Distributor: Description and Operation

Ignition Distributor Assembly Components:

Distributor 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 Engine Control Module (ECM) controls distributor spark advance (timing) with the Ignition Control (IC) system.

OPERATION
The Distributor Ignition system has an ignition control module, which is used in conjunction with ignition control system.

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

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

The IC system consists of the DI control module, ECM and connecting wires.

These circuits perform the following functions:

^ Reference (CKT 430) - This provides the ECM with RPM and crankshaft position information. If this circuit becomes open or grounded, the engine will not run, because the ECM will not operate the injectors. If the engine cranks but will not run, refer to Chart A-3 Engine Cranks But Will Not Run. Related Tests, Information and Procedures

^ Distributor Reference Low (CKT 453) - This wire is grounded in the distributor and provides a reference ground from the DI control module to the ECM for use by the ECM to compare with reference high voltage. If it is open, it may cause poor performance.

^ Bypass (CKT 424) - At about 400 RPM, the ECM applies 5 volts to this circuit to switch spark timing control from the DI control module to the ECM. An open or grounded bypass circuit will set a Diagnostic Trouble Code (DTC) 42 and the engine will run at base timing, plus a small amount of advance built into the DI control module.

^ IC (CKT 423)- After bypass voltage is applied, the ECM uses this circuit to trigger the DI control module. The ECM uses the distributor reference signal to base its calculation of the amount of spark advance needed, under present engine conditions. If the base timing of the engine is incorrect, the entire spark curve will be incorrect.