1988 JEEP CHEROKEE 6 cylinder

Bolt loose .............. A ........... Require tightening or
replacement of bolts.
Bolt missing ............ C ... Require replacement of bolts.
Corroded, affecting
sealability ............ A .. Require repair or replacement.
Cylinder head threads
stripped ............... A ... Require repair or replacement
of cylinder head.
Gasket leaking .......... A ........... Require tightening or
replacement of gasket.
Heat stove bent ......... B ........... (1) Require repair or
replacement of stove.
Heat stove broken ....... A ......... ( 1) Require replacement
of stove.
Heat stove corroded,
affecting structural
integrity .............. 1 ......... ( 1) Suggest replacement
of stove.
Heat stove missing ...... C ......... ( 1) Require replacement
of stove.
Manifold broken ......... A .. Require repair or replacement.
Manifold cracked ........ B .. Require repair or replacement.
Manifold warped ......... A .. Require repair or replacement.
Out of specification .... B .. Require repair or replacement.
Stud broken ............. A .... Require replacement of stud.
Stud missing ............ C .... Require replacement of stud.
Stud threads damaged .... A ... Require repair or replacement
of stud.
Stud threads stripped
(threads missing) ...... A .... Require replacement of stud.
( 1) - Stove may not be available separately; this may require
replacement of manifold.









\









\









\









\









\









\









\


MECHANICAL EFE DEVICES
See HEAT RISERS (MECHANICAL EFE DEVICES) .
MUFFLERS AND RESONATORS
MUFFLER AND RESONATOR INSPECTION








\









\









\









\









\









\









\


Condition Code Procedure
Body shell distorted,
affecting performance or
structural integrity ... A ............ Require replacement.
Corrosion hole .......... A ............ Require replacement.
Missing ................. C ............ Require replacement.
Mounting bracket broken . A .. Require repair or replacement.
Mounting bracket
cracked ................ B .. Require repair or replacement.
Nipple cracked .......... A .. Require repair or replacement.
Nipple loose ............ B ............ Require replacement.
Outer wrap peeling
(exhaust not leaking) .. 1 ............ Suggest replacement.
Plugged ................. A ............ Require replacement.
Puncture (other than a
drain hole) ............ A ............ Require replacement.
Rattling or knocking noise
from inside muffler .... B ............ Require replacement.
Seam open (exhaust

ECU is a digital microprocessor computer. Data sensors
provide the ECU with engine operating information in varying
electrical signals. ECU calculates this information and corrects
air/fuel ratio, ignition timing, and emission control as needed to
maintain efficient engine operation. Other ECU output signals control
upshift indicator light (manual transmission only), ignition module
dwell and A/C clutch operation.
UPSHIFT INDICATOR
On vehicles equipped with a manual transmission, ECU
controls upshift indicator light. Indicator light is normally
illuminated when ignition is turned on without engine running.
Indicator light is turned off when engine is started.
Indicator light will be illuminated during engine operation
in response to engine load and speed. If transmission is not shifted,
ECU will turn light off after 3 to 5 seconds. A switch located on
transmission prevents indicator light from being illuminated when
transmission is shifted to highest gear.
DATA SENSORS & SWITCHES
Manifold Absolute Pressure (MAP) Sensor
MAP sensor is located in engine compartment on firewall,
behind engine. MAP sensor monitors manifold vacuum via a vacuum line
from intake manifold to sensor.
MAP sensor supplies an electrical signal which keeps ECU
informed of manifold vacuum and barometric pressure conditions. This
information is combined with data supplied by other sensors to
determine correct air/fuel ratio.
Oxygen Sensor
Oxygen (O2) sensor is mounted in exhaust manifold where it
is exposed to exhaust gas flow. Its function is to monitor oxygen
content of exhaust gases and to supply ECU with a voltage signal
directly proportional to this content.
If oxygen content of exhaust gases is high (lean air/fuel
mixture), voltage signal to ECU is low. As oxygen content decreases
(mixture becomes richer), signal voltage increases.
In this way, ECU is kept constantly informed of air/fuel
ratio. ECU can then alter fuel injector "on" time, in response to
these signals, to obtain best air/fuel ratio of 14.7:1 under any
given operating conditions.
O2 sensor is equipped with a heating element that keeps
sensor at proper operating temperatures. Maintaining correct sensor
temperatures at all times guarantees a more accurate signal to ECU.
By using an O2 heater, fuel control system may also enter "closed
loop" operating mode sooner and maintain this mode, even during
periods of extended idle.
Temperature Sensors
There are 2 temperature sensors used on this system.
Manifold Air Temperature (MAT) sensor, mounted in intake manifold,
measures temperature of incoming air/fuel mixture and Coolant
Temperature Sensor (CTS), located on left side of cylinder block just
below the exhaust manifold, measures temperature of engine coolant.
Information provided by these 2 sensors to ECU allows ECU to
demand slightly richer air/fuel mixtures and higher idle speeds
during cold engine operation.
Throttle Position Sensor (TPS)
TPS is regulated by movement of throttle shaft. It is

mounted on throttle body and senses angle of throttle blade opening.
A voltage signal of up to 5 volts at wide open throttle is
produced by TPS. Voltage varies with throttle angle changes. This
signal is transmitted to ECU where it is used to adjust air/fuel
ratio during acceleration, deceleration, idle, and wide open throttle
conditions.
A dual TPS is used on models with automatic transmissions.
This dual TPS not only provides ECU with input voltages but also
supplies automatic transmission control unit with input signals
relative to throttle position.
Knock Sensor
Knock sensor (detonation sensor) is located on lower left
side of cylinder block just above oil pan. Knock sensor picks up
detonation vibration from engine and converts it to an electrical
signal for use by ECU.
ECU uses this information to determine when a change in
ignition timing is required. Knock sensor allows for engine operation
on either "premium" unleaded or "regular" unleaded fuel.
When knock occurs, ECU retards ignition timing in one or
more cylinders until detonation is eliminated.
Speed Sensor
Speed sensor is secured by special shouldered bolts to
flywheel/drive plate housing. Speed sensor is nonadjustable and
preset at factory. Speed sensor senses TDC and engine speed by
detecting flywheel teeth as they pass pick-up coil during engine
operation. See Fig. 2.
Flywheel has a large trigger tooth and notch located 12
small teeth before each TDC position. When a small tooth and notch
pass the magnetic core in sensor, concentration and collapse of the
magnetic field created induces a small voltage spike into sensor
pick-up coil windings. These small voltage spikes are sent to ECU,
allowing ECU to count the teeth as they pass sensor.
When a large tooth and notch pass magnetic core in sensor,
increased concentration and collapse of the magnetic field induces a
higher voltage spike than smaller teeth. Higher spike indicates to
ECU that a piston will soon be at TDC position, 12 teeth later.
Ignition timing for cylinder is either advanced or retarded by ECU
based upon "sensor input".
Fig. 2: Speed Sensor Operation
Courtesy of Chrysler Motors.
Engine Switches
Several switches provide operating information to ECU. These

3) Voltage should be 4.5-5.5 volts. Same voltage should also
be at terminal C-14 of ECU wire harness connector. Using Diagnostic
Tester (M.S. 1170), test MAP sensor ground circuit at terminal D-3
and terminal "A" of sensor connector.
4) Using an ohmmeter, test MAP sensor ground circuit at ECU
connector between terminal D-3 of ECU connector and terminal B-11
with an ohmmeter.
5) If ohmmeter or diagnostic tester indicates an open
circuit, inspect for a defective sensor ground connection, located on
right side of cylinder block. If ground connection is good, ECU may
need to be replaced.
Fig. 5: MAP Sensor Test Points
Courtesy of Chrysler Motors.
O2 Sensor Heating Element Test
Disconnect O2 sensor connector. Connect an ohmmeter to
terminals "A" and "B" only (marked on the connector) of O2 sensor
connector. Resistance should be between 5 and 7 ohms. Replace sensor
if ohmmeter indicates an infinity reading.
CTS Test

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
\
\
\
\
\
\
\


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
\
\
\
\
\
\
\

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

See FUEL INJECTOR TEST PROCEDURE chart in this article.
REMOVAL & INSTALLATION
COOLANT TEMPERATURE SENSOR (CTS)
Removal & Installation
Drain cooling system. Remove air cleaner assembly.
Disconnect CTS wire connector. Remove CTS from engine block. Install
replacement CTS and connect CTS wire connector. Install air cleaner
assembly. Fill cooling system.
OXYGEN SENSOR
Removal & Installation
Raise and support vehicle. Disconnect O2 sensor wire
connector. Remove O2 sensor from exhaust manifold. Install O2 sensor
and tighten to 30 ft. lbs. (41 N.m). Connect O2 sensor wire connector
and lower vehicle.
KNOCK SENSOR
Removal & Installation
Raise and support vehicle. Disconnect knock sensor wire
connector. Remove knock sensor from cylinder block. Install knock
sensor and connect knock sensor wire connector. Lower vehicle.
SPEED SENSOR
Removal & Installation
Disconnect speed sensor wire connector. Remove 2 speed
sensor retaining bolts at transmission housing. Install speed sensor
and connect speed sensor wire connector.
STARTER MOTOR RELAY
Removal & Installation
Disconnect negative battery cable. Identify, tag and
disconnect wiring to relay. Remove relay retaining screws and remove
relay from inner fender panel. Install replacement relay and connect
relay wires. Connect negative battery cable. Test relay operation.
MANIFOLD AIR/FUEL TEMPERATURE (MAT) SENSOR
Removal & Installation
Disconnect wire connector from MAT sensor. Remove MAT sensor
from intake manifold. To install, reverse removal procedure.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
Removal & Installation
Disconnect wire connector, vacuum hose, and retaining nuts
from MAP sensor. Remove sensor from firewall. To install, reverse
removal procedure.
FUEL INJECTOR
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.

GEN ER AL C O OLIN G S YSTE M S ER VIC IN G

1988 J e ep C hero ke e
GENERAL INFORMATION
General Cooling System Servicing
* PLEASE READ THIS FIRST *
The following article is for general information only.
Information may not apply to all years, makes and models. See specific
article in the ENGINE COOLING section.
DESCRIPTION
The basic liquid cooling system consists of a radiator, water
pump, thermostat, electric or belt-driven cooling fan, pressure cap,
heater, and various connecting hoses and cooling passages in the block
and cylinder head.
MAINTENANCE
DRAINING
Remove radiator cap and open heater control valve to maximum
heat position. Open drain cocks or remove plugs in bottom of radiator
and engine block. In-line engines usually have one plug or drain cock,
while "V" type engines will have 2, one in each bank of cylinders.
CLEANING
A good cleaning compound removes most rust and scale. Follow
manufacturer's instructions in the use of cleaner. If considerable
rust and scale has to be removed, cooling system should be flushed.
Clean radiator air passages with compressed air.
FLUSHING
CAUTION: Some manufacturers use an aluminum and plastic radiator.
Flushing solution must be compatible with aluminum.
Back Flushing
Back flushing is an effective means of removing cooling
system rust and scale. The radiator, engine and heater core should be
flushed separately.
Radiator
To flush radiator, connect flushing gun to water outlet of
radiator and disconnect water inlet hose. To prevent flooding engine,
use a hose connected to radiator inlet. Use air in short bursts to
prevent damage to radiator. Continue flushing until water runs clear.
Engine
To flush engine, remove thermostat and replace housing.
Connect flushing gun to water outlet of engine. Flush using short air
bursts until water runs clean.
Heater Core
Flush heater core as described for radiator. Ensure heater
control valve is set to maximum heat position before flushing heater.

complete cooling system. Test components as follows, following tool
manufacturer's instructions.
Radiator Cap
Visually inspect radiator cap, then dip cap into water and
connect to tester. Pump tester to bring pressure to upper limit of cap
specification. If cap fails to hold pressure, replace cap.
Fig. 2: Testing Radiator Pressure Cap
Cooling System
1) With engine off, wipe radiator filler neck seat clean.
Fill radiator to correct level. Attach tester to radiator and pump
until pressure is at upper level of radiator rating.
2) If pressure drops, inspect for external leaks. If no leaks
are apparent, detach tester and run engine until normal operating
temperature is reached. Reattach tester and observe. If pressure
builds up immediately, a possible leak exists from a faulty head
gasket or crack in head or block.
NOTE: Pressure may build up quickly. Release any excess pressure
or cooling system damage may result.
3) If there is no immediate pressure build up, pump tester to
within system pressure range (on radiator cap). Vibration of gauge
pointer indicates compression or combustion leak into cooling system.
Isolate leak by shorting each spark plug wire to cylinder block. Gauge
pointer should stop or decrease vibration when leaking cylinder is
shorted.