1988 JEEP CHEROKEE Service Repair Manual

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IG NIT IO N S YSTE M - 4 .0 L W /S O LID S TA TE IG NIT IO N ( S SI)
1 988 J e ep C hero ke e
DISTRIBUTORS & IGNITION SYSTEMS
Jeep Solid State Ignition
4.0L 6-Cylinder
DESCRIPTION
IGNITION SYSTEM GENERAL DESCRIPTION
The Solid State Ignition (SSI) system features a solid state
Ignition Control Module (ICM)/ignition coil assembly, Electronic
Control Unit (ECU), distributor and engine speed sensor. Other
components include the battery, ignition switch, starter solenoid,
spark plugs and wires, cap and rotor, resistance wire, by-pass wire
and a knock sensor. A sync pulse signal generator (stator) inputs the
firing order to the ECU.
Fig. 1: 4.0L 6-Cylinder Firing Order
IGNITION CONTROL MODULE (ICM)
The ignition control module is mounted to the ignition coil.
See Fig. 2 . Based on control system inputs, the ECU triggers the
ignition coil via the ignition control module. The ECU is able to
advance or retard ignition timing by controlling the ignition coil
through the ignition control module.
The ICM consists of a solid state ignition circuit, an
integrated ignition circuit and an integrated ignition coil that can
be removed and serviced separately if necessary.

The ECU provides an input signal to the ICM. The ICM has only
two outputs:
* Tach signal to the tachometer and diagnostic connector
* High voltage from the coil to the distributor cap
Fig. 2: View of Ignition Control Module/Ignition Coil Assembly
IGNITION CONTROL MODULE (ICM) ELECTRICAL FEED CONNECTIONS
Electrical feed to the ICM is through terminal "A" of
Connector No. 1 on the module. See Fig. 3.
NOTE: Electrical supply only occurs with the ignition switch in
the START and RUN position.

Fig. 3: Ignition Control Module Connector Identification
Courtesy of Chrysler Motors.
Terminal "B" of Connector No. 1 is grounded at the engine oil
dipstick bracket along with the ECU ground wire and Oxygen (O2) sensor\
ground.
The tachometer output signal wire of the ICM is connected to
Pin No. 1 of the "D1" Diagnostic connector. The wire is routed to the
diagnostic connector through a short section of the ECU harness, the
engine, and the instrument panel harness. This type of routing
eliminates any potential electrical interference from occurring in the
various ECU circuitry.
Ignition firing signals from ECU terminal "27" are
transmitted through terminal "B" of Connector No. 2 on the ICM. the
ignition signal from the ECU is received by the ICM in the form of a 5
volt square wave. As the leading edge of the wave contacts the
ignition circuitry in the ICM, the ICM charges the coil primary
windings.
When coil saturation occurs, the module circuitry opens the
primary windings to collapse the magnetic field in the windings. This
induces the coil secondary windings which is then transmitted to the
spark plug via the coil wire, distributor cap, and rotor.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
The MAP sensor reacts to absolute pressure in the intake
manifold and provides an input signal to the ECU. As the engine load
changes, manifold pressure varies, which causes the MAP sensor
resistance to change, resulting in a different input voltage to the
ECU. The input voltage level supplies the ECU with information
relating to ambient barometric pressure during engine start-up or
regarding engine load while the engine is running. The ECU calculates
this information and adjusts the air-fuel mixture accordingly.
The MAP sensor is mounted under the hood on the firewall and
is connected to the throttle body with a vacuum hose. See Fig. 4.

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

stream See Fig. 6. The MAT sensor provides an input voltage to the
ECU. As the temperature of the air-fuel stream in the manifold varies,
resistance changes, resulting in a different input voltage to the ECU.
Fig. 6: Location of Manifold Air Temperature (MAT) Sensor
Courtesy of Chrysler Motors.
ENGINE SPEED SENSOR (CRANKSHAFT POSITION SENSOR - CPS)
The engine speed sensor is attached to the flywheel cover
housing and provides an input signal to the ECU relating to crankshaft
speed, angle, and position. See Fig. 7. The ECU converts crankshaft
speed input into engine RPM and converts crankshaft angle to piston

position. The engine speed sensor senses TDC, BDC, and engine speed by
detecting the flywheel teeth as they pass by the sensor during engine
operation. The engine speed sensor is non-adjustable.
The flywheel has three trigger notches, 120
apart. See
Fig. 7 . There are 20 small teeth between each trigger notch. Each
large trigger notch is located 12 small teeth before each Top Dead
Center (TDC) position of the corresponding pistons.
Fig. 7: Location of Engine Speed Sensor
Courtesy of Chrysler Motors.
When a small tooth and notch pass the magnet core in the
sensor, the concentration, followed by the collapse of the magnetic

flux induces a small voltage spike to the sensor pickup coil winding.
These small voltage spikes enable the ECU to count the teeth as they
pass the sensor. When a large trigger tooth and notch pass the
magnetic core in the sensor, the increased concentration, and then
collapse of the magnetic flux induces a higher voltage spike into the
sensor pickup coil winding. See Fig. 8.
Fig. 8: Engine Speed Sensor Operation
Courtesy of Chrysler Motors.
The higher voltage spike is an indication to the ECU that a
piston will reach its TDC position, 12 teeth later. See Fig. 9. The