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IGNITION SYSTEM - 4.0L W / SOLID STATE IGNITION (SSI)
1988 Jeep Cherokee
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.
Othercomponents 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 controlsystem 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 aninput 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 onthe 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 isthen 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 inputvoltage 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 ofManifold 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...