1988 JEEP CHEROKEE instrument panel

emission control devices. This permits optimum engine performance
with minimum emissions.
OPERATION
The engine control system is divided into 6 sub-systems:
electronic control unit (also called the ECU or computer), sensors and\
switches, fuel control, emission control, idle speed control, and
ignition advance control.
ELECTRONIC CONTROL UNIT (ECU)
The ECU is located under the instrument panel, above the
accelerator pedal. It receives information from the 13 engine sensors
or switches to determine engine operating conditions at any particular
moment. The ECU responds to these signals by sending a control signal
to the fuel injector, fuel pump, ignition control module, idle speed
actuator (ISA) motor, EGR solenoid, and canister purge solenoid. It
also controls the Load Swap relay, and on Man. Trans. models, the up-
shift indicator lamp.
SENSORS & SWITCHES
Exhaust Gas Oxygen (EGO) Sensor
The amount of oxygen in exhaust gases varies according to the
air/fuel ratio of the intake charge. The exhaust gas oxygen sensor,
located in the exhaust pipe, detects this content and transmits a low
voltage signal to the ECU.
The outer surface of the sensor is exposed to exhaust gases,
the inner surface to outside air. The difference in the amount of
oxygen contacting the inner and outer surfaces of the sensor creates a
pressure, which results in a small voltage signal. This signal, which
is a measure of the unburned oxygen in the exhaust gas, is transmitted
to the ECU.
If the amount of oxygen in the exhaust system is low (rich
mixture), the sensor voltage signal will be high. If the mixture is
lean, the oxygen sensor will generate a low voltage signal.
The sensor has a heating element that keeps the sensor at
proper operating temperature during all operating modes.
Manifold Air/Fuel Temperature (MAT) Sensor
The manifold air/fuel temperature sensor is installed in the
intake manifold. This sensor provides a voltage signal to the ECU
representing the temperature of the air/fuel mixture in the intake
manifold. The ECU compensates for air density changes during high
temperature operation.
Coolant Temperature Sensor (CTS)
The coolant temperature sensor is located in the intake
manifold coolant jacket. This sensor provides a voltage signal to the
ECU. The ECU uses this signal to determine engine temperature. During
cold engine operation, the ECU responds by enriching the air/fuel
mixture delivered to the injector, compensating for fuel condensation
in the intake manifold, controlling engine warm-up speed, increasing
ignition advance, and inhibiting operation of the EGR system.
Manifold Absolute Pressure (MAP) Sensor
The MAP sensor detects absolute pressure in the intake
manifold as well as ambient atmospheric pressure. This information is
supplied to the ECU, through voltage signals, as an indication of
engine load. The sensor is attached to the plenum chamber near the
hood latch. A vacuum line from the throttle body supplies the sensor

These small voltage spikes give the ECU information for calculating
engine speed.
When a large tooth and notch pass the magnetic core of the
sensor, the concentration and collapse of the magnetic field created
induces a higher voltage spike into the sensor pick-up coil windings.
This higher voltage spike indicates to the ECU that a piston will soon
be at TDC (12 teeth later).
The ECU uses speed sensor information for advancing or
retarding ignition timing, dependent upon input from other sensors and
switches.
A/C "ON" Switch
This switch signals the ECU that the A/C system is energized.
This signal is used by the ECU to increase idle speed.
Starter Motor Relay
Although no switch or sensor is used, the ECU requires this
information for fuel enrichment during starting. This information is
passed on the the ECU through the starter motor relay.
Distributor Reference Signal
Although no switch or sensor is used, the ECU requires this
information for ignition timing and fuel supply. This information is
relayed to the ECU through a "synch pulse" provided by the distributor
stator. This allows the ECU to synchronize injector opening with
intake valve opening.
ELECTRONIC CONTROL MODULE (ECM)
The Electronic Control Unit (ECU) is the "brain" of the
engine control system. The ECU is located in the passenger
compartment, under left side of dash to right of steering column.
The ECU senses engine operating conditions, processes input
signals, and controls the various systems that affect vehicle
performance.
DIAGNOSTICS
The engine control system has diagnostic capabilities. Using
System Tester (M.S. 1700), diagnostics can be performed on the system
to more accurately determine faulty components. The diagnostic
connectors are located in the engine compartment at the right shock
tower (or under left side of instrument panel).
NOTE: Information for diagnosing the engine control system using
the System Tester (M.S. 1700) was not available from
manufacturer. Only component testing is provided.
ELECTRONIC SPARK ADVANCE
Based upon engine operating conditions received from the
sensors and switches, the ECU controls spark timing. The ECU triggers
the ignition coil through the ignition control module.
IDLE SPEED CONTROL
Idle speed is controlled by the ECU through the idle speed
stepper motor. A latch relay is used to program the stepper motor for
engine start-up. The relay is located on the right inner fender panel.
See Fig. 1 . The ECU energizes the latch relay when engine is in
cranking mode, and keeps the relay energized for 3-5 seconds after the
engine is stopped. In this way, the stepper motor can provide the










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g grams 

GND or GRND Ground 

GRN Green 

GRY Gray 

Ga. Gauge 

Gals. gallons 

Gov. Governor 











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"H" ABBREVIATION TABLE
"H" ABBREVIATION TABLE







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ABBREVIATION DEFINITION 










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H/D Heavy Duty 

HAC High Altitude Compensation 

HC Hydrocarbons 

HEDF High Speed Electro Drive Fan relay or circuit 

HEGO Heated Exhaust Gas Oxygen Sensor 

HEGOG HEGO Ground circuit 

HEI High Energy Ignition 

HLDT Headlight 

HO High Output 

HP High Performance 

HSC High Swirl Combustion 

HSO High Specific Output 

HTR Heater 

HVAC Heating 

Headlt. Headlight 

Hg Mercury 

Hgt. Height 

Htr. Heater 

Hz Hertz (Cycles Per Second) 











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"I" ABBREVIATION TABLE
"I" ABBREVIATION TABLE







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ABBREVIATION DEFINITION 










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I.D. Inside Diameter 

IAC Idle Air Control 

IACV Idle Air Control Valve 

IC Integrated Circuit 

ID Identification 

IDM Ignition Diagnostic Monitor 

IGN Ignition system or circuit 

ILC Idle Load Compensator 

In. Hg Inches of Mercury 

INCH Lbs. Inch Pounds 

INFL REST Inflatable Restraint 

INJ Injector or Injection 

IP Instrument Panel 

IPC Instrument Panel Cluster 

ISA Idle Speed Actuator 

ISC Idle Speed Control 

ISS Idle Stop Solenoid 

ITS Idle Tracking Switch 

IVSV Idle Vacuum Switching Valve 

\003
A/C -H EA TE R S YSTE M - M AN UAL

1988 J e ep C hero ke e
1988 Manual A/C-Heater Systems
JEEP
* PLEASE READ THIS FIRST *
CAUTION: When discharging air conditioning system, use only approved
refrigerant recovery/recycling equipment. Make every attempt
to avoid discharging refrigerant into the atmosphere.
DESCRIPTION
On Cherokee, Comanche and Wagoneer, the Climate Control
System is an integral assembly combining air conditioning, heating and
ventilating capabilities for models with air conditioning. The A/C
registers are built into instrument panel.
On Wrangler models, a dual flow unit with the capability of
recirculating air or drawing air from outside is used. It incorporates
under-dash registers and uses a blend-air type system.
The A/C components include a compressor, condenser, receiver-
drier, a sight glass (in the receiver-to-evaporator line) and
evaporator housing. Evaporator housing contains evaporator core,
expansion valve, thermostat, wiring and drain tube. Blower motor and
fan are located in blower housing, which is attached to evaporator
housing.
Vents provided in evaporator housing are adjustable to direct
air to different positions in vehicle's interior. System controls
include fan and temperature control knobs built into lower evaporator
housing assembly or temperature and mode levers on control panel
integrated with instrument panel.
OPERATION
A/C SYSTEM CONTROLS
Temperature & Fan Control Knobs
(Wrangler)
The right rotary control knob allows selection of desired
temperature. Knob has "OFF" "1", "2" and "3" positions for different
levels of cooling. The left rotary fan switch (knob) controls blower
motor speed. Knob has "OFF", "LOW", "MED" and "HI" positions. Fan will
operate unless mode control lever is in "OFF" position.
The A/C fan switch is a 3-speed unit used in conjunction with
a blower motor resistor. Fan switch controls low, medium and high
speed operation. Switch may be serviced by removing access plate
located on lower evaporator core housing, below control panel.
The A/C temperature control switch has a thermostat unit
built-in. Cooling adjustment is done by turning knob in a clockwise
rotation. For fast, efficient system operation in hot weather, vehicle
should be purged of hot air by driving about 2-3 city blocks with at
least one window open. During this time, temperature control knob
should be rotated clockwise to "MAX" position and fan control knob
placed in "HI" position. This allows evaporator to pre-cool and avoid
typical first blast of warm air.
Temperature Control Panel
(Cherokee, Comanche & Wagoneer)
The upper temperature control (mode) lever includes "MAX",
"NORM", "BI-LEVEL", "VENT", "HEAT" and "OFF" positions. At the far end

Discharge system (slowly to prevent loss of compressor oil)
using approved refrigerant recovery/recycling equipment. Disconnect
evaporator and condenser lines from receiver-drier. Detach mounting
screws from receiver-drier bracket. Remove receiver-drier.
Installation
To install, reverse removal procedure. Evacuate, leak test,
recharge. Check A/C system for proper operation.
Fig. 1: Exploded View of Wrangler Evaporator Housing
Courtesy of Chrysler Motors.
EVAPORATOR-BLOWER HOUSING &
HEATER & EVAPORATOR CORES
NOTE: The following procedures are for vehicles with A/C-heater
systems only. For replacement of heater cores on vehicles
with heater systems only, see HEATER SYSTEMS, JEEP article.
Removal (Cherokee, Comanche & Wagoneer)
1) Disconnect battery ground. Discharge A/C system using
approved refrigerant recovery/recycling equipment. Disconnect A/C
hoses from expansion valve. Drain cooling system. Disconnect heater
hoses at core tubes. Disconnect blower motor wires and vent tube.
2) Remove console (if equipped). Remove lower instrument
panel. Disconnect electrical connectors at A/C relay, blower motor
resistors and A/C thermostat. Disconnect vacuum hose at vacuum motor.
Cut plastic retaining strap holding evaporator-blower housing to
heater core housing.
3) Disconnect blend-air heater control cable. Detach clip at
rear of blower housing flange and remove retaining screws. Remove

housing attaching nuts from studs on engine compartment side of dash
panel. Remove evaporator drain tube.
4) Remove right kick panel. Detach instrument panel support
bolt. Gently pull on right side of dash, then rotate housing downward
and toward rear of vehicle to disengage housing studs from dash panel.
Remove evaporator-blower housing.
5) To remove heater core, detach retaining screws. Remove
heater core by pulling straight out of housing. To remove evaporator
core, detach top housing retaining screws, then remove top of
evaporator housing.
6) Remove thermostatic switch and capillary tube. Detach 2
evaporator retaining screws and lift evaporator core from housing.
Remove expansion valve from evaporator.
Installation
1) To install components, reverse removal procedure. When
installing evaporator core, install thermostatic switch and capillary
tube before installing top of housing.
2) When installing heater core, ensure seal is properly
cemented in place to prevent it from moving when blower assembly is
installed. Evacuate, leak test, recharge and check A/C system
operation.
NOTE: The evaporator housing mounting location is similar to
Wrangler, except blower motor is mounted horizontally to
firewall.
Removal (Wrangler)
1) Discharge system using approved refrigerant
recovery/recycling equipment. Disconnect inlet line at compressor.
Disconnect hose, at quick-disconnect coupling, between receiver-drier
and evaporator. Remove hose clamps and dash grommet retaining screws.
See Fig. 2 .
Fig. 2: Wrangler Evaporator Housing Mounting Screw Locations
Courtesy of Chrysler Motors.
NOTE: It is not necessary to discharge system to service blower
motor. Evaporator housing may be lowered from instrument

panel to gain access to blower motor mounting screws.
2) Remove screws attaching evaporator housing to instrument
panel and mounting bracket. Lower evaporator housing and pull hoses
and grommet through opening.
Installation
To install, reverse removal procedure. Evacuate, leak test,
recharge and check A/C system for proper operation.
EXPANSION VALVE
Removal (Cherokee, Comanche & Wagoneer)
Discharge A/C system using approved refrigerant
recovery/recycling equipment. Disconnect A/C hoses from expansion
valve. Disconnect expansion valve from evaporator core inlet and
outlet tubes. Remove expansion valve.
Installation
To install, reverse removal procedure. Evacuate, leak test,
recharge and check A/C system operation.
NOTE: The expansion valve is pre-set and should not be adjusted. A
faulty valve requires replacement.
Removal (Wrangler)
1) Discharge system using approved refrigerant
recovery/recycling equipment. Remove evaporator housing. Remove
insulation wrapped around suction line and expansion valve.
2) Mark capillary tube location on suction line. Disconnect
inlet and outlet connections, capillary tube clamp and equalizer tube.
Remove expansion valve.
Installation
1) Clean suction line to provide positive contact with
replacement expansion valve capillary tube. Connect inlet and outlet
hoses. Clamp capillary tube securely at marked position and connect
equalizer tube.
2) Wrap expansion valve and line with insulation. Install
evaporator housing assembly. Evacuate, leak test, recharge and check
A/C system operation.
TEMPERATURE CONTROL THERMOSTAT
Removal (Cherokee, Comanche & Wagoneer)
1) Disconnect battery ground. Remove console (if equipped).
Remove lower instrument panel. Disconnect electrical connection at
thermostat. See Fig. 3.
2) Remove capillary tube retaining screw. Remove thermostat
retaining screws. Remove capillary tube from tube guide hole.
Installation
1) To install, feed capillary tube through tube guide hole
until Red tape on capillary tube just enters hole in housing. Install
capillary tube retaining screw and clip.
2) Install thermostat retaining screws and attach electrical
connection. To complete installation, reverse removal procedure.
CAUTION: Handle capillary tube with care to avoid bends or kinks
which could cause thermostat to malfunction.
Removal (Wrangler)
Remove evaporator housing assembly. Remove lower cover.

system will store a diagnostic trouble code in the TCM memory if
certain electronic problems exist. If electronic problem goes away,
diagnostic trouble code will be erased from TCM memory after ignition
has been cycled approximately 75 times.
NOTE: Diagnostic trouble code may be referred to as fault code.
Diagnostic trouble codes can be retrieved using a Diagnostic
Readout Box-II (DRB-II). After repairing an electrical system problem,\
stored diagnostic trouble code must be cleared from TCM memory.
The TCM is located behind right side of instrument panel on
Cherokee. See Fig. 1.
Fig. 1: Transmission Control Module (TCM) I.D. (Cherokee)
Courtesy of Chrysler Corp.
VALVE BODY SOLENOIDS
Valve body solenoids, mounted on the valve body, are output
devices controlled by signals received from the TCM. See Fig. 2. The
No. 1 and 2 valve body solenoids control transmission shifts while the
No. 3 valve body solenoid is used for torque converter lock-up.
When No. 1 and 2 valve body solenoids are energized,
solenoid plunger moves from seat. This opens the drain port and
releases line pressure. When either valve body solenoid is de-
energized, plunger closes the drain port.
The No. 3 valve body solenoid operates in reverse. When No. 3
valve body solenoid is de-energized, solenoid plunger moves away from
seat. This opens the drain port and releases line pressure. When No. 3
valve body solenoid is energized, the plunger closes the drain port.