1988 JEEP CHEROKEE engine

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

sensor. With engine warm, resistance should be less than 1000 ohms.
2) Connect one ohmmeter lead to sensor connector terminal.
Connect other lead, in turn, to ECU harness connector terminals 32
and 14. Repair wiring harness if resistance is greater than 1 ohm.
COOLANT TEMPERATURE SENSOR
1) Disconnect wiring harness from CTS sensor. Test
resistance of sensor. If resistance is not 185-100,700 ohms (3400
ohms at 70
F; 1600 ohms at 100F), replace sensor. With engine warm,
resistance should be less than 1000 ohms.
2) Connect one ohmmeter lead to sensor connector terminal.
Connect other lead, in turn to ECU harness connector terminals 15 and
32. Repair wiring harness if an open circuit is indicated.
WIDE OPEN THROTTLE (WOT) SWITCH
1) Disconnect wiring harness from WOT switch. Connect
ohmmeter leads to switch terminals, and manually open and close the
switch. When switch is closed, resistance should be infinite. A low
resistance should be indicated at wide open position. Test switch
operation several times. Replace WOT switch if defective. Reconnect
wiring harness.
2) With ignition switch "ON", connect voltmeter between pin
6 and pin 7 (ground) of diagnostic connector D2. Voltage should be
zero with switch in wide open position and greater than 2 volts in
any other position.
3) If voltage is always zero, test for short circuit to
ground in wiring harness or switch. Check for open circuit between
pin 8 of ECU connector and the switch connector. Repair or replace
wiring harness as necessary.
4) If voltage is always greater than 2 volts, test for an
open wire or connector between the switch and ground. Repair as
required.
CLOSED THROTTLE SWITCH
NOTE: It is important that all testing be done with the idle speed
actuator (ISA) motor plunger in the fully extended position
(as it would be after a normal engine shut down). If it is
necessary to extend the motor plunger to test the switch, an
ISA motor failure can be suspected. Refer to ISA motor test.
1) With ignition on, connect voltmeter positive lead to pin
13 of diagnostic connector D2. Attach negative lead to pin 7. Voltage
should be close to zero at closed throttle and greater than 2 volts
at any position other than closed throttle.
2) If the voltage is always zero, test for a short circuit
to ground in the wiring harness or switch. Test for an open circuit
between pin 25 of ECU connector and throttle switch.
3) If voltage is always more than 2 volts, test for an open
circuit in the wiring harness between the ECU and switch connector.
Also check for open circuit between the switch connector and ground.
Repair or replace wiring harness as needed.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
1) Inspect MAP sensor vacuum hose connections at sensor and
throttle body. Repair as required. Test MAP sensor output voltage at
MAP sensor connector pin B (as marked on sensor body) with the
ignition switch "ON" and engine off. See Fig. 15. Output voltage
should be 4.0-5.0 volts.

NOTE: Voltage should drop 0.5-1.5 volts with hot engine, at idle.
Fig. 6: MAP Sensor Terminal Identification
If markings on MAP sensor vary from illustration, use markings on
sensor.
2) Test voltage at pin 33 of ECU connector for 4.0-5.0 volts
to verify wiring harness condition. Repair if required.
3) With ignition on, check for MAP sensor supply voltage of
4.5-5.5 volts at sensor connector pin C. Similar voltage should be
present at pin 16 of ECU connector. Repair or replace wiring harness
if required. Test for sensor ground between pin 17 of ECU connector
and pin A of sensor connector.
4) Using an ohmmeter, check for ground from pin 17 of ECU
connector to pin 2. If an open circuit is indicated, check for a
defective sensor ground on the flywheel housing near the starter
motor.
5) If ground is good, the ECU must be replaced. Before
replacing ECU, check to see if pin 17 of ECU connector is shorted to
12 volts. If so, correct the condition and test ECU before replacing.
Refer to the ELECTRONIC CONTROL UNIT TEST.
ELECTRONIC CONTROL UNIT

1) If all components have been checked and/or repaired, but
a system failure or problem still exists, the ECU may be at fault.
However, the ECU is a very reliable unit and must always be the final
component replaced if a doubt exists concerning the cause of a system
failure.
2) The only way to confirm an ECU malfunction is to take the
unit to an AMC dealer to have it tested. This is the only sure way to
avoid replacing a good ECU.
SYSTEM DIAGNOSIS
PRELIMINARY CHECKS
Be sure fuel is actually reaching the injector. Make sure no
air is entering the intake or exhaust system above the catalytic
converter. Before assuming an engine control system malfunction,
inspect the following systems to ensure components are in good
condition and are operating properly.
* All support systems and wiring.
* Battery connections and specific gravity.
* Electrical and vacuum connections on components and sensors.
* Emission control devices.
* Ignition system.
* Vacuum hoses.
CAUTION: Never connect or disconnect a component without turning the
ignition switch off. Never apply more than 12 volts or AC
voltage to system terminals. Disconnect battery cables
before charging it. Remove ECU if temperatures are expected
to exceed 176
F (80C), such as in a paint shop bake oven.
DIAGNOSTIC TEST CHARTS
Following are 6 different diagnostic test flow charts,
providing the shortest means of testing the system. These include:
* Ignition Switch "OFF" Chart - Tests system power for ECU
memory keep-alive voltage.
* Ignition Switch "ON" Power Chart - Tests system power
function and fuel pump power function.
* Ignition Switch "ON" Input Chart - Tests closed throttle
(idle) switch, wide open throttle (WOT) switch, manifold
absolute pressure (MAP) sensor, park/neutral switch, coolant
temperature sensor (CTS), manifold air/fuel temperature (MAT\
)
sensor and the respective switch or sensor circuits.
* System Operational Chart - Tests engine start-up and fuel
injector circuits, plus function of closed loop air/fuel
mixture, coolant temperature sensor, manifold air/fuel
temperature sensor, knock sensor and closed loop ignition
retard/advance, EGR valve and canister purge solenoid, idle
speed actuator, and A/C control.
* Basic Engine Chart - Indicates possible failures within other
engine related components.
* Man. Trans. Up-shift Chart - Tests up-shift indicator lamp
function on manual transmission vehicles.