ECU JEEP RENIX AND SYSTEMS XJ 1.988 – 1.

990 ( THEORETICAL-PRACTICAL AID )

By: Ramón Miranda ( ramon.miranda811@hotmail.com )

Greetings Colleagues. At the end of the 1980s, commercial car designers ( Equipped with explosion engines ), in order to
improve fuel dosing and the obligation to reduce pollutants, started alternative systems to the carburettor and
electronically controlled ( ECU = Engine Control Utit ), which add control algorithms that use the information from
various sensors located in the engine to handle their different phases of operation, simultaneously satisfying the
requests of the driver and protection of the environment. The Jeep with the Renix system was among the pioneers of
the multi-point injection system ( An injector per piston ), which was upgraded in Jeep models after the year 1991 and
currently used by most commercial cars. In this article I indicate technical information and the main differences with
respect to the later models, which can affect the correct functioning, as well as contribution of ideas to correct possible
damages. It should be noted that this information is freely usable, based on knowledge coupled with experiences of the
author and sustained on the theoretical basis of Jeep manuals.

CONTENT
1. Jeep Renix ECU system.
1.1 Basic engine diagram, ECU and connectors.
1.2 Switched injectors with open collector P-N-P outputs.
1.3 Wiring length.
1.4 Setting the TPS sensor.
 1.5 Absence of the oxygen sensor.
1.6 Components of the ECU prone to damage.
1.7 Description of ECU inputs.
1.8 Electronic diagram of major components of the ECU.
1.9 Description of ECU outputs.
2. Radiator blower motor.
3. Alternator with built-in regulator.
4. Relocate high consumption feeds.
5. Other images and annexes to facilitate diagnostics ( Air conditioning system, 1987 - 1990 Cherokee – Comanche
- Wagoneer ( 4.0L ) engine shematics, Engine shematics II, ECU connector pin identification II, Starter and
alternator diagrams, Headlamp switchs ).
6. To end.

ECU RENIX JEEP SYSTEM 1988 – 1999

BASIC ENGINE DIAGRAM
 SWITCHED INJECTOR WITH OPEN COLLECTOR P-N-P OUTPUTS
The main difference with respect to any multipoint injection system is the injector activator circuit, which in this case the
ECU switches them to the positive potential of the feed ( PNP open-collector outputs ) and the common electrodes of
the injector group are Connected to ground ( Chassis ). At present it is the reverse, ie the common electrodes of the
injector group are supplied with + 12VDC from the contacts of a relay generally identified as ASD ( Auto Shut Down ) and
in the ECU each injector is grounded from NPN outputs Which connect to the negative power potential.

In this ECU Renix design, the PNP ( Open Collector ) output circuit for the injector group is supplied with + 12VDC at
terminals C11 and D10, from the fuel pump relay contacts ( FUEL PUPM RELAY ) . This makes the ECU vulnerable to short
circuits in wiring, injectors or connectors, earthing, etc. To avoid this, it is suggested to adapt an automotive fuse for
lower current of 5 Amps, installed in the cable that feeds the circuit of injectors ( Terminals C11 and D10 of the ECU ).
The following pictures show damages for this reason (3rd and 4th image) and defects due to bad repairs ( 1st and 2nd
image ).

The main cause of these problems is the deterioration or damage of motor wiring ( In many countries they use a glue for
cables passing over the motor ), a possible solution to minimize these damages in the ECU is to separate and relocate
the motor wiring , So that it is as far apart as possible from the block. In Jeep models after 1991, motor cabling is
protected against heat by means of a thermal insulation board.
 WIRING LENGTH
Another of the main problems in these Jeeps models is the loss ( Attenuation ) of the voltage because of the length in
the wiring, in the following image I show ( Red color line ) the route of most power cables + 12VDC, since The positive
distribution center ( Start relay screw ) to the cab and fusible.

The yellow line represents the ignition wire that feeds the coil module, among others. ( No devices are indicated:
ignition switch, fuse holder, cab wiring connector, etc. ). To solve or minimize these problems, in different forums Jeeps
suggest adding grounding to the chassis at multiple points of negative wiring. I particularly suggest adding grounding
inside the ECU, between the thick tracks of the printed circuit ( Electrically connected to terminals B11 and B12 of the
connector ) and one of the screws that secure the card. In the following pictures I gave examples of repaired ECUs and
the proper adaptation of the grounding to the chassis.
To improve the spark quality and minimize cold start problems, I suggest adapting a relay that directly feeds the
ignition coil module with + 12VDC directly from the positive battery terminal ( with its fuse ). If only the cold start is to
be improved, the appropriate relay coil must be operated from the starter relay coil or from the cable that powers the
starter motor solenoid.

If it is necessary to improve the spark quality at all times, it is necessary to cut the yellow ( Thick ) wire that feeds the
coil module of the ignition and with it to drive the coil of the relay adapted, so that the contacts of the same relay feed
locally Ignition coil module. This adaptation also allows the addition of anti-theft security when interrupting one of the
two coil cables of the adapted relay ( Can be chassis or ground ).

Motor diagram with adapted relay ( Spark enhanced ):

 TPS SENSOR ADJUSTMENTS
Another difference and almost exclusive to the Renix control of these Jeeps is the angle adjustment on the throttle
position sensor ( THROSTLE POSITION SENSOR = TPS ). In later models the ECU algorithm corrects the TPS calibration
(caused by wear on the acceleration body, cams and sensor actuators), but in these Renix controls the readjustment is
necessary. Jeep service manuals suggest measuring the TPS signal ( Cable arriving at the ECU's C7 terminal ) with a digital
voltmeter and adjusting the TPS angle until it indicates + 0.85VDC at minimum throttle opening.

ABSENCE OF THE OXYGEN SENSOR

In many countries it is difficult to acquire the oxygen sensor because they are resistive type and whose operation is
different from other Jeep models after 1991 and any injection system. It is usually disconnected by producing constant
faults, damage to the threads or fracture when disassembling. The absence of the sensor prevents the ECU from
correcting the correct air-fuel mixture and therefore generates a slight lean mixture. The relay that heats this sensor also
bypasses the gas pump's series resistance ( BALLAST RESISTOR ), in the presence of the maximum throttle opening mode
( Only if the oxygen sensor is in good condition ).

To normalize the appropriate mixture or to taste, "temporarily" I suggest replacing the sensor, by a potentiometer of
25 KΩ ( Anything between 20 KΩ and 50 KΩ ). In the next photograph on the right I show the adaptation.
In order to replace the relay drive which increases the pressure in the fuel pump, a simple electronic circuit ( Inside or
outside the ECU ) must be added, which will practically not be necessary with the performance recovered by the motor
after the adjustment and adjustment Of the potentiometer that replaces the oxygen signal.

ECU COMPONENTS PROPOSED TO BE DAMAGED

The main advantage of the Renix Jeep 88-90 ECU is that they are repairable because they come installed inside the
driver's cab, so they are not designed for extreme dirt, grease, humidity, etc. As with any ECU, power components are
often damaged by short circuits or the like.

* The group of transistors for injectors was originally Motorola trademark, reference 5849-
11, but any T0-220 encapsulation PNP type transistors with gains ( hfe ) greater than 200,
examples: BD912, BD744, 2N6490, 2N6491 , TIP2955, etc. Although I have known about
using transistors Darlington TIP115, TIP116 and TIP117, they are of higher gains ( Between
500 and 1000 ) and with good results.
* For auxiliary drives ( Relays, EGR solenoid, SHIFT lamp, etc. ) originally used are transistors with RCA 715 references,
but any NP-type TO-220 encapsulation transistors with gains greater than 200 can be used, examples: TIP110, TIP111,
TIP112, ECG261, NTE261, etc., although the transistors Q16 and Q19 corresponding to the outputs A6 and A11 are not
used and are available for exchange.

In times when I replaced the Q21 ( FUEL PUMP RELAY ) transistor that drives the fuel pump relay, depending on the gain,
I had to lower the value of resistor R47 ( By 4.7 KΩ ).

* As a + 5VDC memory controller, originally used the LM2931A, but can be replaced by any + 5VDC controller (
Examples: LM7805, ECG960 or similar ), TO-220 encapsulation, but I particularly suggest the LM340 T5, LM2940- 5 or
NTE1951 which, like the original, are + 5VDC controllers protected against short circuits, overheating, overtemperatures
and overvoltages.

The drivers for the IAC stepper motor are S452-2 integrated circuits and are available in the market.

TTL integrated circuits ( PC74HC00P, PC74HC244P, PC74HC373P, etc. ) and many other components are easily available
from electronics retailers, such as diodes, resistors, capacitors, crystal resonators.

INPUTS DESCRIPTIONS
The Jeep Renix 88-90 ECU has four + 12VDC power inputs:

1. B7 = BATTERY or memory ( Voltage permanently switched off for ECU reset ).

2. B8 = IGNITION ( Voltage when switching the ignition switch, feeds the ECU while the engine is off, or the FUEL
PUMP RELAY and + B LATCH RELAY relays are de-energized ).
3. B10 = LATCHED ( Main ECU power, + 12VDC from the relay contacts + B LATCH RELAY ).
 4. C11 and D10 = INJECTOR FEED ( Supply the power circuit of the injector group, + 12VDC from the FUEL PUMP
RELAY relay contacts ).
5. B11 and B12 = Main negative potential of the ECU ( Chassis, ground, GROUD, GND, etc. ).

Three analog inputs for resistive sensors or thermistors ( Internally with a voltage or voltage divider, where the sensor
is part of the same divider, ie with the sensor disconnected there is +5 VDC present and the voltage drops as the sensor
goes down Its resistive or ohmic value, if it approaches zero ohms, the voltage at the sensor will approach zero volts ):

1. C8 = Intake air temperature sensor, or AIR TEMPERATURE SENSOR located on manifold or intake manifold.
2. D9 = Oxygen Sensor ( OXIGEN SENSOR = Located in the manifold of the exhaust, internally includes a heating
resistor ).
3. C10 = Coolant temperature sensor ( COOLANT TEMPERATURE SENSOR = Located on the engine block, driver side
).

The following graphs taken from different Jeep workshop manuals indicate the circuit plus the behavior of intake and
coolant temperature sensors:

Two analog inputs for sensors with voltage output:

1. C7 = TPS sensor ( THROSTLE POSITION SENSOR = Approximate to + 5VDC in maximum throttle or throttle
opening position Approximate to 0VDC in minimum opening position ). Mainly determines the opening or pulse
time in fuel injectors, the longer pulse time, the greater amount of fuel injected.
2. C8 = MAP sensor ( MANIFOLD ABSOLUTE PRESSURE = Approximate to + 5VDC with minimum motor torque and
tends to approach 0VDC as motor torque increases ). It mainly displaces the angle at which the spark is
generated, to increase the motor torque.

Four + 12VDC digital inputs ( Not shown in the above diagram ):

 1. C2 = Air conditioning thermostat ( A / A REQUEST ). The + 12VDC enters when closing the thermostat contacts or
when the air conditioning temperature requires the compressor to be operated. Similarly, the ECU commands a
slight increase in engine RPM ( Increases IAC valve steps ).
2. C3 = START SIGNAL. The + 12VDC enters when the motor start relay is activated.
3. C4 = PARK and NEUTRAL position of the automatic transmission ( SPEED CONTROL ). The ground signal comes
from the shift lever selector, in order to increase IAC steps to increase engine RPM when selecting speeds. In
synchronous transmission cars and even in some automatic models, I have observed this point not wired ( ? ).
4. D2 = Low air-conditioning refrigerant gas pressure ( A / A SELECT ). The + 12VDC will enter whenever the low
pressure valve is closed. If it detects low refrigerant gas pressure, it opens the contact and therefore will not
operate the air conditioning compressor.

Two inputs for exclusive digital sensors (In later models sensors with digital or TTL outputs are used, the sensors in
these Renix controls pick up the signal and in the ECU they are interpreted digitally):

1. C1 and D1 = Crankshaft speed or position sensor ( ENGINE SPEED SENSOR ). It constantly detects impulses while
the crankshaft rotates and whenever it detects absence of impulses or lack of recess in the actuator fixed to the
rack, it indicates to the ECU that the piston comes in ascent and that in approximately followed 12 pulses, it
must generate the spark of the power on. At other angles of the crankshaft and combined with the sync sensor,
generates the activation sequence of the injectors. In the following graphs the original signals with respect to
the digital interpretation of the ECU are appreciated.

2. C5 = Camshaft position or synchronism sensor ( SYNC SENSOR ). The sensor actuator is a 1/2 moon which
generates a signal with two logic states ( H and L, ò 1 and 0 ) in the sensor, where each logic state equals 180
degrees or 1/2 turn in the Cams For each cycle of the engine the crankshaft rotates twice, while the camshaft
only makes one turn. The signals in the speed sensor are repeated for the pistons 1 and 6 - 5 and 2 - 3 and 4, the
synchronism sensor discriminates the first and second turns of the crankshaft to give result to the sequence 1-5-
3-6- 2-4 fuel injection.
An exclusive entry for detonation detector probe:

1. D8 and D16 = Knock Sensor ( Not indicated in the above diagram ) It consists of a device that oscillates at a
stable frequency which is affected in the presence of strong impacts transmitted to the motor block.

DESCRIPTION OF OUTPUTS
The Jeep Renix 88-90 ECU features six P-N-P digital pickup outputs for the fuel injector group ( They switch to positive
potential + 12VDC of the car and its operation is described at the beginning of the file ):

1. B1 = Injector # 1 ( The one closest to the front of the car ).

2. A3 = Injector # 2.
3. A1 = Injector # 3.
4. A4 = Injector # 4.
5. B2 = Injector # 5.
6. A2 = Injector # 6.

Eight open collector N-P-N digital outputs ( Switch to the negative potential of the car = Ground ) for the operation of
relays, solenoids, lamps or any other device.
 1. A5 = Fuel pump relay ( FUEL PUMP RELAY ). It activates the relay when it passes the ignition switch, upon
receiving the start signal ( 3 ) and remains held while the engine is running. If no signal is received from the
speed sensor ( Motor stopped ), it will de-energize after approximately 2 seconds. You can re-energize the relay
if it detects engine rotation ( Start the engine ).
2. A6 = Available (I do not know how to operate).
3. A7 = Oxygen sensor heater relay ( OXIGEN SENSOR RELAY ). Operate the relay in the presence of maximum
throttle or throttle opening mode. Contact N.C. ( Normally closed ) relay energize the oxygen sensor heater
element and NO. ( Normally open ) from the same relay, bypass the BALLAST RESISTOR series of the fuel pump
for the purpose of increasing the power and pressure in that fuel pump. This behavior is only possible if the
oxygen sensor is in normal operation.
4. A8 = SHIFT indicator light on the dashboard ( UPS SHIFT IND ). Only in cars with manual transmission (
Synchronous Boxes ), will energize the pilot light directly, to instruct the driver to perform ( Raise ) the speed
change.
5. A9 = LATCH or main ECU relay ( + B LATCH RELAY ). The operation is similar to that of output A5 ( FUEL PUMP
RELAY ), with the only difference that deenergizes a second later, ie first the pump relay is de-energized and the
LATCH relay is almost immediately de-energized.
6. A10 = EGR solenoid solenoid ( EGR EVAPORATOR CANISTER SOLENOID ). Energize the solenoid valve from the
ignition switch and while the engine is running in idle modes, maximum acceleration mode and constant speed.
Deenergize the solenoid valve only in acceleration mode.
7. A11 = Available ( Accel Mode or EGR ). Behavior similar to that of output A10. In 4x4 competitions, a rectifier
diode ( 1N4001 or similar ) can be added to output A7 ( OXIGEN RELAY ), this in order to increase the pressure in
the fuel pump whenever the engine is accelerated ( Do not drive in maximum opening mode Throttle valve or
throttle ). I warn that the permanent use of this adaptation can prematurely damage the fuel pump.

8. A12 = Air-conditioning compressor clutch relay ( A / A RELAY ).

Three sensor outputs:

1. C14 = Power supply + 5VDC for MAP sensor ( MAP SUPLY ).

2. C15 = power supply for the TPS sensor ( TPS SUPLY ). + 5.1VDC in ECU model or type S101 140 102B and +
4.8VDC in ECU model S101 140 102A.
 3. C16 = Power supply for camshaft sensor ( SYNC SUPLY ). Approximately + 7.5VDC with sensor connected and +
8.2VDC with sensor disconnected.
4. D3 = Negative potential for analog sensors ( SENSOR GROUND ).

Four exclusive output terminals to move the IAC stepper motor.

1. B3 and B4 = Winding AB of the motor. With the motor IAC disconnected they must measure B3 = + 0.3VDC and
B4 = + 10VDC.
2. B5 and B6 = CD winding of the motor. With the motor IAC disconnected they must measure B5 = + 0.3VDC and
B6 = + 10VDC.

An exclusive output for switching ignition coil module:

1. D13 = IGN INTERFERENCE OUT, made the "B" terminal of connector No 2 ( ECU ESQUARE WAVE COIL INTERFACE
) on the coil module. Normally it measures + 5VDC and commutes to ground whenever the coil generates
ignition sparks. The following graph shows the connectors of the coil module:

2. Terminal "C" of connector No 1 ( TACH SIGNAL ), is an output of the coil module, used for the RPM indicator of
the motor, located in the cluster or panel of the board, usually measures + 12VDC and also commutes to ground
Each time it generates ignition sparks.

One serial communication port:

1. D11 = RX SERIAL DATA IN.

2. D12 = TX SERIAL DATA OUT.

RADIATOR BLOWER MOTOR

The Renix Jeep 88-90 ECU does not control the engine cooling system. The COOLING FAN RELAY that feeds the
RADIATOR BLOWER MOTOR is operated from a RADIATOR TEMPERATURE SWITCH.

The two-diode device ( FAN DIODE ASSEMBLY ) allows the COOLING FAN relay coil to be operated independently from
the air conditioning system and from the RADIATOR TEMPERATURE SWITCH valve.

Each time the ECU ( FROM ENGINE ELECTRONIC CONTROL UNIT ) drives the A / C relay coil CLUTCH, the NO. Of this relay
will energize the clutch solenoid in the air conditioning compressor and by means of the FAN DIODE ASSEMBLY will also
energize the coil of the COOLING FAN relay independently and if it affects the drive from the radiator temperature valve.
The CLUTCH DIODE diode suppresses the high voltage generated at the disconnection ( Counter-electromotive force ) of
the clutch solenoid.

The switch, indicator light and colored lines belong to an adaptation of its own, which allows the actuation and
monitoring of the electric fan drive from the panel inside the car cabin.

ALTERNATOR WITH BUILT-IN REGULATOR

Like many cars of the time, these early Jeep XJ models use alternator with built-in electronic controller. Despite having
SLIP PLATE, this can represent possible vulnerability problems by immersing the underside of the engine in water or
mud. In XJ models after 1991, the electronic alternator control circuit was incorporated into the ECU or PCM.
If it is necessary to relocate the regulator to a high and safe place in the body, this modification will allow to use
alternator of model XJ after 1991. In the following photograph I show an example of relocation, where also in case of
adaptations for different models XJ or any other Type of car, I show in the connector two cables ( F and L ), the main one
is F ( Field ) and it must be wired up to + 12VDC ignition. On models XJ 88 - 90 the L ( Ligth ) terminal reaches the cluster
on the dashboard, but for other XJ models or cars, it will be used to install pilot light fault indicator.

This adaptation requires that one of the two rings ( Coal ) of the rotor is grounded. On XJ alternators after 1991 or other
brands, it is necessary to make such a connection.
It is necessary to add two wires in the alternator ( Positive coal and F1 phase ), in the next photograph right, cables
adapted in phases F1 and F2 are observed. It is not necessary to adapt in phase F2, particularly in this case used for the
purpose of taking AC voltage to eventually install a voltage-boosting transformer ( Disassembled from a single-battery 12
VDC UPS, disembodied by damage ) , Allowing power tools of 120 volts or 220 volts AC in remote areas ( Drill, emery,
cautin, polisher, light bulbs, blender, etc. ). This additional adaptation requires basic electromechanical knowledge, in
particular the RMP control of the motor to maintain the frequency of the output voltage in the alternator between 50 Hz
or 60 Hz.
 RELOCATE HIGH CURRENT ACCESSORIES
At the time, in many automotive electrical designs, the distribution of + 12 VDC power supplies comes from the vehicle's
main switch ( IGNITION SWITCH ), causing possible premature damage to the switch. ( Hot spots. In XJ models after 1991
a distribution center + 12VDC was incorporated by means of relays ). If you want to reduce this load, I suggest to
incorporate a relay for more than 80 Amps, whose coil is operated from the ACC ( Accessory ) terminal of the IGNITION
SWITCH and whose contacts feed the air conditioning system ( Blower motor and compressor clutch ), by means of a
cable taken directly from the positive battery terminal ( With its respective maxi-fuse ).
OTHER IMAGES AND ANNEXES TO FACILITATE DIAGNOSTICS
DIAGRAM OF AIR CONDITIONING SYSTEM
 TO END
The present file is the first version translated into the English language of the article originally written in Spanish
language. To contribute to knowledge, the intention is to provide theoretical and practical support to owners and
technical personnel working with cars controlled by Jeep Renix systems. If you want to complement knowledge about XJ,
I suggest reading the file "DIAGNOSTICOS ELECTRICOS-CAJAS JEEP CHEROKEE XJ.pdf" available on the WEB.

This and many similar articles that I have written, are of free use, I eventually modify them and the last update will
always be available at www.qrz.com/db/YY5RM ( Amateur Radio Page ). In case of comments, please write to me at
ramon.miranda811@hotmail.com I hope this is useful. QRV.

Ramón Miranda

12-04-2016.