1988 JEEP CHEROKEE Service Repair Manual

Fig. 2: 4.0L & 4.2L Timing Marks & Firing Order
Magnetic probe located at 9.5 degrees ATDC on 4.2L.
IGNITION TIMING (4.0L)
Information not available from manufacturer.
IGNITION TIMING (4.2L)
NOTE: Engines are equipped with a receptacle for a magnetic probe
timing light, located 9.5 degrees ATDC. Do not use this
location to check timing with a conventional light, use a
magnetic probe timing light.
Standard Timing Procedure
1) Apply parking brake, and place transmission in Neutral
(Park for automatic transmissions). Operate engine to normal operating\
temperature. Turn ignition off. Install timing light.
2) Connect tachometer. Disconnect and plug vacuum hose at
distributor. Disconnect vacuum switch assembly wire connector, located
on top of valve cover. Start engine and increase engine speed to 1600
RPM.
3) Compare timing to specification. See
IGNITION TIMING SPECIFICATIONS table. To adjust timing, loosen
distributor clamp bolt and turn distributor. Recheck timing after
clamp bolt is tightened.
Alternate Timing Procedure
1) Apply parking brake and place transmission in Neutral
(Drive for automatic transmissions). Operate engine to obtain normal
operating temperature. Turn ignition off. Install timing light.
2) Disconnect and plug vacuum switch hose (Red and Black

wires connected to switch). Disconnect distributor vacuum advance hose
and connect hose to vacuum switch. Disconnect wire connector from
knock sensor, located in cylinder head.
3) Using a jumper wire, ground knock sensor wire connector to
engine block. Start engine. With engine at idle speed (solenoid
energized), check timing. Adjust timing to one degree more than
specification (if required). See Emission Control Label.
IGNITION TIMING SPECIFICATIONS (DEGREES BTDC@RPM) - 4.2L









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Application Man. Trans. Auto. Trans.
50 State ............. 9 @ 1600 ............... 9 @ 1600
High Altitude ........ 16 @ 1600 ............. 16 @ 1600









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HOT (SLOW) IDLE RPM
4.0L
1) Apply parking brake and place transmission in Neutral
(Drive for automatic transmissions). Operate engine to normal
operating temperature. Turn ignition off.
2) Ensure all accessories are off. Connect positive
tachometer lead to terminal "D1-1" and negative lead to "D2-7" of
diagnostic connectors. See Fig. 3. Disconnect air stepper motor
connector and TPS wiring connector. See Fig. 4.
Fig. 3: Diagnostic Connector Terminal Identification
Courtesy of Chrysler Motors.

Fig. 4: Installing Air Stepper Motor Exerciser
Courtesy of Chrysler Motors.
3) Install Air Stepper Motor Exerciser (Ele.CT01) on the air\
stepper motor connector. See Fig. 4. Install the Red terminal of the
air stepper exerciser on the positive battery post and the Black
terminal on the negative battery terminal.
4) Start engine and place stepper motor exerciser switch on
the "LOW" position and note engine RPM. Idle speed should be 750F50
RPM. Disconnect and remove air stepper motor exerciser.
5) If adjustment is required, turn ignition off. Remove the
welch plug from the RPM adjusting screw cavity. See Fig. 5. Turn RPM
adjusting screw inward 2 turns. Start the engine and note engine RPM.

Fig. 5: Adjusting Idle RPM
Courtesy of Chrysler Motors.
6) Adjust idle RPM screw to obtain idle speed of 750F50 RPM.
Once correct idle RPM is obtained, seal the welch plug cavity with RTV
sealant. Reconnect air stepper motor and TPS.
4.2L
1) Warm engine to normal operating temperature. Apply parking
brake. Place automatic transmission in Drive (Neutral on manual
transmission). Disconnect and plug vacuum hose from vacuum actuator.
Disconnect solenoid wire connector.
2) Adjust curb idle screw to obtain correct curb idle. See
CURB IDLE SPEED (RPM) table. Apply 10-15 in. Hg vacuum to vacuum
actuator. With throttle positioner fully extended, adjust screw on
throttle lever, to set vacuum actuator RPM to specification. See
VACUUM ACTUATOR & SOLENOID IDLE (RPM) table. Disconnect vacuum pump.
3) Apply battery voltage to solenoid with a jumper wire. Turn
A/C on (if equipped). Open throttle, allowing solenoid to fully
extend. Adjust solenoid adjusting screw to obtain solenoid idle RPM.

See VACUUM ACTUATOR & SOLENOID IDLE (RPM) table. Reconnect solenoid
connector and vacuum hose.
CURB IDLE SPEED (RPM) TABLE - 4.2L









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Application Man. Trans. Auto. Trans.
50 State ................ 680 ...................... 600
High Altitude ........... 700 ...................... 650









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VACUUM ACTUATOR & SOLENOID IDLE (RPM) TABLE - 4.2L








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Application Vacuum Actuator Solenoid Energized
Man. Trans. ............ 1100 ...................... 900
Auto. Trans. ............ 900 ...................... 800









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Fig. 6: Adjusting 4.2L Idle RPM
Courtesy of Chrysler Motors.
NOTE: Idle mixture adjustment is not part of a normal tune-up. DO
NOT adjust mixture unless carburetor has been disassembled

or vehicle fails emissions testing.
IDLE MIXTURE (TACHOMETER (LEAN DROP) PROCEDURE)
NOTE: On 4.2L engines, ensure idle speed and timing are set prior
to adjusting the idle mixture. If mixture adjustment time
exceeds 3 minutes, run engine at 2000 RPM in Neutral for one
minute, and resume adjustment. On 4.0L engines, idle mixture
adjustment is not possible.
4.2L
1) Remove carburetor and locate roll pins blocking idle
mixture screws. Drill through throttle body on closed end of roll pin
hole. Drive pins out with punch. Reinstall carburetor. Install
tachometer.
2) Operate engine to normal operating temperature, and adjust
curb idle speed. Place automatic transmission selector in Drive
(Neutral for manual transmissions). Turn mixture screws inward until
RPM drops. Turn screws outward until highest RPM is reached.
3) Turn mixture screws inward to obtain the correct decrease
in RPM. See LEAN DROP (RPM) table. Adjust both screws equally. When
mixture is correctly adjusted, replace roll pin to block adjustment
screws.
NOTE: If final RPM differs more than 30 RPM from specified curb
idle speed, reset curb idle, and repeat mixture adjustment.
LEAN DROP (RPM) TABLE









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Application Man. Trans. Auto. Trans.
4.2L .................... 50 ........................ 50









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THROTTLE POSITION SENSOR (TPS)
NOTE: Adjustment of TPS only applies to the 4.0L models. It may be
necessary to remove throttle body from intake manifold, to
access sensor wiring harness.
Checking & Adjusting - 4.0L (Automatic Transmission)
1) Locate the square TPS connector. Note connector terminal
identification stamped on the back of the connector. Turn ignition on.
2) Connect voltmeter through back of wiring harness
connector. Connect negative voltmeter lead to terminal "D" and
positive voltmeter lead to terminal "A" to check input voltage. DO NOT
disconnect TPS connector.
3) Hold throttle plate closed against idle stop and note
voltage. Input voltage should be approximately 5 volts. Disconnect
voltmeter positive lead and connect to terminal "B" to measure output
voltage.
4) With throttle plate closed, measure the output voltage.
The output voltage should be approximately 4.2 volts. If output
voltage is not within specification, loosen TPS retaining screws.
5) Partially tighten one retaining screw. Rotate TPS to
obtain correct output voltage. Tighten retaining screws once correct
voltage is obtained.
Checking & Adjusting - 4.0L (Manual Transmission)
1) Turn ignition on. Connect voltmeter through back of wiring
harness connector. Connect negative voltmeter lead to terminal
"B" and positive voltmeter lead to terminal "A". DO NOT disconnect TPS
connector.

2) Hold throttle plate in the closed throttle position
against idle stop and note input voltage reading. Input voltage should
be approximately 5.0 volts.
3) Disconnect positive lead from terminal "A" and connect to
terminal "C" to check output voltage. Output voltage should be checked
with throttle plates fully closed.
4) Output voltage should be approximately 0.8 volts. If
output voltage is not within specification, loosen TPS bottom
retaining screw and pivot sensor for a large adjustment or top
retaining screw for a fine adjustment.
5) Adjust sensor to obtain correct output voltage. Tighten
retaining screws. Remove voltmeter.
COLD (FAST) IDLE RPM
4.2L
Disconnect and plug EGR valve vacuum hose. With engine
running at normal operating temperature, place fast idle screw on
second step of fast idle cam and against shoulder of high step. Turn
screw to adjust fast idle speed.
FAST IDLE SPEED (RPM) TABLE









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Application Man. Trans. Auto. Trans.
4.2L ................... 1700 ..................... 1700









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AUTOMATIC CHOKE SETTING
Choke coil cover is riveted in place and no adjustment is
necessary or possible.
SERVICING
EMISSION CONTROL
See EMISSIONS section.
SPECIFICATIONS
IGNITION
Distributor
All vehicles use a Motorcraft breakerless solid state
distributor.
PICK-UP COIL RESISTANCE TABLE - OHMS @ 75
F (24C)








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Application Specification
All Models ....................................... 400-800









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TOTAL SPARK ADVANCE TABLE @ 2000 RPM








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Application W/ Vac. Advance W/O Vac. Advance
4.0L ................ N/A .......................... N/A
4.2L ............... 30.5
 ................... 7.5-12.5

(1) - Information not available from manufacturer.








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IGNITION COIL
IGNITION COIL OUTPUT TABLE @ 1000 RPM








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Application Output
All Models .................................. 24KV Minimum









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IGNITION COIL RESISTANCE TABLE








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Temperature Primary Ohms Secondary Ohms
75
F (24C) .......... 1.13-1.23 ............. 7700-9300
200F (93C) ............ 1.5 ................... 12,000








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FUEL SYSTEM
CARBURETORS & FUEL INJECTION TABLE








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Application Model
4.0L ................................................ MPFI
4.2L ................................... Carter BBD 2-Bbl.









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BATTERY
BATTERY SPECIFICATIONS TABLE








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Application Cold Cranking Reserve Capacity
** ( 1) Amps Minutes
Standard ................ 421 ....................... 75
Optional ................ 452 ....................... 81
( 1) - At 0
F (-18C).








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STARTER
All models equipped with Mitsubishi starters.
STARTER SPECIFICATIONS TABLE









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Application Volts Amps Test RPM
All Models ...... 11.22 .......... 80 ........... 2500









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ALTERNATOR
All models use Delco-Remy solid state alternators with
internal voltage regulator.
ALTERNATOR SPECIFICATIONS TABLE









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Application Field Current Rated