1988 JEEP CHEROKEE check engine

Roll-Over Check Valve
When valve is inverted, a stainless steel ball pushes a
plunger against its seat, blocking fuel flow through valve.
Charcoal Canister
All models are equipped with a dual-purge type canister. An
inlet is provided for carburetor fuel bowl vapors and another one for
fuel tank vapors. The outlet is connected to intake manifold vacuum.
A secondary purge fourth nipple connects to carburetor ported vacuum.
During engine operation, manifold vacuum draws fresh air
through inlet filter in bottom of canister and purges stored vapors.
When ported vacuum reaches 12 in. Hg, secondary purge circuit is
opened and canister purges at a much higher rate.
Carburetor Bowl Vent
The carburetor bowl vent provides an outlet for fuel vapors
when engine is not operating. During engine operation, fuel bowl is
vented to inside of air cleaner. The bowl is automatically closed by
mechanical link to throttle when engine is started.
MAINTENANCE
No adjustments are required with this system. Replace air
inlet filter (if equipped) in bottom of charcoal canister every
30,000 miles. Regular inspection should be made and defective
components replaced as necessary.

* Battery connections and specific gravity.
* Engine Compression.
* Electrical connections on components and sensors.
* Emission control devices.
* Ignition system.
* All vacuum line, fuel hose and pipe connections.
General Precautions
In order to prevent injury to operator or damage to system
or component parts, use following techniques:
* Turn ignition off before connecting or disconnecting any
component parts.
* DO NOT apply DC voltage greater than 12 volts or any AC voltage
to system.
* Disconnect battery cables before charging.
* Remove ECU from vehicle if ambient temperature could exceed
176

F (80
C).
* DO NOT modify or circumvent any system functions.
SYSTEM TESTING
Fuel System Test
WARNING: Always relieve residual fuel pressure in fuel delivery
system before opening system. To prevent chance of personal
injury, cover fittings with shop towel while disconnecting
fittings.
1) Remove cap from pressure test port located in fuel rail.
See Fig. 3 . Connect Fuel Pressure Gauge (J-34730-1) to pressure
fitting.
2) Start vehicle. Pressure should be approximately 31 psi
(2.1 kg/cm
) with vacuum hose connected to pressure regulator and
39 psi (2.6 kg/cm) with vacuum hose removed from pressure
regulator.
3) Check fuel pump flow rate. A good fuel pump will deliver
at least one liter of fuel per minute with fuel return line pinched
off. If fuel pump does not pump adequately, inspect fuel system for a
plugged fuel filter or filter sock.
Fig. 3: Fuel System Pressure Test Components
Courtesy of Chrysler Motors.
4) Fuel pump flow rate can be checked by connecting one end
of an old A/C gauge hose to fuel test port on fuel rail and inserting

1) Disconnect wire harness connector from CTS sensor. Test
resistance of sensor with a high impedance digital ohmmeter.
Resistance should be less than 1000 ohms with engine warm. See
TEMPERATURE-TO-RESISTANCE VALUES table.
2) Test resistance of wire harness between ECU terminal D-3
and sensor connector terminal. Repeat test at terminal C-10 of ECU
and sensor connector terminal. Repair wire harness if an open circuit
is indicated.
MAT Sensor Test
1) Disconnect wire harness connector from MAT sensor. Test
resistance of sensor with a high impedance digital ohmmeter.
Resistance should be less than 1000 ohms with engine warm. Replace
sensor if resistance is not within specified range. See
TEMPERATURE-TO-RESISTANCE VALUES table.
2) Test resistance of wire harness between ECU wire harness
connector terminal D-3 and sensor connector terminal. Repeat test
with terminal C-8 at ECU and sensor connector terminal. Repair wire
harness if an open circuit or resistance is greater than one ohm is
indicated.
TPS Test
See THROTTLE POSITION SENSOR TEST PROCEDURE chart in this
article.
TEMPERATURE-TO-RESISTANCE VALUES (CTS & MAT SENSOR) TABLE
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F
C Ohms
(Approximate)
212 .......................... 100 ........................... 185
160 .......................... 70 ............................ 450
100 .......................... 38 ........................... 1600
70 ........................... 20 ........................... 3400
40 ............................ 4 ........................... 7500
20 ........................... -7 ......................... 13,500
0 ............................ -18 ........................ 25,000
-40 .......................... -40 ....................... 100,700
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Knock Sensor Test
1) Start engine until engine reaches normal operating
temperature. Connect Diagnostic Tester (M.S. 1700). Observe knock
value on tester. Using tip of screw driver, gently tap on cylinder
block next to knock sensor and observe knock sensor value on tester.
2) Knock sensor value should increase while tapping on
cylinder block. If knock sensor value does not increase while tapping
on cylinder block, check knock sensor for proper connection. If
connection is good, replace knock sensor.
Speed Sensor Test
Disconnect speed sensor connector from ignition control
module. Place an ohmmeter between terminals "A" and "B". (Marked on
connector) Reading should be 125-275 ohms with engine hot. Replace
sensor if readings are not within specifications.
Idle Speed Stepper (ISS) Motor
1) Set parking brake and block drive wheels. Route all
tester cables away from cooling fans, drive belts, pulleys and
exhaust system. Always allow engine speed to return to normal before
disconnecting testing tools.
2) With ignition off, disconnect ISS motor connector at

throttle body. Connect Exerciser Tool (Part No. 8980 002 646) into
ISS motor. See Fig. 6.
3) Connect Red clip to battery positive cable. Connect
Black clip to battery negative cable. Red light on exerciser tool
will illuminate when properly connected. Start engine.
NOTE: When switch on exerciser tool is in "High" or "Low"
position, light on exerciser tool will flash indicating
voltage pulses are being sent to ISS motor.
4) Move exerciser tool switch to "High" position. Engine
speed should increase. Move switch to "Low" position. Engine speed
should decrease. If engine speed increases or decreases while using
exerciser tool, ISS motor is functioning properly.
5) Disconnect exerciser tool and reconnect ISS motor
connector. If engine speed does not change, turn ignition off and
remove ISS motor from throttle body.
6) With ignition off, switch exerciser tool between "High"
and "Low" positions. Check movement of ISS motor pintle. Pintle
should move in and out. If pintle does not move, replace ISS motor.
7) Start engine and test new ISS motor for proper
operation. If pintle operates properly, check ISS motor bore in
throttle body for blockage and clean as necessary. Reinstall ISS
motor into throttle body.
Fig. 6: ISS Motor Test Connector
Courtesy of Chrysler Motors.
Fuel Injector Test

\003
FU EL IN JE C TIO N S YSTE M - T B I

1 988 J e ep C hero ke e
1988 Electronic Fuel Injection
JEEP/RENIX THROTTLE BODY INJECTION
2.5L Cherokee, Comanche, Wagoneer, Wrangler
DESCRIPTION
The Throttle Body Injection (TBI) system is a single
injector system that introduces fuel into throttle body from above
throttle plate. Fuel injector, located within throttle body, is
controlled by the Electronic Control Unit (ECU).
The ECU is a sealed microprocessor that receives input
signals from several sensors and other related engine components.
Based on these inputs, ECU generates output signals that control and
adjust air/fuel mixture and ignition timing as necessary for proper
engine performance.
ECU also controls engine idle speed, emission control
systems, upshift indicator light (manual transmission only), and A/C
compressor clutch.
OPERATION
ELECTRONIC CONTROL UNIT (ECU)
On Cherokee, Comanche and Wagoneer, ECU is located under
instrument panel, above accelerator pedal. On Wrangler, ECU is
located behind glove box. Input information from various engine
sensors to ECU is used to determine engine operating conditions and
needs. Battery voltage input is used to ensure that correct output
voltage is supplied by ECU during fluctuations in battery voltage.
FUEL INJECTOR
Fuel injector is mounted in throttle body so that fuel is
injected into incoming airflow. When injector solenoid is energized,
armature and plunger move upward against spring. Check ball above
injector nozzle moves off seat and opens small orifice at end of
injector.
Fuel supplied to injector is forced around ball and through
orifice, resulting in fine spray of fuel. Volume of fuel injected is
dependent only on length of time that injector is energized by ECU,
as fuel pressure is constant at injector. During cold engine starts,
extra fuel is supplied so richer mixture will aid in starting.
FUEL PRESSURE REGULATOR
Fuel pressure regulator is integral part of throttle body.
Pressure regulator has a spring chamber that is vented to same
pressure as tip of injector. Because differential pressure between
injector nozzle and spring chamber is same, only the length of time
that injector is energized controls volume of fuel injected.
Fuel pump delivers more fuel than is required by engine.
Excess fuel goes to fuel tank from pressure regulator via fuel return
hose. Fuel pressure regulator function is mechanical and ECU does not
control it.
FUEL PUMP

Electric roller type fuel pump is located in fuel tank.
Integral check valve is used to maintain pressure in fuel delivery
system after pump stops running. Fuel pump operation is controlled by
ECU through a fuel pump relay.
IDLE SPEED ACTUATOR (ISA) MOTOR
ISA motor acts as movable idle stop to change throttle stop
angle. Both engine idle speed and deceleration throttle stop angle
are set by ISA. ECU controls ISA motor by providing appropriate
voltage outputs to produce idle speed or throttle stop angle required
for engine operating condition.
OXYGEN (O2) SENSOR
Oxygen sensor is equipped with a heating element that keeps
sensor at proper operating temperature at all times. Oxygen sensor is
located in exhaust pipe.
Maintaining proper sensor temperature at all times, system
enters "Closed Loop" operation sooner and remains in "Closed Loop"
during periods of extended idle. Electrical feed to oxygen sensor is
through ignition switch.
The ECU receives sensor voltage signal which varies with
oxygen content in exhaust gas. Signal is used by ECU as reference for
setting air/fuel mixture ratio. ECU varies voltage to injector both
to compensate for battery voltage fluctuations and to change duration
of injector opening for control of air/fuel mixture.
MANIFOLD AIR/FUEL TEMPERATURE (MAT) SENSOR
MAT sensor provides a signal to ECU that changes depending
upon temperature of air/fuel mixture in intake manifold. During high
temperature conditions, ECU will compensate for changes in density of
air.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
MAP sensor measures absolute pressure in intake manifold.
Both mixture density and ambient barometric pressure are supplied to
ECU by MAP sensor. Sensor is mounted in middle of firewall in engine
compartment. Sensor receives manifold pressure information through
vacuum line from throttle body. See Fig. 1.
Fig. 1: Manifold Absolute Pressure (MAP) Sensor
Courtesy of Chrysler Motors.
COOLANT TEMPERATURE SENSOR (CTS)

IDLE SPEED ACTUATOR (ISA) MOTOR
1) Adjust ISA motor plunger to establish initial position of
plunger only if motor has been removed or replaced. Remove air filter
elbow and start engine. Run engine until engine reaches normal
operating temperature. Turn A/C off (if equipped).
2) Connect tachometer leads to diagnostic connector D1,
attaching negative lead to terminal D1-3 and positive lead to
terminal D1-1. See Fig. 4. Turn ignition off. ISA motor plunger
should move to fully extended position.
3) When ISA motor plunger is fully extended, disconnect ISA
motor wiring connector and start engine. Engine speed should be
3300-3700 RPM. If incorrect, turn hex head screw at end of plunger to
provide engine speed of 3500 RPM.
4) Fully retract ISA motor by holding closed throttle (idle)\
switch plunger inward as throttle is opened. Closed throttle switch
plunger should not touch throttle lever in closed position. If
contact is made, check linkage and/or cable for binding or damage.
Repair as necessary.
5) Connect ISA motor wiring harness connector and turn
ignition off for 10 seconds. ISA motor should move to fully extended
position. Start engine. Engine speed should be 3500 RPM for short
period of time and then decrease to normal idle speed.
6) Turn ignition off. Disconnect tachometer. After final
adjustment of ISA motor, use thread penetrating sealant (Loctite 290)
on adjustment screw to prevent movement and maintain adjustment.
NOTE: If adjustment screw must be moved after thread sealant
hardens, loosen threads by heating screw with flameless heat
such as soldering gun. DO NOT use flame or torch type of
heat as damage to ISA motor will result.
FUEL PRESSURE REGULATOR
WARNING: Always relieve residual fuel pressure in fuel delivery
system before opening system. To prevent chance of personal
injury, cover fittings with shop towel while disconnecting
fittings.
1) Replacement fuel pressure regulator must be adjusted to
establish correct pressure. Remove air filter elbow and hose. Connect
tachometer leads to diagnostic connector D1, attaching negative lead
to terminal D1-3 and positive lead to terminal D1-1. See Fig. 4.
Remove screw plug and install fuel pressure test fitting.
NOTE: Fuel pressure test fitting is not included with throttle
body. Fitting (8983 501 572) must be obtained separately.
2) Connect fuel pressure gauge to test fitting. Start engine
and increase speed to approximately 2000 RPM. Turn Torx head screw at
bottom of regulator to set correct pressure. Turning screw inward
increases pressure and turning screw outward decreases pressure. See
Fig. 3 .
3) All models require fuel pressure of 14.5 psi (1.0 kg/cm
).
Install lead sealing ball to cover regulator adjustment
screw after adjusting fuel pressure. Turn ignition off. Remove
measuring equipment and test fitting. Install original plug screw and
air filter assembly.

3) Return throttle plate to closed throttle position (M/T),
or maintain throttle plate in closed position (A/T). Check sensor
output voltage. To do so, disconnect voltmeter positive lead from
terminal "C" and connect it to terminal "A" (top) of sensor (M/T), o\
r
from terminal "A" and connect it to terminal "B" (A/T).
4) Move and hold throttle plate in wide open throttle
position (M/T), or maintain throttle plate in closed position (A/T).\
Ensure throttle linkage contacts stop. Note voltmeter reading. Output
voltage should be 4.6-4.7 volts (M/T), or .2 volt (A/T).
5) If output voltage is incorrect, loosen bottom sensor
retaining screw and pivot sensor in adjustment slot for a coarse
adjustment. Loosen top sensor retaining screw for fine adjustments.
Tighten screws after adjustment.
TESTING & TROUBLE SHOOTING
PRELIMINARY CHECKS & PRECAUTIONS
Subsystem Checks
Before testing fuel injection system for cause of
malfunction, check that following subsystems and components are in
good operating condition:
* Battery and charging system.
* Engine state of tune.
* Emission control devices.
* Fuel system pressure and delivery volume.
* Wiring connectors at components.
General Precautions
In order to prevent injury to operator or damage to system
or component parts, use following techniques:
* Turn ignition off before connecting or disconnecting any
component parts.
* DO NOT apply DC voltage greater than 12 volts or any AC voltage
to system.
* Disconnect battery cables before charging.
* Remove ECU from vehicle if ambient temperature could exceed
176

F (80
C).
* DO NOT modify or circumvent any system functions.
RESISTANCE & VOLTAGE TESTS
MAT Sensor
1) Disconnect wiring from MAT sensor. Using high input
impedance digital volt-ohmmeter (DVOM), check resistance of sensor.
Resistance should be less than 1000 ohms when engine is warm. Replace
sensor if it does not fall within range shown in
TEMPERATURE-TO-RESISTANCE VALUES table.
2) Test resistance in wiring harness between ECU connector
terminal No. 32 and sensor connector terminal. Also test resistance
in wiring harness between ECU harness terminal No. 14 and sensor
connector terminal. See Fig. 5. Repair wiring harness if open circuit
or resistance greater than one ohm is indicated.
Coolant Temperature Sensor (CTS)
1) Disconnect wiring harness from CTS. Using high input
impedance digital volt-ohmmeter (DVOM), check resistance of sensor.
Resistance should be less than 1000 ohms when engine is warm. Replace
sensor if it does not fall within range shown in
TEMPERATURE-to-RESISTANCE VALUES table.