engine JEEP CHEROKEE 1988 Service Owner's Manual

MAT, MAP, EGO, speed and knock sensors. It also is informed of
throttle, gear (automatic transaxle models) and A/C control position.
When the ECU receives deceleration input from the closed
throttle (idle) switch, it grounds the EGR valve/canister purge
solenoid. This interrupts vacuum to EGR valve and canister purge
function. The injector is grounded, and during rapid deceleration,
the ECU may stop injection for a short period of time. The ECU also
controls engine idle speed and throttle stop angle.
WIDE OPEN THROTTLE MODE
During wide open throttle mode, the ECU receives inputs from
the CST, MAT, MAP, EGO, speed and knock sensors. It also monitors
throttle position.
When the ECU receives deceleration input from the closed
throttle (idle) switch, it grounds the EGR valve/canister purge
solenoid. This interrupts vacuum to EGR valve and canister purge
function. The EGO sensor input is not accepted by the ECU. The
injector is grounded and amount of fuel is precisely controlled.
IGNITION SWITCH "OFF" MODE
When ignition switch is turned "OFF", the ECU ceases to
provide ground for the injector and all fuel injection stops. The ECU
causes the idle speed actuator to fully extend for the next start up.
The ECU then deactivates.
COMPONENT TESTING
NOTE: When test calls for volt-ohmmeter, use of a high impedance
digital type is required.
Fig. 5: Diagnostic Connectors D1 and D2 Terminal Identification
1) Disconnect wiring harness connector from the MAT sensor.
Test resistance of the sensor with an ohmmeter. If resistance is not
185-100,700 ohms (3400 ohms at 70
F; 1600 ohms at 100F), replace

sensor. With engine warm, resistance should be less than 1000 ohms.
2) Connect one ohmmeter lead to sensor connector terminal.
Connect other lead, in turn, to ECU harness connector terminals 32
and 14. Repair wiring harness if resistance is greater than 1 ohm.
COOLANT TEMPERATURE SENSOR
1) Disconnect wiring harness from CTS sensor. Test
resistance of sensor. If resistance is not 185-100,700 ohms (3400
ohms at 70
F; 1600 ohms at 100F), replace sensor. With engine warm,
resistance should be less than 1000 ohms.
2) Connect one ohmmeter lead to sensor connector terminal.
Connect other lead, in turn to ECU harness connector terminals 15 and
32. Repair wiring harness if an open circuit is indicated.
WIDE OPEN THROTTLE (WOT) SWITCH
1) Disconnect wiring harness from WOT switch. Connect
ohmmeter leads to switch terminals, and manually open and close the
switch. When switch is closed, resistance should be infinite. A low
resistance should be indicated at wide open position. Test switch
operation several times. Replace WOT switch if defective. Reconnect
wiring harness.
2) With ignition switch "ON", connect voltmeter between pin
6 and pin 7 (ground) of diagnostic connector D2. Voltage should be
zero with switch in wide open position and greater than 2 volts in
any other position.
3) If voltage is always zero, test for short circuit to
ground in wiring harness or switch. Check for open circuit between
pin 8 of ECU connector and the switch connector. Repair or replace
wiring harness as necessary.
4) If voltage is always greater than 2 volts, test for an
open wire or connector between the switch and ground. Repair as
required.
CLOSED THROTTLE SWITCH
NOTE: It is important that all testing be done with the idle speed
actuator (ISA) motor plunger in the fully extended position
(as it would be after a normal engine shut down). If it is
necessary to extend the motor plunger to test the switch, an
ISA motor failure can be suspected. Refer to ISA motor test.
1) With ignition on, connect voltmeter positive lead to pin
13 of diagnostic connector D2. Attach negative lead to pin 7. Voltage
should be close to zero at closed throttle and greater than 2 volts
at any position other than closed throttle.
2) If the voltage is always zero, test for a short circuit
to ground in the wiring harness or switch. Test for an open circuit
between pin 25 of ECU connector and throttle switch.
3) If voltage is always more than 2 volts, test for an open
circuit in the wiring harness between the ECU and switch connector.
Also check for open circuit between the switch connector and ground.
Repair or replace wiring harness as needed.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
1) Inspect MAP sensor vacuum hose connections at sensor and
throttle body. Repair as required. Test MAP sensor output voltage at
MAP sensor connector pin B (as marked on sensor body) with the
ignition switch "ON" and engine off. See Fig. 15. Output voltage
should be 4.0-5.0 volts.

NOTE: Voltage should drop 0.5-1.5 volts with hot engine, at idle.
Fig. 6: MAP Sensor Terminal Identification
If markings on MAP sensor vary from illustration, use markings on
sensor.
2) Test voltage at pin 33 of ECU connector for 4.0-5.0 volts
to verify wiring harness condition. Repair if required.
3) With ignition on, check for MAP sensor supply voltage of
4.5-5.5 volts at sensor connector pin C. Similar voltage should be
present at pin 16 of ECU connector. Repair or replace wiring harness
if required. Test for sensor ground between pin 17 of ECU connector
and pin A of sensor connector.
4) Using an ohmmeter, check for ground from pin 17 of ECU
connector to pin 2. If an open circuit is indicated, check for a
defective sensor ground on the flywheel housing near the starter
motor.
5) If ground is good, the ECU must be replaced. Before
replacing ECU, check to see if pin 17 of ECU connector is shorted to
12 volts. If so, correct the condition and test ECU before replacing.
Refer to the ELECTRONIC CONTROL UNIT TEST.
ELECTRONIC CONTROL UNIT

1) If all components have been checked and/or repaired, but
a system failure or problem still exists, the ECU may be at fault.
However, the ECU is a very reliable unit and must always be the final
component replaced if a doubt exists concerning the cause of a system
failure.
2) The only way to confirm an ECU malfunction is to take the
unit to an AMC dealer to have it tested. This is the only sure way to
avoid replacing a good ECU.
SYSTEM DIAGNOSIS
PRELIMINARY CHECKS
Be sure fuel is actually reaching the injector. Make sure no
air is entering the intake or exhaust system above the catalytic
converter. Before assuming an engine control system malfunction,
inspect the following systems to ensure components are in good
condition and are operating properly.
* All support systems and wiring.
* Battery connections and specific gravity.
* Electrical and vacuum connections on components and sensors.
* Emission control devices.
* Ignition system.
* Vacuum hoses.
CAUTION: Never connect or disconnect a component without turning the
ignition switch off. Never apply more than 12 volts or AC
voltage to system terminals. Disconnect battery cables
before charging it. Remove ECU if temperatures are expected
to exceed 176
F (80C), such as in a paint shop bake oven.
DIAGNOSTIC TEST CHARTS
Following are 6 different diagnostic test flow charts,
providing the shortest means of testing the system. These include:
* Ignition Switch "OFF" Chart - Tests system power for ECU
memory keep-alive voltage.
* Ignition Switch "ON" Power Chart - Tests system power
function and fuel pump power function.
* Ignition Switch "ON" Input Chart - Tests closed throttle
(idle) switch, wide open throttle (WOT) switch, manifold
absolute pressure (MAP) sensor, park/neutral switch, coolant
temperature sensor (CTS), manifold air/fuel temperature (MAT\
)
sensor and the respective switch or sensor circuits.
* System Operational Chart - Tests engine start-up and fuel
injector circuits, plus function of closed loop air/fuel
mixture, coolant temperature sensor, manifold air/fuel
temperature sensor, knock sensor and closed loop ignition
retard/advance, EGR valve and canister purge solenoid, idle
speed actuator, and A/C control.
* Basic Engine Chart - Indicates possible failures within other
engine related components.
* Man. Trans. Up-shift Chart - Tests up-shift indicator lamp
function on manual transmission vehicles.

Fig. 23: Chart 4-K - System Operational Test (Cont.)
Fig. 24: Chart 5 - Basic Engine Test

THROTTLE BODY ASSEMBLY
Removal
1) Remove throttle return spring, throttle cable and cruise
control cable, if equipped. Disconnect wiring harness connector from
injector, WOT switch, and ISA motor. Remove fuel supply and return
pipes from throttle body.
2) Identify and tag vacuum hoses for installation later.
Disconnect vacuum hoses from throttle body assembly. Remove throttle
body-to-manifold retaining nuts from studs.
3) Remove throttle body assembly from intake manifold. If
being replaced, transfer ISA motor and WOT switch and bracket
assembly to new throttle body.
Installation
To install, reverse removal procedure using a new gasket
between components. Adjust ISA motor and WOT switch.
FUEL INJECTOR
Removal
Remove air cleaner assembly, injector wire connector, and
injector retainer clip screws. Using a pair of small pliers, gently
grasp center collar of injector (between electrical terminals), and
carefully remove injector with a lifting and twisting motion. Note
back-up ring fits over upper "O" ring.
Installation
1) Lubricate new lower "O" ring with light oil and install
in housing bore. Lubricate new upper "O" ring with light oil and
install in housing bore. Install back-up ring over upper "O" ring.
2) Position new injector in fuel body, and center nozzle in
lower housing bore. Seat injector with a pushing and twisting motion.
Align wire connector terminals properly. Install retainer clip and
screws. Connect injector wire connector.
FUEL PRESSURE REGULATOR
Removal
Remove 3 retaining screws, securing pressure regulator to
fuel body. After noting location of components for reassembly
reference, remove regulator assembly.
Installation
Position pressure regulator assembly with a new gasket.
Install 3 retaining screws, securing regulator to throttle body.
Adjust regulator. Operate engine and inspect for leaks.
IDLE SPEED ACTUATOR, MOTOR & WIDE OPEN THROTTLE SWITCH
NOTE: Closed throttle (idle) switch is integral with ISA and motor
assembly.
Removal
1) Remove air cleaner assembly. Disconnect throttle return
spring, throttle cable and cruise control cable, if equipped.
Disconnect wiring harness connector from ISA motor and WOT switch.
2) Remove ISA motor and WOT switch bracket from throttle
body. Remove motor-to-bracket retaining nuts. See Fig. 23. Do not
remove nuts from motor studs.
CAUTION: Do not attempt to remove ISA motor attaching nuts without

nuts. Install motor and WOT switch bracket assembly on throttle body.
2) Connect wiring harness connector to ISA motor and WOT
switch. Connect the throttle return spring, throttle cable and cruise
control cable. Adjust ISA motor and WOT switch. Install air cleaner
assembly.
NOTE: After replacing or reinstalling the original ISA motor, be
sure motor plunger is fully extended before starting the
engine. If plunger is not fully extended, the closed
throttle switch may open prematurely, causing idle speed to
drop to approximately 400 RPM.
3) Start engine with throttle at 1/4 open position. This
prevents ISA plunger from retracting. Stop engine. When ignition is
turned off, the motor plunger will fully extend. After installation
is complete, adjust ISA as required.
ELECTRONIC CONTROL UNIT
Removal & Installation
Locate ECU in passenger compartment, below glove box. Remove
retaining screws and mounting bracket. Remove the ECU, and disconnect
wiring harness connector from ECU. Reverse removal procedure to
install.
OXYGEN SENSOR
Removal
Disconnect the wire connector from sensor, and unscrew
sensor from exhaust pipe adapter. Clean adapter threads.
Installation
1) Apply anti-seize compound to sensor threads. Do not allow
compound to adhere to any other part of sensor. Hand start the sensor
into place and tighten. Check that wire terminal ends are properly
seated in connector. Connect wire.
2) Do not push the rubber boot over sensor body lower than
1/2" (13 mm) above base of sensor. If the sensor wire should break,
sensor must be replaced. These wires cannot be spliced or otherwise
repaired.
MANIFOLD AIR TEMPERATURE & MANIFOLD ABSOLUTE PRESSURE SENSORS
Removal & Installation
Disconnect wiring harness connector from sensor. Disconnect
vacuum hose from MAP sensor. Remove sensor. Clean MAT sensor manifold
threads, and wrap with Teflon tape. To install, reverse removal
procedure.
COOLANT TEMPERATURE SENSOR (CTS)
Removal & Installation
Allow engine to cool and release pressure from cooling
system. Remove wiring harness from sensor. Remove sensor at rear of
intake manifold, and plug hole to prevent excessive coolant loss. To
install, reverse removal procedure and replace lost coolant.
EGR VALVE & CANISTER PURGE SOLENOID
Removal & Installation
Disconnect wiring harness and vacuum hose from solenoid.
Remove solenoid and bracket as an assembly. Replace solenoid as an

assembly. To install, reverse removal procedure.
ADJUSTMENTS
NOTE: The following adjustment procedures should not be necessary
during normal vehicle operation or maintenance. Adjustment
of the listed components should only be required when a
faulty component is replaced with a new one.
IDLE SPEED ACTUATOR (ISA) MOTOR
1) With air cleaner removed, air conditioner off (if
equipped) and engine at normal operating temperature, connect a
tachometer to terminals 1 (+) and 3 (-) of the small diagnostic
connector D1. See Fig. 16. Turn ignition off and observe ISA motor
plunger. The plunger should move to fully extended position.
2) With ISA plunger fully extended, disconnect wire
connector and start engine. Engine speed should be 3300-3700 RPM. If
not, turn hex head screw on end of plunger until correct speed is
obtained.
3) Hold closed throttle switch plunger all the way in while
opening throttle. Release the throttle. Throttle lever should not make
contact with the plunger. If contact is made, inspect throttle
linkage and/or cable for binding or damage. Repair as needed.
4) Reconnect ISA motor wire connector and turn ignition off
for 10 seconds. Motor should move to fully extended position. Start
engine. Engine speed should be 3300-3700 RPM for a short time and
then fall to normal idle. Turn off engine and remove tachometer.
5) When final adjustments have been made, apply thread
sealer to adjustment screw threads to prevent movement. Install air
cleaner.
WIDE OPEN THROTTLE SWITCH
1) Remove the throttle body assembly from the engine, and
loosen 2 WOT switch retaining screws. Hold throttle in wide open
position, and attach a Throttle Angle Gauge (J-26701) to flat surface
of the throttle lever.
2) Rotate scale to align the 15
mark with the pointer.
Level the gauge. Rotate scale to align zero with the pointer, and
close the throttle enough to center bubble. This positions the
throttle at 15
before wide open throttle.
3) Adjust the WOT switch lever on the throttle cam so that
the plunger is just closed at 15
position. Tighten the retaining
screws, and remove the gauge.

2) Connect accurate fuel pressure gauge to fuel pressure
test fitting. Start engine and accelerate to 2000 RPM. Turn torx head
adjustment screw on bottom of fuel regulator to obtain 17.3 psi (1.2
kg/cm(C)) of fuel pressure.
NOTE: Turning screw inward increases pressure; turning screw
outward decreases it.
3) After specification is reached, install a lead seal ball
to cover regulator adjustment screw. Turn ignition switch off, and
disconnect tachometer. Disconnect fuel pressure gauge, remove test
fitting, and install original plug screw. Install air cleaner
assembly.

\003
4.0 L 6 -C YL - V IN [ M ] & 4 .2 L 6 -C YL - V IN [ C ]

1 988 J e ep C hero ke e
1988 ENGINES
Chrysler Motors 4.0L & 4.2L 6-Cylinder
Jeep; Cherokee, Comanche, Wagoneer, Wrangler
ENGINE CODING
ENGINE IDENTIFICATION
NOTE: For engine repair procedures not covered in this article,
see ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION article
in the GENERAL INFORMATION section.
The Vehicle Identification Number (VIN) is located on upper
left side of dash and is visible through windshield. The fourth
character identifies the engine size. The tenth character identifies
the model year.
ENGINE IDENTIFICATION CODES TABLE









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Application VIN Code
4.0L 6-Cylinder MPFI ........................ M
4.2L 6-Cylinder 2-Bbl. ...................... C









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SPECIAL ENGINE MARKS
Some engines are produced at factory with oversize or
undersize components. These engines are identified by a letter code
stamped on a boss between ignition coil and distributor. Letters are
decoded as follows:
* "B" indicates all cylinder bores are .010" (.25 mm)
oversize.
* "C" indicates all camshaft bearing bores are .010" (.25 mm)
oversize.
* "M" indicates all main bearing journals are .010" (.25 mm)
undersize.
* "P" indicates all connecting rod journals are .010" (.25 mm)
undersize.
REMOVAL & INSTALLATION
ENGINE REMOVAL
See ENGINE REMOVAL in this section.
INTAKE & EXHAUST MANIFOLD (4.0L)
WARNING: Fuel system is under pressure. Use care when removing fuel
lines to prevent personal injury.
Removal
1) Disconnect negative battery cable. Remove air inlet from
throttle plate assembly. Disconnect throttle, cruise control and
throttle valve (A/T models) cables (if equipped).
2) Disconnect and mark all vacuum and electrical connectors