Ignition System: Description and Operation

Fig. 2 HEI-EST Distributor:

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

OPERATION AND CONSTRUCTION
The HEI-EST system consists of an electronic distributor, an externally mounted coil and an Electronic Control Module (ECM).

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 coolant temperature.
The distributor has four wires that connect the IC module to the ECM. These circuits perform the following functions:

Circuit 430 "DISTRIBUTOR REFERENCE HIGH" - provides the ECM with rpm and crankshaft position information.

Circuit 453 "REFERENCE GROUND LOW" - may be grounded in the distributor and makes sure the ground circuit between the IC module and ECM has no voltage drop which could affect performance. If it is open, it may cause poor performance.

Circuit 424 "BYPASS" - At about 400 rpm the ECM applies 5.0 volts to this circuit to switch spark timing control from the IC module to the ECM. An open or grounded bypass circuit will set a Code 42 and the engine will run at base timing, plus a small amount of advance controlled by the IC module.

Circuit 423 "IC Control" - circuit triggers the IC module. The ECM 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 form that point. Therefore, if the base timing is set incorrectly, the entire spark curve will be incorrect. An open or ground, in the EST circuit, will set a Code 42 and cause the engine to run on the HEI module timing. This will cause poor performance and poor fuel economy. A ground may set a Code 42 and the engine may not run.

A secondary spark advance system is incorporated into the system to signal the ignition module in case of ECM failure. The HEI-EST system does not use vacuum or mechanical advance.

No resistance wire is used as this is a full 12 volt system.

NOTE:
Never connect a wire directly between the Tach terminal of the distributor connector and ground as this will damage the electronic circuitry of the module. When using a timing light to adjust ignition timing, the connection should be made at the No. 1 spark plug. Forcing foreign objects through the boot at the No. 1 spark plug terminal will damage the boot and cause engine misfire.