1988 JEEP CHEROKEE coolant temperature

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.

NOTE: When diagnosing fuel system problems using following
procedures, no specialized service equipment is needed.
Following diagnostic procedures are NOT applicable if
special tester M.S. 1700 is used.
Six different test flow charts are used to fully evaluate
fuel injection system:
TEST 1: IGNITION OFF
This test checks that system power provides for ECU memory
keep-alive voltage.
TEST 2: IGNITION ON: POWER
This test checks system power function and fuel pump power
function.
TEST 3 & 3A: IGNITION ON: INPUT
These tests check the following components and their
circuits: closed throttle (idle) switch, Throttle Position Sensor
(TPS), MAP sensor, A/T gear selector switch, Coolant Temperature
Sensor (CTS) and MAT sensor. Coolant temperature and MAT sensors are
tested in cold condition. This procedure also checks all interrelated
wiring circuits as well.
TEST 4 & TEST 4A: SYSTEM OPERATIONAL
These tests check engine start-up circuit, fuel injector,
"Closed Loop" air/fuel mixture function, coolant temperature sensor
function, MAT sensor function, detonation sensor "Closed Loop"
ignition retard/advance function, EGR valve and canister purge
solenoid function, idle speed control and A/C control functions.
TEST 5: BASIC ENGINE
This test indicates failures in related engine components
that are not part of fuel injection system.
TEST 6: MANUAL TRANSMISSION UPSHIFT
This test checks upshift indicator light function on vehicles
with manual transmissions.
Fig. 5: Jeep/Renix Fuel Injection ECU Connector
Courtesy of Chrysler Motors.
REMOVAL & INSTALLATION

COOLANT TEMPERATURE SENSOR (CTS)
Removal & Installation
Allow engine to cool down. Disconnect CTS wiring harness.
Remove CTS from intake manifold and rapidly plug hole to prevent
coolant loss. Install replacement CTS and connect CTS wiring harness.
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.
Removal
Remove air cleaner assemby. Remove injector wiring
connector. Remove injector retainer screws and clip. Using small
pliers, carefully grasp center collar of injector between electrical
terminals and carefully remove injector with lifting/twisting motion.
Discard both "O" rings. See Fig. 6.
Installation
1) Using light oil, lubricate new upper and lower "O" rings.
Install "O" rings in housing bore. Install back-up ring over upper
"O" ring. Position replacement injector in fuel body.
2) Center nozzle in lower housing bore and use a
pushing/twisting motion to seat injector. Align wire connectors in
proper orientation. Install retainer clip and screws. Connect
injector wiring. Install air cleaner.
Fig. 6: Fuel Injector & Throttle Body Assembly
Courtesy of Chrysler Motors.
FUEL PRESSURE REGULATOR

REFILLING
To prevent air from being trapped in engine block, engine
should be running when refilling cooling system. After system is full,
continue running engine until thermostat is open, then recheck fill
level. Do not overfill system.
TESTING
THERMOSTAT
1) Visually inspect thermostat for corrosion and proper
sealing of valve and seat. If okay, suspend thermostat and thermometer
in a 50/50 mixture of coolant and water. See Fig. 1. Do not allow
thermostat or thermometer to touch bottom of container. Heat water
until thermostat just begins to open.
2) Read temperature on thermometer. This is the initial
opening temperature and should be within specification. Continue
heating water until thermostat is fully open and note temperature.
This is the fully opened temperature. If either reading is not to
specification, replace thermostat.
Fig. 1: Testing Thermostat in Anti-Freeze/Water Solution
PRESSURE TESTING
A pressure tester is used to check both radiator cap and

HEA TE R S YSTE M

1988 J e ep C hero ke e
1988 HEATER SYSTEMS
Jeep
DESCRIPTION
Jeep vehicles use a blend-air type heater system. Wrangler
models have a constant flow type system with engine coolant
continuously flowing through heater core. Cherokee and Comanche use
heater control valves. Coolant flow to heater core on these models can
be stopped or diverted.
On all models, temperature of heated air entering passenger
compartment is controlled by regulating amount of air flowing through
heater core and then blending it with a controlled amount of cool air
by-passing heater core.
CONTROL PANEL OPERATION
Control panel on all models includes fan switch, sliding
temperature control lever and sliding mode lever.
DEFROSTER CONTROL
Defroster control operates heater housing door regulating
heater and defroster operation by directing flow of air through
defroster hose or floor outlets. When control knob is pushed in,
blended air will enter passenger compartment through floor heat duct.
When control is pulled completely out, door directs all heated air to
windshield defroster outlets. Any intermediate position divides air
flow between windshield and floor outlets.
TEMPERATURE CONTROL LEVER (CHEROKEE, COMANCHE & WRANGLER)
Lower control lever operates blend-air door in heater core
housing. At full right position, all air is directed through heater
core, providing maximum heat flow. At full left position, all air is
directed around heater core providing fresh air. Control can be set in
any intermediate position to provide a blend of heated and unheated
air. With control lever in "COOL" position, water valve will close
(except Wrangler). Mode control lever must be in "HEAT" or defroster
mode before any air can enter vehicle.
MODE CONTROL LEVER (CHEROKEE & COMANCHE)
Mode control lever includes "BI-LEVEL", "VENT", "HEAT" and
"OFF" positions. At far end of scale, a symbol for defroster indicates
defrost position. In "BI-LEVEL" position, a mixture of floor heat and
defroster air is obtained.
FAN CONTROL
Fan control is a 4-position control switch, regulating blower
motor and air flow for heat and defrost. Switch has "LOW", "HIGH" and
2 intermediate positions. Fan will remain on unless mode lever is
placed in "OFF" position.
CONTROL CABLES ADJUSTMENT

Fig. 4: View of Manifold Absolute Pressure (MAP) Sensor
Courtesy of Chrysler Motors.
COOLANT TEMPERATURE SENSOR (CTS)
The coolant temperature sensor is installed in the engine
water jacket on the left side of the engine. See Fig. 5. It provides
an input voltage to the ECU. As coolant temperatures vary, the Coolant
Temperature Sensor resistance changes, resulting in a different input
voltage to the ECU. The ECU calculates this information and adjusts
the following:
* Adjust fuel injector pulse width. Colder coolant

temperatures will result in longer injector pulse width
and richer air-fuel mixtures.
* Compensate for fuel condensation in the intake manifold.
* Control engine warm-up idle speed.
* Increase ignition advance when the coolant is cold.
* Energize the EGR valve solenoid, thus preventing the flow
of vacuum to the EGR valve.
Fig. 5: Location of Coolant Temperature Sensor (CTS)
Courtesy of Chrysler Motors.
MANIFOLD AIR TEMPERATURE (MAT) SENSOR
The Manifold Air Temperature (MAT) sensor is installed in the\
intake manifold with the sensor element extending into the air-fuel

If connector locking tabs weaken or break, press together
firmly and bind with electrical tape or a harness tie strap to assure
good connection.
COMPONENT LOCATIONS
COMPONENT LOCATIONS








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Component Location
Coolant Temperature
Sensor (CTS) .................... On left side of engine
Engine Speed Sensor .... Attached to flywheel cover housing
Ignition Control Module ......... Mounted to ignition coil
Knock Sensor ................ On lower left side of engine
Manifold Absolute Pressure
(MAP) Sensor ................ Under the hood, on firewall
Manifold Air Temperature
(MAT) sensor .......................... On intake manifold
Sync Pulse Signal Generator ........... Inside distributor









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TESTING
BASIC DIAGNOSTIC PROCEDURES
This section contains information for determining individual
system component performance. Diagnosis of ECU/Engine Control System
is performed using the DRB-II Diagnostic Tester. See the appropriate
Computerized Engine Control System article in this section.
COOLANT TEMPERATURE SENSOR (CTS) TEST
1) Disconnect the CTS wire harness connector from the CTS.
See Fig. 13 .
Fig. 13: Location of Coolant Temperature Sensor (CTS)
Courtesy of Chrysler Motors.
2) Using a high input impedance (digital) volt-ohmmeter, tes\
t
the resistance of the sensor. Resistance should be less than l000 ohms