Ignition System: Description and Operation

Ignition Control Module:

PURPOSE
The Distributor Ignition (DI) system controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide improved engine performance, fuel economy, and control of exhaust emissions, the control module controls distributor spark advance (timing) with the Ignition Control (IC) system.

Only the IC system will be described here. Additional information on the distributor ignition system is found in Engine Electrical.

OPERATION
The distributor ignition system has a distributor module with four terminals for the IC system that are connected by the control module.

To properly control ignition/combustion timing, the control module needs to know:

^ Crankshaft position.
^ Engine speed (RPM).
^ Engine load (manifold pressure or vacuum).
^ Atmospheric (barometric) pressure.
^ Engine coolant temperature.

The IC system consists of the distributor module, a control module, and connecting wires. The four terminals for IC are lettered in the module.

The distributor four terminal connector is lettered A-B-C-D.

These circuits perform the following functions:

^ Terminal "A" - Reference ground lo - This wire may be grounded in the distributor. It makes sure the ground circuit, between the module and control module has no voltage drop which could affect performance. If it is open, it may cause poor performance.

^ Terminal "B" - Bypass - At about 400 RPM, the control module applies 5 volts to this circuit to switch spark timing control from the module to the control module. An open or grounded bypass circuit will set a DTC 42 and the engine will run at base timing, plus a small amount of advance built into the module.

^ Terminal "C" - Distributor reference hi - This provides the control module with RPM and crankshaft position information.

^ Terminal "D" - IC - This circuit triggers the module. The control module does not know what the actual timing is, but it does know when it gets the reference signal. It then advances or retards the spark from that point. Therefore, if the base timing is set incorrectly, the engine spark curve will be incorrect.