is operating to
inform the driver.
TRAC OFF Switch and

Indicator Lights
The TRAC OFF switch
allows the driver to control
the TRAC system operation.
Section 12
ABS & TRAC The ABS & TRAC actuator is contained in one housing and has twelve
Actuator 2−position solenoid valves which control hydraulic pressure to the
brake calipers. In addition there are two pumps controlled by one
motor, two reservoirs and two regulator valves.
Of the twelve solenoid valves there are:

• Two Master Cut Solenoid Valves.
• Two Reservoir Cut Solenoid Valves.
• Four Pressure Holding Valves.
• Four Pressure Reduction Valves.
The Master Cut Solenoid Valve opens and closes the hydraulic
circuit between the master cylinder and the ABS pressure holding
valve 2−position solenoid and on to the front brake caliper. Its normal
position is spring loaded in the open position. Its construction and
operation are the same as the pressure holding valve.
The Reservoir Cut Solenoid Valve opens and closes the hydraulic
circuit from the master cylinder to the actuator pump. Its normal
position is spring loaded in the closed position. Its construction and
operation are the same as the pressure reduction valve.
The Pressure Regulator Valve regulates brake fluid pressure

generated by the actuator pump.
When the TRAC system is activated the Master Cut Solenoid Valve
and Reservoir Cut Solenoid Valve control the brake system to the drive
wheels while the Pressure Holding Valve and Pressure Reduction
Valves of the ABS system modulate the pressure in three phases:
Pressure Increase, Pressure Holding and Pressure Reduction.

ABS & TRAC Actuator

Section 12
Normal Operation During normal operation when the TRAC system is not activated all
Mode actuator valves are OFF. The Master Cut Solenoid Valve is open
allowing fluid from the master cylinder to flow through the Pressure
Holding Valve to the wheel cylinder. In this mode the brakes function
just like a system without ABS or TRAC.
Normal Operation
Mode

Pressure Increase During sudden acceleration or driving on a slippery surface, if the drive
Mode wheels start to slip the ABS & TRAC ECU causes the actuator to go
into pressure increase mode.
• The Master Cut Solenoid is ON blocking the brake circuit to the
master cylinder.
• The Reservoir Cut Solenoid Valve is ON opening the master
cylinder to the pump.
• The pump is turned ON generating pressure and sending it
through the Pressure Holding Valve and on to the wheel cylinder.
Pressure Increase
Mode
Section 12
Pressure Holding When fluid pressure in the wheel cylinder circuit is optimized by an
Mode increase or decrease in pressure, the ABS & TRAC ECU controls the
system as follows:
• The Pressure Holding Valve is turned ON blocking pressure from
the pump.
• The Reservoir Cut Solenoid Valve is turned OFF, blocking
additional fluid from the master cylinder
• The pump continues to rotate.
Pressure Holding
Mode

Pressure Reduction When fluid pressure in the wheel cylinder needs to be reduced:
Mode
• Reservoir Cut Solenoid Valve is OFF and spring loaded in the
closed position blocking fluid from the Master Cylinder to the
Pump.
• Master Cut Solenoid is ON, blocking the master cylinder from the
wheel cylinder.
• Pressure Reduction Valve is turned ON, allowing fluid pressure to
flow to the reservoir and pump, and allowing the wheel to turn.
Pressure Reduction
Mode
Section 12
Supra TRAC The operation of the TRAC system on the Supra is similar to the
Camry and Avalon however there are distinct differences between the
two systems:
• Most notable is the separate TRAC actuator and ABS actuator in
1993.5 through 1995.
• Engine torque is controlled via a sub−throttle actuator which
controls the sub−throttle ahead of the valve in the throttle body.
• Beginning with 1996 production, brake actuation is no longer
utilized for Supra traction control.
Once activated, the TRAC System reduces engine torque and rear wheel
speed as necessary to bring the vehicle under control. The ABS ECU,
TRAC ECU and ECM all work together to provide traction control. ABS
speed sensors are monitored by the TRAC ECU which in turn controls a
sub−throttle plate and applies the rear brakes. The ECM also retards
engine timing while the ABS modulates pressure at the rear brakes.
Supra TRAC
Component Locations
The TRAC System shares
some ABS components to
control braking functions.

Operation Of The TRAC OFF switch is located on the instrument panel above the
Components center console. It allows the driver to activate or deactivate the TRAC
system when the switch is depressed. The system defaults to ON when
the ignition switch is cycled.
The TRAC OFF indicator light goes on when any one of the following
occur:
• the TRAC system is deactivated by the TRAC OFF switch.
• a TRAC related problem is detected with the engine.
• an ABS related problem is detected. The TRAC indicator light
indicates when:
• the system is operating.
• a malfunction occurs in the system (it remains illuminated to warn
the driver).
• the TRAC ECU is set to the diagnostic mode (the light blinks the
trouble code).
A change in January of 1996 included a SNOW feature which modifies

the engine torque control when selected. This feature ensures reduced
engine torque in anticipation of a lower coefficient of friction between
the tire and the road surface. The driver selects this feature by
pressing the SNOW button. Like the TRAC OFF switch, cycling the
ignition switch defaults the system to normal operation.
TRAC OFF Switch

and Indicator Lights
Section 12
Sub-Throttle The Sub−Throttle Actuator uses a step motor located between the main
Valve Motor throttle valve and air cleaner. It is fitted on the throttle body and
controls the position of the sub−throttle valve based on commands
made by the TRAC ECU thus controlling the engine output. By
controlling the sub−throttle plate, engine management controls engine
torque reducing wheel spin.
Sub-Throttle Valve
Motor
The TRAC ECU controls
the position of the
sub-throttle valve.
The sub−throttle valve motor consists of a permanent magnet, a coil, a

rotor shaft and pinion gear. It is a step motor that is rotated by a signal
from the ABS & TRAC ECU. The pinion gear rotates a cam gear, fitted
on the sub−throttle valve shaft end, controlling the sub−throttle valve
opening angle.
Sub-Throttle
Actuator Operation
The position of the sub-
throttle valve controls the
incoming volume of air to
control engine torque.

Sub-Throttle Position This sensor is fitted to the sub−throttle valve shaft. It converts the
Sensor sub−throttle valve opening angle to a voltage signal and sends this
signal to the TRAC ECU via the ECM (Engine ECU). The sensor is built
and operates in the same way as the main Throttle Position Sensor.
Sub-Throttle
Position Sensor
Converts the sub-throttle
valve opening angle to a
voltage signal and sends
this signal to the ECM
and TRAC ECU.
TRAC Pump The function of the TRAC Pump is to generate brake fluid pressure
necessary for applying the rear disc brakes when the TRAC system is
operating. It draws brake fluid from the master cylinder reservoir,
pressurizes and directs it to the TRAC brake actuator. It is a
motor−driven, three chamber radial pump.
TRAC Pump
Generates brake fluid
pressure necessary for
applying the rear disc
brakes when the TRAC
system is operating.
Section 12
TRAC Brake Actuator The TRAC Brake Actuator consists of two cut solenoid valves and
three spring loaded valves which regulate the brake fluid pressure in
the right and left rear wheels. The rear wheels are controlled
independently through the ABS actuator based on signals from the
ABS ECU.
The Master Cylinder Cut Solenoid Valve opens and closes the
hydraulic circuit from the master cylinder or TRAC pump to the ABS
actuator. When the TRAC system is operating, it supplies the brake
fluid pressure from the TRAC pump to the disc brake cylinders via the
ABS actuator. It also prevents the fluid from flowing out of the ABS
actuator pump to the master cylinder.
The Reservoir Cut Solenoid Valve is located between the return side
of the ABS 3−position solenoid and the master cylinder. It returns the
fluid from the disc brake cylinders back to the master cylinder reservoir.
The Pressure Regulator Valve controls the brake fluid pressure

generated by the TRAC pump.
The Relief Valve relieves the systems highest pressure should a

malfunction occur.
The Check Valve prevents fluid from flowing out of the disc brake
cylinder to the TRAC pump.
TRAC Brake Actuator

Consists of two cut solenoid
valves and three spring
loaded valves which regulate
the brake fluid pressure in
the right and left rear wheels.

TRAC System
Hydraulic Circuit
Section 12
TRAC Operation Dialing normal operation (TRAC not activated) all solenoid valves of
the TRAC brake actuator remain inactive when the brakes are applied.
As the brake pedal is depressed, brake fluid pressure generated by the
master cylinder is applied to the disc brake cylinders, via the master
cylinder cut solenoid valve, and the 3−position solenoid valves in the
ABS actuator. When the brake pedal is released, fluid pressure returns
from the disc brake cylinders to the master cylinder.
During vehicle acceleration (TRAC operative) when a rear wheel slips

the TRAC system controls the engine output and braking of the rear
wheels to help prevent wheel slippage.
The brake fluid pressure applied to the right and left rear wheels is
controlled separately according to 3 control modes:
• Pressure Increase.
• Pressure Holding.
• Pressure Reduction.
Pressure Increase When a rear wheel starts to slip, just as the accelerator pedal is being
Mode depressed:
• All the solenoid valves in the TRAC Brake Actuator are activated
by signals received from the ABS ECU.
• The 3−Position Solenoid Valves in the ABS actuator are engaged in
the pressure increase mode.
• The Master Cylinder Cut Solenoid Valve is activated (ports A" and
C" open), and brake fluid pressure generated by the TRAC pump is
applied to the disc brake cylinders via the Master Cylinder Cut
Solenoid Valve and the 3−Position Solenoid Valves in the ABS
actuator.
• The Reservoir Cut Solenoid Valve is also activated (open) allowing
fluid to flow back to the master cylinder reservoir.
• The TRAC pump discharge pressure is maintained constant by the
Pressure Regulator Valve.

Pressure Increase
Mode
Brake fluid pressure
generated by the
TRAC pump is applied to
the disc brake cylinders via
the master cylinder cut
solenoid valve and the
3-position solenoid valves
in the ABS actuator.
Section 12
Pressure Holding When the brake fluid pressure for the rear brake cylinders is increased
Mode or decreased as required, the system switches to the holding mode.
This mode change is performed by engaging the 3−position solenoid
valve in the ABS actuator to the holding mode. This results in blocking
the TRAC pump pressure from flowing to the disc brake cylinder
through port D.
Pressure Holding
Mode
This mode change is
performed by engaging the
3-position solenoid valve in
the ABS actuator to the
holding mode closing port D.

Pressure Reduction When decreasing pressure applied to the rear brake cylinders, the ABS
Mode ECU engages the 3−position solenoid valve in the ABS actuator to the
pressure reduction mode. Fluid pressure applied to the brake cylinder
returns to the master cylinder reservoir from the 3−position solenoid
valve and reservoir in the ABS actuator to the Reservoir Cut Solenoid
Valve, thus alleviating the brake fluid pressure.
Pressure Reduction
Mode
The ABS ECU engages the
3-position solenoid valve in
the ABS actuator in the
pressure reduction mode.
Section 12
Wheel Speed Control The TRAC ECU constantly receives signals from the 4 speed sensors
and calculates the speed of each wheel. At the same time, it estimates
the vehicle speed based on the speed of the 2 front wheels and sets a
target control speed.
When the accelerator pedal is depressed on a slippery road, the rear

wheels (driving wheels) begin to slip and the rear wheel speed exceeds
the target control speed. The TRAC ECU then sends a close signal to
the sub−throttle valve motor.
At the same time, ABS ECU sends a signal to the TRAC brake actuator
and causes it to supply brake fluid pressure to rear disc brake
cylinders, changing the rear disc brakes in the TRAC mode.
The 3−position solenoid valves of the ABS actuator are modulated to

control rear brake fluid pressure to prevent wheel slippage.
TRAC Wheel Speed

Control
When the rear wheels
begin to slip, the TRAC ECU
sends a sub-throttle valve
close signal. The ABS ECU
sends a signal to the
TRAC brake actuator to
supply brake fluid pressure
to the rear disc brake.

Initial Check After completing the Initial Check of the ABS system, the ABS ECU
Function cycles the solenoid valves of the TRAC actuator and operates the TRAC
pump.
When the shift lever is in park or neutral range with the main throttle
valve fully closed and the ignition key is turned from ACC to the ON
position, the TRAC ECU drives the sub−throttle valve motor to fully
close the sub−throttle valve.
This Initial Check occurs once per key cycle.
TRAC Actuator and

Pump Check
The ABS ECU checks the
TRAC actuator and the
TRAC pump. The TRAC
ECU drives the sub-throttle
valve motor to fully close the
sub-throttle valve.
Section 12
Self-Diagnosis If a malfunction occurs in any of the signal systems, the TRAC

indicator light in the Combination Meter will light and alert the driver
that a malfunction has occurred. The TRAC ECU will also store codes
for each of the malfunctions.
Diagnostic trouble codes are accessed when the following conditions are
met:
• Ignition switch is turned on.
• Tc and E1 terminals in the Data Link Connector 1 or 2 [Check
Connector or TDCL] are jumpered.
Data Link Connectors

Jumper connectors E1 and TC to
access diagnostic codes.
Diagnostic Trouble Diagnostic trouble code(s) are indicated in the same fashion as ABS
Codes codes. The light blinking pattern code for 12 and 31 are shown in the
example below. If two or more malfunctions are indicated at the same
time, the lowest numbered diagnostic trouble code will be displayed
first. There is a 2.5 second pause between codes and a longer 4 second
pause before the codes are repeated.
Diagnostic Trouble
Codes
Diagnostic trouble code(s)
are indicated in the same
fashion as ABS codes.

ABS and TRAC The diagnostic chart on the following page shows the ABS diagnostic
Related Diagnostic codes on the left and a general description of components and related
Codes circuits on the right. Following the diagnostic code, the indicator lights
identify whether the ABS or TRAC systems monitor the specific
component. Additionally, the TRAC OFF light will illuminate if the
fault causes the TRAC system to be turned OFF.
For example, code 11 (open circuit in the solenoid relay circuit) will
cause the ABS indicator light to turn ON. The ABS solenoid is not
monitored by the TRAC ECU so it will not illuminate however, it will
cause the TRAC system to be switched OFF and therefore the TRAC
OFF light will illuminate.
If any of the diagnostic codes 31 through 34 and 41 are detected by

both ABS and TRAC systems, the TRAC light will flash.
Section 12
ABS/TRAC Related
Diagnostic Code
Chart
This chart identifies
diagnostic codes common
to ABS and TRAC Systems.
ÁÁÁÁÁÁÁÁÁÁÁÁÁ
Indicator Lights
Code No at
Code No.
TRAC Diagnosis
No. ABS TRAC TRAC ECU*2
OFF
11*1 Open circuit in solenoid relay circuit.
12*1
13*1
14*1

−
−

Short circuit in solenoid relay circuit.
Open circuit in pump motor relay circuit.
− Short circuit in pump motor relay circuit.
15 Open circuit in TRAC solenoid relay circuit.
16 − Short circuit in TRAC solenoid relay circuit.
17 − Open circuit in TRAC motor relay circuit.
18 − 43 Short circuit in TRAC motor relay circuit.
21*1 − Open or short circuit in 3−position solenoid of front right wheel.
22*1 − Open or short circuit in 3−position solenoid of front left wheel.
23*1 − Open or short circuit in 3−position solenoid of rear right wheel.
24*1 − Open or short circuit in 3−position solenoid of rear left wheel.
Open or short circuit in master cylinder cut solenoid valve
25
circuit of TRAC brake actuator.
Open or short circuit in reservoir cut solenoid valve circuit of
27
TRAC brake actuator.
31*1 *3 31, 43 Front right wheel speed sensor signal malfunction.
32*1 *3 32, 43 Front left wheel speed sensor signal malfunction.
33*1 *3 33, 43 Rear right wheel speed sensor signal malfunction.
34*1
35*1
36*1

*3
−

34, 43
43 ÁÁÁÁÁÁÁÁÁÁÁÁÁ
Rear left wheel speed sensor signal malfunction.
Open circuit in front left and rear right speed sensors.
− Open circuit in front right and rear left speed sensors.
Low battery voltage (9.5 V or lower) or abnormally high
41*1 *3 41, 43
battery voltage (17 V or higher).
44*1 − − Lateral acceleration sensor signal malfunction.
51*1
55

−
−
Pump motor locked or open circuit.
Fluid level of brake master cylinder reservoir dropped causing
master cylinder reservoir level warning switch to go on.
58 − Open circuit in TRAC motor.
43 Open or short circuit in circuit which inputs TRAC system
61 −
operation to ABS ECU.
Malfunction in ABS ECU (Involving vehicle speed signal
62*4 −
input inside ABS ECU).
Always
Malfunction in ABS ECU.
ON*1
Diagnostic trouble code indicated

− Not applicable
*1 Both the code number and description of diagnosis are identical to those of the ABS ECU without
the TRAC system (2JZ−GE engine model).
*2 To find out which of the indicator lights the TRAC ECU uses to output the codes shown in the chart,
refer to the chart for the diagnostic items of TRAC ECU shown on page 194.
*3 The indicator light flashes only if the same diagnosis is also detected by the TRAC ECU.
*4 The ABS ECU deletes the stored code No.62 when it detects the malfunctions numbered from No.31
to No.36 (wheel speed sensor signal malfunction).

TRAC Related The diagnostic codes in the chart below are specifically TRAC related. The
Diagnostic Codes speed sensor codes are similar to ABS codes. If both indicator lights are
ON however, begin your diagnosis in the Repair Manual ABS section first.
In addition, codes 44 through 48 which identify the main throttle position
sensor and the sub−throttle position sensor, begin your diagnosis in the
engine control system to determine whether the ECM has the same
diagnostic codes stored first before pursuing diagnosis of the TRAC system.
TRAC Related
Diagnostic Code Chart
The diagnostic codes
in the chart are
specifically TRAC related.
Indicator Lights
Code Code No.
No at
TRAC Diagnosis
TRAC ECU*1
No. ABS TRAC
OFF
24 − − − Open or short circuit in step motor circuit of sub−throttle actuator
25 − − − Step motor does not move to a position decided by TRAC ECU.
26 − − − Leak at sub−throttle position sensor or stuck sub−throttle valve.
31
32
*2
*2

*2
*2
31
Front right wheel speed sensor signal malfunction.
Front left wheel speed sensor signal malfunction.
33
34
*2
*2

*2
*2
33
Rear right wheel speed sensor signal malfunction.
Rear left wheel speed sensor signal malfunction.
Low battery voltage (9.5 V or lower) or abnormally high
41 − − −
battery voltage (17 V or higher).
43
44

−

−
−
−
Malfunction in ABS ECU.
Engine speed signal (NE) is not input from the ECM*
[Engine ECU] during TRAC control.
Short circuit in 1DL signal circuit of the main throttle
45 − − −
position sensor.
Open or short circuit in VTA1 signal circuit of the main
46 − − − −
throttle position sensor.
47 − − −
Open or short circuit in IDL signal circuit of the sub−throttle
position sensor.
Open or short circuit in VTA2 signal circuit of the
48 − − −
sub−throttle position sensor.
Malfunction in engine control system causes malfunction
51 − − −
indicator lamp [CHECK ENGINE warning lamp] to go on.
53
61
−
−

−
−
−
− ÁÁÁÁÁÁÁÁÁÁÁÁÁ
Malfunction in communication circuit to ECM* [Engine ECU].
Malfunction in communication circuit to ABS ECU.
Always
ON
−
Diagnostic trouble code indicated

− Not applicate
−
Malfunction in TRAC ECU.
*1 To find out which of the indicator lights the ABS ECU uses to output the codes shown in the chart, refer to the chart
for the diagnosis of ABS ECU shown on page 193.
*2 The indicator light flashes only if the same diagnosis is also detected by the ABS ECU.
*3 ECM (Engine Control Module)
Section 12
Clearing Diagnostic Diagnostic trouble codes in the TRAC ECU can be cleared after repairs
Codes are completed with the following steps:
1. Jumper terminals Tc and E1 in the DLC1 or DLC2 [Check
Connector or TDCL].
2. Turn ignition switch ON.
3. Depressing the brake pedal 8 or more times within 3 seconds.
4. Check that the warning light shows the normal code.
5. Remove the jumper wire.
Clearing Diagnostic
Trouble Codes

Fail-Safe When a malfunction occurs while the TRAC system is inoperative, the
TRAC ECU immediately turns OFF the TRAC motor relay and TRAC
solenoid relay, and stops TRAC system operation.
When the TRAC system is operative, the TRAC ECU continues control,
stops the control, or fully opens the sub−throttle valve depending on the
type of malfunction.
After the TRAC system becomes inoperative, the engine and brake
system operates in the same way as models without the TRAC system.
Section 12
Bleeding Procedure In order to bleed air from the TRAC actuator a new TRAC
Sub−Harness SST (09990−00330) is required to operate the pump
motor. The harness is connected to the TRAC pump connector and the
other end is connected to the battery to power the pump motor.
1. Disconnect the connector from the TRAC pump.
2. Connect the harness to the pump connector.
3. Connect a vinyl tube to the bleeder plug of the TRAC actuator and
loosen the bleeder.
4. Start the engine.
5. Connect the harness leads to the battery terminals.
• Allow the pump to run for 60 seconds.
• Close the bleeder plug.
• Allow the pump to run for 30 additional seconds.
6. Check fluid level.
7. Reconnect the TRAC pump to the vehicle harness.
TRAC Bleeding
Harness
To bleed air from the TRAC
actuator a TRAC Sub-
Harness SST is required to
operate the pump motor.


TRAC and ABS Diagnostic System
In this Worksheet you will practice the use of the ABS and TRAC warning lights.

• Repair Manual.
• Jumper Wire SST.
Procedure:
1. Disconnect the sub-throttle actuator connector (S4) and the TRAC actuator connector (T3).
2. Start the engine and note the condition of the ABS warning and TRAC indicator lights in the following chart.
ABS Warning TRAC Indicator TRAC OFF Master Warning

Light Light Light Light
3. Next, output diagnostic codes by turning the ignition switch ON, pulling short pin from DLC1, and connecting
terminals Tc to E1 at DLC1 or DLC2.
4. Record the codes in the chart below and indicate which light outputs each code.
5. Refer to the Repair Manual and record the malfunction condition indicated by each code.
ABS Warning TRAC Indicator ABS TRAC

Malfunction Condition
Light Light Code No. Code No.
Section 12
6. Which TRAC conditions (codes) were output by the ABS Diagnostic system?
7. Why do you think a TRAC related problem would output an ABS code?
8. Erase codes by connecting terminals Tc to E1 at DLC1 or DLC2, turn ignition switch ON, and press the
brake pedal at least 8 times in 3 seconds.
9. Reinstall short pin to DLC1.


Traction Control System Operation
In this Worksheet you will verify the operation of the TRAC System.

• Vehicle Lift or Floor Jack.
• Jack Stands.
Preparation:
• Mount the vehicle on a lift and raise the wheels six inches from the floor.
• For safety considerations make sure no one is standing to the front or rear of the vehicle.
• Make sure that the lift does not interfere with the rotating wheels.
• Make sure that the TRAC OFF switch is in the enabled position.
Procedure:
1. Start engine and place the transmission in Drive Range.
2. From idle, quickly depress the throttle and hold momentarily.
a. What immediately happened to engine RPM?
b. What was the maximum engine RPM achieved?
3. From idle, depress the throttle and hold momentarily a second time. What immediately happened to the
drive wheel speed?
4. How were both of these TRAC functions accomplished?
5. During TRAC operation what happened to the TRAC Indicator?

Section 12


TRAC Control System Bleeding
In this Worksheet you will practice the use of the ABS and TRAC warning lights.

• Repair Manual.
• TRAC Sub-Harness SST (09990-00330).
Procedure:
1. Disconnect the TRAC pump connector and attach the TRAC sub-harness to the pump.
2. Connect a vinyl tube to the bleeder plug of the TRAC actuator, then loosen the bleeder plug.
3. Start the engine.
4. Connect the sub-harness leads directly to the battery terminals to operate the TRAC pump.
5. Close the bleeder screw after 60 seconds.
6. Allow the pump to run for 30 seconds after tightening the bleeder screw.
7. Listen for pump operation.

Section 12

Appendix A
GLOSSARY OF TERMS
Ampere - The unit for measuring the rate of electrical current flow in a circuit.
A
Anti-squeal Shims - A single or multiple metal plates located between the brake
pad and caliper to reduce brake squeal.
Arcing - A grinding process that machines drum brake lining to the proper
curvature for a given drum size
Asbestos - The generic name of a group of minerals used in brake friction materials
and made up of individual fibers. Poses a serious health hazard if inhaled or
ingested.
Atmospheric Pressure - The pressure on the earth’s surface caused by the weight of
air in the atmosphere. At sea level - 14.7 psi.
Automatic Adjusters - Brake adjusters that use shoe movement or parking brake
application, to continually reset the lining to drum clearance.
Backing Plate - A pressed steel plate attached to the vehicle suspension. The wheel
B cylinder and shoes are mounted to the backing plate. Braking torque is transferred
from the brake shoes through the backing plate to the suspension.
Brake Dust - The dust created when brake friction materials wear during brake
application.
Brake Fade - The partial or total loss of braking power occurring when excessive
heat is absorbed by brake components reducing friction.
Brake Lines - The network of steel tubing and rubber hoses used to transmit brake
hydraulic pressure from the master cylinder to the wheel cylinders.
Caliper - Mounted to the steering knuckle or suspension and houses the piston or
C pistons. Converts the action of hydraulic pressure on the piston to mechanical force
used to apply brake pads against the rotor.
Coefficient of Friction - A numerical value expressing the amount of friction
between two objects. Obtained by dividing force by the weight of an object.
Compensating Port - The opening between the fluid reservoir and pressure side of
the master cylinder piston.
Cup Seal - Circular rubber seals with a depressed center surrounded by a raised
sealing lip. Seals in one direction only allowing fluid to bypass it in the opposite
direction.
Disc Brake - Brake system which uses brake pads rubbing against the sides of a
D brake rotor to generate friction to stop a vehicle.
Drum - Rotating part of the drum brake assembly which turns with the wheel.
Brake shoes are forced to contact the drum creating friction necessary to stop the
vehicle.
Dual Servo Brake - A drum brake that has servo action in forward and reverse
directions.
Noise, Vibration, and Harshness - Course 472 207

Appendix A
Energy - The capacity or ability to do work.

E
Equalizer - A bracket or cable guide in parking brake linkage used to ensure both
brakes receive equal application force.
Friction - The resistance to motion between two surfaces in contact.

F
Friction Modifiers - Additives used to alter the friction coefficient of a brake lining
material.
Gas Fade - Brake fade caused by hot gases and dust particles that reduce friction in
G a brake system under hard prolonged braking.
Glazed Lining - An overheated brake lining with a smooth shiny appearance.
Hygroscopic - An affinity or attraction for water.
Inertia - The property of a body at rest to remain at rest, and a body in motion to
I remain in motion in a straight line unless acted upon by an outside force.
Intermediate Lever - A parking brake linkage component used to increase parking
brake application force.
Kinetic Energy - The energy of mass in motion.

K
Lateral Runout - Side to side movement of the friction surfaces of a brake rotor.
L
Leading-Trailing Brake - A non−servo brake with one shoe energized and one
de−energized. The brake assembly works as well in forward or reverse. (see self
energizing action)
Lining Fade - Brake fade caused a drop in the lining coefficient of friction as a result
of excessive heat under hard prolonged braking.
Lockheed Master Cylinder - A master cylinder design having a compensating port
and inlet port.
Master Cylinder - Converts mechanical pressure from the brake pedal into hydraulic
M pressure for the wheel cylinders.
Mechanical Fade - Brake fade caused by heat expansion of the brake drum away
from the brake shoes.
Parallelism - A measurement of the two rotor surfaces that are an equal distance
P apart at every point around the circumference.
Pad Wear Indicator - Attaches to the brake pad and projects beyond the metal
backing to contact the rotor when the lining has worn. The squealing sound warns
the driver of worn pads.
208 TOYOTA Technical Training

Glossary of Terms
Pedal Height - The distance from the melt sheet of the floor and the top of the brake
pedal with the pedal retracted. Adjusted with the push rod.
Pedal Freeplay - The travel of the brake pedal from the retracted position to the
point that resistance in the brake pedal is felt as the pushrod contacts the booster or
master cylinder.
Pedal Reserve Distance - The distance from the melt sheet of the floor and the top
of the brake pedal with the pedal depressed.
Portless Master Cylinder - A master cylinder design which does not use a
compensating port. A single passage is open from the reservoir to the cylinder
controlled by a mechanical valve.
Proportioning Valve - A valve in the brake hydraulic system that reduces pressure
to the rear brakes to achieve better brake balance.
Radial Runout - A change in dimension from the center of a round object to its outer
R edge (radius).
Residual Pressure - A constant pressure held in the brake hydraulic circuit when
the brakes are not applied.
Rotor Phase Matching - Repositioning the rotor on the spindle hub to obtain the
least amount of rotor−run−out.
Self Energizing Action - A characteristic of drum brakes in which the rotation of the
S drum increases the application force of a brake shoe by wedging it tighter against
the drum surface.
Servo Brake - A drum brake that uses the stopping power of one shoe to help
increase the application force of the other shoe.
Slip Ratio - The difference between the vehicle’s body speed and the speed of the
wheels measured as a percentage.
Tandem Booster - A vacuum power booster that uses two diaphragms to increase
T brake application force.
Tandem Master Cylinder - A master cylinder design having two pistons providing
pressure to separate hydraulic circuits.
Thickness Variation - Differences in parallelism measurements made on the
circumference of a rotor. If great enough will cause feedback through the brake
pedal.
Tire Slip - The difference between vehicle speed and the speed of the tire tread
moving along the pavement.
Torque - The turning or twisting force applied at the end of a rotation shaft.
Traction - The amount of grip between the tire tread and the road surface
Noise, Vibration, and Harshness - Course 472 209

Appendix A

Appendix B
A.B.S. Comparison Chart
Appendix C
HANDLING ASBESTOS
In 1986 the Occupational Safety and Health Administration (OSHA) established workplace
standards for repair facilities that perform brake and clutch repair to reduce the level of asbestos
in the workplace. The following is a summary of the workplace standard. You should obtain
specific compliance advice from your company’s attorney or OSHA specialist. Additionally there
may be state or local regulations that may be applicable. Encourage your employee to establish
an information and training program for all employees.
Controlling asbestos residue in the workplace is of importance to everyone. Using compressed air
to remove the brake dust from brake assemblies may endanger the health of everyone in the
workplace and should never be done.
When touching a hot exhaust manifold, one knows immediately that continued contact will
result in tissue damage and sustained levels of pain. The immediate response is to pull away
from the source of heat; not so with substances such as asbestos. Damage caused by asbestos
may have a latency period of 15 to 30 years before symptoms occur and can be diagnosed.
Asbestos does not melt, burn, breakdown, dilute or digest, it remains indestructible inside the
body. Controlling asbestos residue is the only rational course of action.
Special vacuum cleaner equipment recommended by OSHA, utilize High Efficiency Particulate
Air (HEPA) filters that are very efficient in removing asbestos fibers. Most asbestos fibers in
brake dust are smaller than four tenths of a micron in size. Therefore, a special vacuum and
filter system is required to prevent these fibers from getting airborne. A regular shop vacuum is
insufficient for containing these small fibers and should never be used for this purpose as it will
further broadcast the asbestos throughout the shop. Asbestos can spread 75 feet from the point of
origin if a shop vacuum is used.
Some of the systems recommended by OSHA encase the brake assembly and allow the technician
to blow the brake dust loose with a regulated internal air nozzle, while the system vacuum
cleaner draws the dust into its filter. Once the brake dust has been removed, the brake assembly
can be worked on. Also vacuum the dust from the brake drum using the OSHA recommended
vacuum, before servicing it.
There are other OSHA approved systems consisting of a low velocity solvent which moistens the
brake dust until it is stuck together and collected in a tray or basin. Using a brake cleaner
propellant or water to wash down the brake dust should not be done as it will also cause some of
the dust to become airborne. Later, when the cleaner or water evaporates, the dust again may
become airborne.
In all cases avoid breathing asbestos when performing clutch and brake services. Make every
effort to effectively collect the dust in these operations with OSHA approved methods. If you
wear a respirator, make sure that it is OSHA approved for working with asbestos and that is fits
properly around the corner of the face. Even if you use a respirator for your protection, you must
also use an approved collection system to protect others in the workplace.
Toyota Brake Systems - Course 552 213

Appendix C

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Section 1

Lesson Objectives 1. Describe the cycle of heat as it applies to automotive brakes.

Fundamental The most important safety feature of an automobile is its brake

The basic principle of brake operation is the conversion of energy.

Friction is the resistance to movement between two objects in contact

The amount of friction produced is proportional to the pressure between

2 LEXUS Technical Training

The coefficient of friction is a measurement of the friction between

However 100 pounds of rubber pulled across a concrete floor may

The coefficient of friction varies in the two examples above based on

4 LEXUS Technical Training

There are primarily two types of brake fading caused by heat;

A feature of hydraulic theory can be seen in the illustration below

Another important distinction to make is that liquids cannot be

6 LEXUS Technical Training

Fluid pressure is indicated in pounds per square inch (psi). It is

In the series of examples below we are examining working force and

By contrast piston C will have an output force of twice that of piston A

Working Force and

Hydraulic brakes deliver equal braking force to all wheels with a

Brake Fluid Brake fluid is specifically designed to be compatible with its

The Federal Motor Vehicle Safety Standard (FMVSS) states that by

One of the negative characteristics of polyglycol is that it is

Extra caution should be taken with containers of brake fluid because it

Silicone is purple in color. It is not hygroscopic and therefore has

8 LEXUS Technical Training

Toyota recommends the exclusive use of Polyglycol DOT 3 brake fluid

10 LEXUS Technical Training

The reservoir tank is made mainly of synthetic resin, while the

Conventional On front−engine rear−wheel−drive vehicles, one of the chambers

Diagonal Piping for

12 LEXUS Technical Training

The Secondary Piston is pushed to the right by the force of Secondary

Piston. Hydraulic pressure in the Primary Chamber moves the

14 LEXUS Technical Training

When the Primary Piston is pushed farther to the left, hydraulic

16 LEXUS Technical Training

Role of the Second

A tandem master cylinder having a single reservoir tank has a separator

Brake Tubing Brake hydraulic components are connected by a network of seamless

Double Flare Tubing

The master cylinder on the 1997 Camry and Avalon incorporates

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Brake Fluid Level

A typical brake warning lamp electrical circuit is shown below. It also

Brake Warning Light

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Lesson Objectives 1. Identify the components of the drum brake system.

Wheel Cylinder The wheel cylinder consists of a number of components as illustrated

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The brake drum must be:

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When the brake pedal is depressed while the vehicle is moving

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Parking Brake The second type of automatic clearance adjustment operates by

The adjusting lever is arranged in such a way as to engage with one

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Lining that is eccentrically ground, that is having clearance at the heel