1988 JEEP CHEROKEE Service Repair Manual

Fig. 5: Testing Oxygen Sensor Heating
Element Resistance
Knock Sensor
Knock sensor must be tested with System Tester (M.S. 1700).
Speed Sensor
Disconnect speed sensor electrical connector from the sensor.
Using an ohmmeter, measure resistance between connector terminals "A"
and "B" (marked on connector). If reading is not 125-275 ohms with
engine at normal operating temperature, replace speed sensor.
"Synch Pulse" (Stator)
1) Using an analog voltmeter, insert positive probe into Blue
wire at distributor connector. Insert negative probe into Gray wire
with tracer at distributor connector.
NOTE: Do not remove connector from distributor. Insert voltmeter
leads from rear of connector. Do not puncture wire.
2) With voltmeter set on "15-Volt A/C" scale, turn ignition
on. Voltmeter should indicate 5 volts. If voltage is indicated, go to
step 4). If no voltage is indicated, check voltmeter leads for good
connection and go to next step.
3) If there is still no voltage reading, turn ignition off
and remove ECU. Reconnect harness and turn ignition off and remove
ECU. Reconnect harness and turn ignition on. Check for voltage between
ECU pin "C-16" and good ground. If voltage still does not register,
test system with System Tester (M.S. 1700).
4) If 5 volts was detected, check for continuity at the
following points:
* Between Blue wire at distributor connector and terminal "C-16"
at ECU.
* Between Gray wire with tracer at distributor connector and
terminal "C-5" at ECU.
* Between Black wire at distributor connector and good ground.

If no continuity is shown, repair wiring harness. If
continuity is shown, reconnect voltmeter as in step 2).
5) While observing voltmeter, crank engine. Voltmeter needle
should fluctuate back and forth during engine cranking. This verifies
proper operation of distributor stator. If voltmeter needle does not
fluctuate, replace stator.
EGR Solenoid Valve
1) Make sure vacuum is present at port "C" of the solenoid.
See Fig. 6 . Remove vacuum connector from ports "A" and "B" of
solenoid. Connect vacuum gauge to port "B". Start engine and run at
idle. Vacuum should not be present at port "B".
2) Disconnect electrical connector from solenoid. Vacuum
should now be present at port "B". Stop engine. Reconnect electrical
connector at solenoid. Remove vacuum gauge and reconnect vacuum hoses.
Relays - General Testing
1) The relays used on the 4.0L engine are all of the same
basic construction and design. Terminal No. 30 is usually connected to
battery voltage ("switched" or battery positive at all times).
Terminal No. 87A is connected to terminal No. 30 in de-energized
position.
2) Terminal No. 87 is connected to terminal No. 30 in the
energized position. This connection supplies battery voltage to the
operated device.
3) Terminal No. 86 is connected to the electromagnet of the
operated device; usually through a "switched" power source. Terminal
No. 85 is connected to the electromagnet of the operated device;
usually grounded through a switch or the ECU.
NOTE: Not all relays have battery voltage applied at terminal No.
30. Some may have battery voltage applied at terminal Nos. 87
or 87A. Check design of relay before testing.
4) When testing relays, there should be continuity between
terminal Nos. 87A and 30 when relay is in de-energized position.
Resistance between terminal Nos. 85 and 86 should be 70-80 ohms for
resistor-type relays and 81-91 ohms for diode-type relays.
Fig. 6: Testing EGR Solenoid Valve
Starter Motor Relay
1) Disconnect electrical connectors from terminals "I" and
"G". Using an ohmmeter, measure resistance between the terminals.

Resistance value should be about 22 ohms. Measure resistance between
either terminal and battery negative post. Resistance value should be
infinite. If resistance values are not as specified, replace relay. If
okay, reconnect electrical connectors.
2) Disconnect electrical connector from "SOL" terminal.
Connect voltmeter between terminal and battery negative post. While
observing voltmeter, turn ignition switch to "START" position. If 12
volts is not indicated, check and repair wiring harness, bulkhead
connector, and/or ignition switch. If battery voltage is present, go
to next step.
3) If battery voltage is present, but starter relay does not
work, place transmission in Park (Neutral for man. trans.) and apply
parking brake. Disconnect electrical connector from terminal "I" (Dark
Green wire) and apply battery voltage to terminal. Using a jumper
wire, touch terminal "G" to ground. If relay does not click, replace
relay. If relay does click, repair ground circuit.
Fig. 7: Design & Function of Relays
REMOVAL & INSTALLATION

ELECTRONIC CONTROL UNIT (ECU)
Removal & Installation
Information not available from manufacturer.
COOLANT TEMPERATURE SENSOR (CTS)
Removal & Installation
Drain cooling system. Remove air cleaner assembly. Disconnect
electrical connector and remove sensor from block. To install, reverse
removal procedure. Refill cooling system.
OXYGEN (O2) SENSOR
Removal & Installation
Raise and support vehicle. Disconnect electrical connector
from sensor. Remove sensor from exhaust manifold. Install and tighten
sensor to 35 ft. lbs. (47 N.m). Reconnect electrical connector. Lower
vehicle.
THROTTLE POSITION SENSOR (TPS)
Removal
Disconnect electrical connector from TPS. Bend lock tabs away
from retaining screws and remove screws. Remove TPS.
Installation
With throttle valve in normal closed position, install TPS.
Install TPS retaining screws. Perform OUTPUT CHECK.
NOTE: Throttle position sensor is nonadjustable. Only output
voltage can be monitored.
Output Check (Manual Transmission Only)
1) Connect negative lead of digital voltmeter to terminal "B"
and positive lead to terminal "A" of TPS connector. Do not disconnect
electrical connector. Insert leads through back of connector. It may
be necessary to remove throttle body from vehicle to gain access to
connector.
2) Turn ignition on, engine off. With throttle closed and
resting against idle stop, input voltage should be about 5 volts. Move
positive lead from terminal "B" to terminal "C" and read voltage
output. Output voltage should be about 0.8 volt with throttle closed
and resting against idle stop.
3) If voltage is not as specified, loosen lower retaining
screw and pivot sensor for large adjustments. Loosen upper retaining
screw and pivot sensor for small adjustments. Adjust sensor so output
voltage reading equals 16 percent of input voltage. If voltage
specifications cannot be obtained, replace sensor. Remove voltmeter.
Tighten screws and bend retaining tabs into position.
ALL OTHER SENSORS, SOLENOIDS & SWITCHES
Removal & Installation
Removal of sensors, switches, and solenoids is accomplished
by disconnecting the electrical and/or vacuum connectors and removing
or detaching the component. To install, reverse removal procedure.
TESTING
NOTE: Refer to Figs. 8-10 for Testing information.

Fig. 8: Throttle Position Sensor Diagnostics

Fig. 9: Fuel Injector Diagnostics

Fig. 10: ECU Connector Pin & Diagnostic Connector Pin Identification
WIRING DIAGRAM

Fig. 11: Comanche Engine Control System Wiring Diagram