CHECK AND CHANGE ABS AND WHEEL SPEED SENSORS

Here you will find useful information and important tips relating to the ABS sensor and wheel speed
sensor in vehicles.

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Wheel speed sensors record the rotation of the wheels. This information is used by numerous safety and assistance
systems as the most important control parameter. Among other things, this page explains the differences between
active and passive wheel speed sensors as well as their functional principle. Workshop professionals will also find
highly detailed step-by-step instructions along with a wealth of practical tips for troubleshooting and replacing wheel
speed sensors.

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Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide
professional support to vehicle workshops in their day-to-day work. The information provided on this
website is intended for use by suitably qualified personnel only.

TABLE OF CONTENTS

1. Basic principles 2. Comparison 3. Symptoms

ABS sensors Inductive speed sensor and active ABS sensor faulty
wheel speed sensors

4. Causes of failure 5. Troubleshooting 6. Repair procedure

Causes of a faulty ABS sensor Checking the ABS sensor Changing the ABS sensor
:
 7. Practical tips
Troubleshooting tree for wheel
speed sensors

1 ABS SENSORS: BASIC PRINCIPLES

Importance of ABS sensors
The increasing complexity of the traffic situation on our roads is placing high demands on car drivers. Driver assistance
systems relieve the burden on the driver and optimise road safety. As a result, state-of-the-art driving assistance systems
are now included as standard on virtually all new European vehicles. This also means that workshops are being faced with
new challenges.

Nowadays, the vehicle electronics play a key role in all comfort and safety equipment. Optimal interaction between
complex electronic systems ensures that the vehicle operates without problems, and this, in turn, increases road safety.
Intelligent communication of data between electronic vehicle systems is supported by sensors. When it comes to driving
safety, speed sensors play a particularly important role, and this is reflected by their varied use in a number of different
vehicle systems.

They are used by the control units in driving assistance systems such as ABS, TCS, ESP, or ACC in order to detect the
wheel speed.

The wheel speed information is also provided to other systems (engine, transmission, navigation, and chassis control
systems) via data lines by the ABS control unit.

As a result of their varied use, speed sensors contribute directly to the driving dynamics, driving safety, driving comfort,
lower fuel consumption, and lower emissions. Wheel speed sensors are often also called ABS sensors as they were used
in vehicles for the first time when ABS was introduced.

Wheel speed sensors can be designed as active or passive sensors, depending on how they operate. A clear and precise
way of distinguishing or categorising them has not been defined.

The following strategy has therefore proven useful in day-to-day workshop activities:
If a sensor is only "activated" when a supply voltage is applied and then generates an output signal, this is an "active"
sensor.
If a sensor operates without additional supply voltage applied, this is a "passive" sensor.

2 INDUCTIVE SPEED SENSOR AND ACTIVE WHEEL SPEED SENSORS: COMPARISON

Inductive speed sensor, passive sensors
Operating principle
The wheel speed sensors are positioned directly above the
impulse wheel, which is connected to the wheel hub or
drive shaft. The pole pin, surrounded by a winding,
connects to a permanent magnet whose magnetic effect
extends to the pole wheel. The rotation of the impulse
wheel and the resulting switching from tooth to tooth space
brings about a change in the magnetic flow caused by the
pole pin and winding. This changing magnetic field induces
a quantifiable, or measurable alternating voltage in the
winding (Figure 1).
 
The frequency and amplitudes of this alternating voltage
are in relation to the wheel speed. Inductive passive
sensors do not require a separate power supply from the
(/techworld/assets/images/10028405a.jpg)
control unit. Since the signal range for signal detection is
defined by the control unit, the amplitude level must be

:
within a specific voltage range. Gap (A) between sensor Figure 2
and impulse wheel is provided by way of the axle design
(Figure 2).

Active wheel speed sensors

Operating principle
The active sensor is a proximity sensor with integrated
electronics that is supplied with a defined voltage from the
ABS control unit. A multipole ring can be used as an
impulse wheel while at the same time being integrated in a
sealing ring of a wheel bearing. Inserted in this sealing ring
are magnets with alternating pole directions (Figure 3). The
magneto-resistive resistors integrated in the electronic
circuit of the sensor detect an alternating magnetic field
when the multipole ring rotates. This sinusoidal signal is
converted by the electronics in the sensor into a digital
 
signal (Figure 4). It is then transferred to the control unit as
a current signal using the pulse-width modulation method.

The sensor is connected to the control unit via a two-pole

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electric connecting cable. The sensor signal is also
transmitted at the same time over a power supply line. The
other line is used as a sensor ground. In addition to
magneto-resistive sensor elements, nowadays Hall sensor

elements are also fitted that permit a larger air gap and Figure 3
respond to the smallest of changes in the magnetic field. If
a steel impulse wheel is installed in a vehicle in place of a
multipole ring, a magnet is also affixed to the sensor
element. When the impulse wheel turns, the constant
magnetic field in the sensor changes. The signal
processing and IC are identical to the magneto-resistive
sensor.

ADVANTAGES OF ACTIVE SENSORS

Wheel speed detection from standstill. This
facilitates speed measurements down to 0.1 km/h,
which is relevant to traction control systems (TCS)
as soon as the vehicle accelerates from a stop.
The Hall sensors detect forward and backward
movements.
The sensor is smaller and lighter in design.
The lack of impulse wheels simplifies the power
transfer linkage.
Sensitivity to electromagnetic interference is less
pronounced.
Changes in the air gap between the sensor and
(/techworld/assets/images/D352.jpg) magnetic ring have no direct impact on the signal.
Virtual insensitivity to vibrations and fluctuations in
temperature.
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Active sensors

3 ABS SENSOR FAULTY: SYMPTOMS

The following system characteristics can be recognised when wheel speed sensors fail:
The ABS warning light illuminates
Fault code is stored
The wheels lock during braking
Incorrect or pseudo regulation interventions
:
 Failure of further systems

4 CAUSES OF A FAULTY ABS WHEEL SPEED SENSOR: CAUSES OF FAILURE

Causes of failure:
Breaks in wiring
Internal short circuits
External damage
Heavy contamination
Increased wheel bearing clearance
Mechanical damage to the encoder wheel

5 CHECKING ABS SENSORS: TROUBLESHOOTING

Generally speaking, a malfunction of the ABS/TCS/ESP
braking system occurs before the wheel speed sensors are
tested.

After the warning light illuminates, the following options are

available for troubleshooting and diagnostics:
Read out the fault memory
Check the supply voltages and signals using a
multimeter and oscilloscope
Visually inspect the wiring and mechanical assemblies  
Diagnostic unit
Read out the fault memory
Evaluate parameters
Compare and evaluate signals from individual wheel
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speed sensors

Multimeter
Voltmeter (check the voltage supply)
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Ohmmeter
Check the internal resistance (the internal resistance
cannot be measured on active sensors)

Oscilloscope
Signal depiction (evaluation of signal curve)

NOTE
Troubleshooting wheel speed sensors can be difficult when it comes to differentiating between active and passive
sensors as these sensors cannot always be easily distinguished by appearance. Here, specific manufacturers'
specifications and the specifications of the respective vehicle manufacturer must be consulted. Due to their
advantageous technical characteristics such as accuracy and low structural size, vehicle manufacturers have mainly
installed active wheel sensors since 1998.

Requirements for reliable diagnostics

Proper documentation in the form of technical data
A suitable diagnostic unit (/techworld/uk/Diagnostics-
tools-and-data-6396/), multimeter, or oscilloscope
Sufficient technical know-how of the technician and
targeted employee training

When performing diagnostics on complex systems, the best

technology alone cannot help repair the vehicle. Only well-
trained personnel can prevent the random replacement of
:
system components, avoid malfunctions in workshop
processes, and strengthen the trusting relationship with the
customer.

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Observe the following general repair notes:
Repair work to brake systems may only be carried out by qualified specialists.

When carrying out any repairs on the brake system, you must follow the maintenance and safety instructions of the vehicle
manufacturer and the product-specific assembly instructions
Observe the respective tightening torques without fail.

6 CHANGING THE ABS SENSOR: REPAIR PROCEDURE

Fault diagnostics in daily workshop service
Using the following example of "Rear left speed sensor
faulty", we will explain the diagnostics work required for an
active wheel speed sensor. Your customer has reported a
functional problem with the ABS system. The ABS warning
light comes on while driving.

In this example, the repair procedure is shown based on a

BMW E46. The schematic illustrations, figures, and
descriptions are intended solely as explanations of the
document text, and cannot be used as the basis for
carrying out installation and repair work.
(/techworld/assets/images/10006060a.jpg)
Preparing the diagnostics:


In order to allow correct allocation of the vehicle, it is
important that the vehicle documents (registration
documents) are included with the job sheet. Customer complaint: The ABS warning light comes on.
Check the battery voltage. Insufficient voltage supply
may cause system failure or result in incorrect
measurements or voltage drops.
Check the system-related fuses. A quick look in the fuse
box might eliminate the first cause of the fault.

Troubleshooting

1 Check the service brake

:
 Run the vehicle on the brake test stand. We
recommend the use of a roller test stand. Potential
defects on the brake mechanics can be identified in this
way, even under gentle braking. Any imbalance in the
brake disc will result in differing wheel speeds when
braking, which will change the wheel speed information
sent to the control unit.
Determine the braking effect.

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
Using the diagnostic unit

2 Visual inspection

Move the vehicle onto the lifting platform.

Check that the wheels are the correct size and have the correct tires.
Check the tire pressure and tread depth.
Check the wheel bearing play and axle suspension.
Check the brake fluid level.
Check the wear on the brake pads.
Check the connectors and wiring of the sensors to ensure they are correctly positioned and secured, and check for
any major damage.

3 Using the diagnostic unit

Connect the diagnostic unit (/techworld/uk/Diagnostics-tools-and-data-6396/) to the 16-pin OBD connector.

Depending on the vehicle manufacturer and date of registration, a different diagnostic socket and an additional
adapter may be required.

Select program.
Select vehicle.
Select fuel type.
Select model.

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
:
 Select required function.
Select system.

Depending on the diagnostic unit being used,

additional information on the system variants installed
in the vehicle may be displayed here. If it is not
possible to clearly allocate the system, it is possible –
without damaging the respective control unit – to set
up diagnostics with the specified control units one after
the other. Only the control unit which was correctly
identified by the diagnostic unit will establish
communication.

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Start fault diagnostics.

A correct connection and sufficient battery voltage are

required in order to establish reliable communication
with the control unit. A battery voltage of 12.69 V as
evaluated by the control unit can clearly be seen here.
Insufficient supply voltage to the control unit could be
an indication of a wiring defect or a defect in the
vehicle battery.

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4 Read out the fault memory

In this example, the fault code "Rear left speed sensor"

was stored. In addition to the numerical code, some
diagnostic units also provide a definition of the fault
code. This makes it easier to decide on the subsequent
diagnostics work.

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5 Evaluate the details

:
 Here, initial information about a possible reason for the
fault is stored. The stated fault code does not
necessarily refer to an actual fault in the component.
Before you start replacing individual components, read
this information carefully in order to determine the next
diagnostics steps.

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6 Read out the parameters/measured value block

The actual values are displayed here for further

analysis. In this case, the faulty signal curve relative to
the rear right sensor can clearly be seen. The visible
irregularities in the signal curve allow you to narrow
down the defect.

Note:
If the signal curve does not show any irregularities, first
delete the fault. Once this is done, carry out a test drive
with the diagnostic unit connected. In the process we
recommend evaluating the parameters at the same time
so that the cause can be narrowed down if the fault
occurs again. (/techworld/assets/images/10006067a.jpg)

7 Check the voltage supply

Here, we recommend taking the measurement directly

at the sensor connector in order to check the entire line
between the control unit and sensor.

Practical tip:
Due to the connector design, it is very difficult to take a

 
reliable measurement directly at the plug contacts. It may
be helpful to produce an adapter from an old identical
sensor.

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8 Check sensor installation point and impulse ring

:
 Remove the sensor
Check the sensor and impulse ring for damage.

In our example, a defect has been found on the sensor

cable. A cable break in the supply cable, caused by
mechanical damage, resulted in a loose connection on

 
the connector housing.

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9 Replace the wheel speed sensor

Clean the sensor installation point. Clean the contact

surface using a wire brush or, if required, with
sandpaper.
Replace the wheel speed sensor. Please make sure
that the sensor cable is properly routed and fixed in
place.
Observe the tightening torque. Comply with any torque
values specified by the vehicle manufacturer.

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10 Read out the fault memory

Delete the stored fault.

As a result of the diagnostics work on the vehicle,
additional faults may be detected by the control unit.
These must be deleted before the test drive.

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11 Carry out a test drive

:
 In order to check the wheel speed signal after having
replaced the sensor, subsequently carry out a test drive
with the diagnostic unit connected and evaluate the
parameters at the same time.

(/techworld/assets/images/10006074a.jpg)

12 Final test

After carrying out the test drive, read out the fault
memory again. As the systems in the vehicle are
networked, a defect in the ABS system will also be
stored in other control units. Here, we recommend
querying all control units and deleting the stored faults.

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NOTE
Please note the vehicle manufacturer's information when carrying out any testing and diagnostic work. Depending on
the manufacturer, additional vehicle-specific testing methods may have to be taken into consideration. Ensuring optimal
interaction between humans and technology has become more important than ever. Only workshop specialists with the
latest expertise and who know how to optimally use their technical equipment will be ready to tackle the challenges of
the future.

7 TROUBLESHOOTING TREE FOR WHEEL SPEED SENSORS: PRACTICAL TIPS

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 open

WHEEL SPEED SENSOR

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