ECU JEEP CHEROKEE 1988 Service Repair Manual

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2.5 L 4 -C YL - V IN [ H ]

1 988 J e ep C hero ke e
1988 CHRYSLER MOTORS ENGINES
2.5L 4-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 the
upper left side of dash, visible through windshield. The fourth
character of the VIN identifies the engine size. The tenth character
identifies the model year.
ENGINE IDENTIFICATION CODES TABLE









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Application VIN Code
2.5L 4-Cylinder TBI .................................... H









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SPECIAL ENGINE MARKS
Some engines are produced at the factory with oversize or
undersize components. These engines are identified by a letter code
stamped between ignition coil and distributor. Letters are decoded as
follows:
* "B" indicates all cylinder bores .010" (.25 mm) oversize.
* "C" indicates all camshaft bearing bores .010" (.25 mm)
oversize.
* "M" indicates all main bearing journals .010" (.25 mm)
undersize.
* "P" indicates all connecting rod journals .010" (.25 mm)
undersize.
REMOVAL & INSTALLATION
ENGINE REMOVAL
See ENGINE REMOVAL article.
INTAKE & EXHAUST MANIFOLDS
NOTE: Throttle body removal is necessary prior to manifold
removal. Throttle body can be separated from the manifold
and secured in a designated area with vacuum hoses attached.
Removal
1) Disconnect negative battery cable. Remove air cleaner, EGR
pipe and PCV hose. Disconnect and mark all vacuum hoses and electrical
connections. Drain cooling system.
2) Remove coolant hoses from intake manifold. Remove A/C
compressor and power steering pump and mounting bracket (if equipped).\

\0032.5 L C EC S YSTE M

1988 J e ep C hero ke e
1988 Computerized Engine Controls
JEEP 4-CYLINDER 2.5L TBI COMPUTERIZED EMISSION CONTROL
Cherokee, Comanche
DESCRIPTION
The computerized engine control system, used on 2.5L models
with throttle body fuel injection, is built around an electronic
control unit (ECU). The ECU is a microprocessor-based computer.
The major function of the system is to reduce emissions. It
accomplishes this through a series of 13 sensors or switches that
constantly monitor several engine conditions. See Fig. 16.
Fig. 1: Vacuum Diagram for Jeep 2.5L CEC System
The computer processes input information from the sensors to
get an accurate picture of engine operation. It then provides output
control signals to regulate air/fuel ratio, ignition, idle speed and

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

with manifold pressure information.
Knock Sensor
The knock (detonation) sensor, located in the cylinder head,
provides an input signal to the ECU whenever detonation occurs. The
ECU then retards ignition advance to eliminate the detonation at the
applicable cylinders.
Speed Sensor
The speed sensor (or crankshaft position sensor) is mounted
at the flywheel/drive plate housing. The sensor detects the flywheel/
drive plate teeth as they pass during engine operation and sends an
electrical signal to the ECU, which calculates engine speed.
The flywheel/drive plate has a large trigger tooth and notch
located 90
and 12 small teeth before each top dead center (TDC)
position. When a small tooth or notch pass the magnetic core in the
sensor, the build-up and collapse of the magnetic field induces a
small voltage signal in the sensor pick-up windings.
The ECU counts these signals representing the number of teeth
as they pass the sensor. When a larger trigger tooth and notch pass
the magnetic core, a higher voltage signal is sent to the ECU. This
indicates to the ECU that a piston will be at the TDC position 12
teeth later. The ECU either advances or retards ignition timing as
necessary according to sensor inputs.
Battery Voltage
Battery voltage input to the ECU ensures that proper voltage
is applied to the injector. The ECU varies voltage to compensate for
battery voltage fluctuations.
Starter Motor Relay
The engine starter motor relay provides an input to the ECU,
indicating the starter motor is engaged.
Wide Open Throttle (WOT) Switch
The WOT switch is mounted on the side of the throttle body.
The switch provides a voltage signal to the ECU under wide open
throttle conditions. The ECU responds to this signal by enriching the
air/fuel mixture delivered to the injector.
Closed Throttle (Idle) Switch
This switch is integral with the idle speed actuator (ISA)
motor. The switch provides a voltage signal to the ECU, which
increases or decreases the throttle stop angle in response to engine
operating conditions.
Transmission Gear Position Indicator
The gear position indicator is mounted on vehicles equipped
with automatic transaxles. It provides a signal to the ECU to
indicate that the transaxle is in a driving mode and not in Park or
Neutral.
Power Steering Pressure Switch
The switch increases the idle speed during periods of high
power steering pump load and low engine RPM.
A/C Switch
The A/C switch sends a signal to the ECU when the air
conditioner is operating and when the compressor clutch must be
engaged to lower the temperature. The ECU, in turn, increases engine
speed to compensate for the added load of the air conditioner.
FUEL CONTROL

An electric in-tank fuel pump supplies fuel through the fuel
filter located under the right rear floor pan to the throttle body,
maintaining a constant operating pressure. Fuel enters the fuel bowl
reservoir of the throttle body through the injector and overflow type
fuel pressure regulator. The fuel pump is controlled by the ECU. A
ballast resistor attached to the right side of the plenum chamber,
reduces fuel pump speed after engine is running. The resistor is
by-passed in the "Start" position.
The fuel injector and fuel pressure regulator are integral
components of the throttle body. The injector is electronically-
controlled by the ECU. See Fig. 15.
Fig. 2: Cross Section View of Injector
The fuel pressure regulator is a diaphragm-operated relief
valve which maintains fuel pressure of 17.3 psi (1.2 kg/cm

). See
Fig. 16. Fuel in excess of this pressure is returned to fuel tank by a

fuel return line. The regulator is not controlled by the ECU. The
regulator's spring chamber is vented to the same pressure as the tip
of the injector.
Fig. 3: Cross Section View of Fuel Pressure Regulator
Since fuel pressure at the injector is kept constant, the
volume of fuel injected is dependent only on the length of time that
the injector is energized. The injection time duration is based on
engine operating conditions, which are provided to the ECU by the

input sensors. During engine start-up, the injector delivers an extra
amount of fuel to aid in starting.
EMISSION CONTROL
Both EGR and canister purge operation are regulated by the
ECU. Regulation of these 2 systems is accomplished through the use of
an electrically-operated vacuum solenoid.
Whenever the solenoid is energized by the ECU, it prevents
vacuum action on the EGR valve and canister. The solenoid is
energized by the ECU during engine warm-up, improving cold
driveability. It is also energized during closed throttle (idle),
wide open throttle and during rapid acceleration or deceleration.
In this way the EGR is prevented from operating until the
engine reaches a predetermined temperature. The canister purge does
not operate until the oxygen sensor warms up and becomes operational.
This prevents an over-rich mixture until the oxygen sensor can
compensate for the extra fuel vapor.
IDLE SPEED ACTUATOR (ISA)
The ISA motor, located on the throttle body, is an
electrically-driven actuator that changes the throttle stop angle by
acting as a movable idle stop. The ECU commands the ISA to control
engine idle speed and maintain a smooth idle during sudden engine
deceleration. It does this by providing the appropriate voltage
outputs to produce the idle speed or throttle stop angle required for
the particular engine operating condition. There is no idle speed
adjustment.
For cold engine starting, the throttle is held open for a
longer period to provide adequate engine warm-up prior to normal
operation. When starting a hot engine, the throttle is open for
shorter time.
Under normal engine operating conditions, engine idle is
maintained at a pre-programmed RPM, which may vary slightly due to
engine operating conditions. Under certain engine deceleration
conditions, the throttle is held slightly open.
IGNITION ADVANCE CONTROL
Under certain engine operating conditions, the predetermined
ignition advance curve is modified. This is accomplished through 2
switching circuits that connect the ECU and the ignition control
module.
ECU-CONTROLLED RELAYS
System Power Relay
Located on the right strut tower, this relay is energized
during engine start up and remains energized until 3 to 5 seconds
after the engine is stopped. This permits the ECU to extend the idle
speed actuator for the next start up and then cease operation. See
Fig. 4.

Fig. 4: Location of ECU-Controlled Relays
Load Swap Relay
The Load Swap Relay is used on models with A/C and power
steering. The relay works in conjunction with the power steering
pressure switch to disengage the A/C compressor clutch.
If the compressor clutch is engaged when the power steering
pressure switch contacts close, the input signal from the switch to
the ECU also activates the load swap relay. The relay contacts open,
cutting off electrical feed to the compressor clutch. The clutch
remains disengaged until the pressure switch contacts reopen and
engine idle returns to normal.
NOTE: The load swap relay does not reengage the compressor clutch
immediately. The relay has a timer that delays energizing
the clutch for .5 second to ensure smooth engagement.
Fuel Pump Control Relay
Battery voltage is applied to the relay through the ignition
switch. The relay is energized when a ground is provided by the ECU.
When energized, voltage is applied to the fuel pump See Fig. 15.
A/C Clutch Relay

The ECU controls the A/C compressor clutch by means of the
A/C clutch relay. See Fig. 15.
UP-SHIFT INDICATOR LAMP
Manual transaxle vehicles are equipped with an up-shift
indicator lamp. The lamp is normally turned on when the ignition
switch is turned "ON", and is turned off when the engine starts.
The lamp will again light during engine operation, according
to engine speed and load conditions. A switch, located on the
transaxle, prevents lamp from lighting when transmission is shifted
to the next highest gear. If the shift of gears is not performed, the
ECU will turn the lamp off after 3-5 seconds.
MODES OF OPERATION
IGNITION SWITCH "ON" MODE
When the TBI system is activated by the ignition switch, the
system power relay is energized, and the fuel pump is energized by
the ECU through the fuel pump relay. The pump will operate for
approximately 1 second, unless the engine is operating or the starter
motor is engaged.
The ECU receives input from the CTS, MAT, and MAP sensors.
The up-shift indicator lamp is illuminated.
ENGINE START-UP MODE
When the starter motor is engaged, the ECU receives inputs
from the CTS and speed sensors, the starter motor relay, and the wide
open throttle switch. The fuel pump is activated by the ECU and
voltage is applied to the injector, with the ECU controlling
injection time.
The ECU determines proper ignition timing from the speed
sensor input. If the wide open throttle switch is engaged, the ECU
will deactivate the injector to prevent flooding.
ENGINE WARM-UP MODE
The ECU receives inputs from the CTS, MAT, MAP, speed, and
knock sensors. It also is informed of throttle, gear (automatic
transaxle models) and A/C control position.
The ECU provides a ground for the injector, precisely
controlling fuel delivery to the engine. The ECU also controls
ignition timing, engine idle speed and throttle stop angle. On
vehicles with manual transmissions, the up-shift indicator lamp is
controlled according to engine speed and load.
CRUISE MODE
During cruising speed, the ECU receives inputs from the CTS,
MAT, MAP, EGO, speed and knock sensors. It is also informed of
throttle, gear (automatic transaxle models), and A/C control position.\
The ECU provides a ground to the injector, precisely
controlling injector time. It also controls idle speed, throttle stop
angle, ignition timing, air/fuel mixture ratio and up-shift indicator
lamp.
DECELERATION MODE
During deceleration, the ECU receives inputs from the CTS,

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