Ignition System: Description and Operation

Ignition System:

PURPOSE
Provides the ECM with engine speed and crankshaft position information. Controls triggering of ignition coil primary windings.

LOCATION
EST / HEI module - within the distributor.

OPERATION
Electronic Spark Timing (EST) is part of the Computer Command Control System. The ECM monitors information from various sensors, computes the desirable spark timing and signals the distributor when timing changes are required. The distributor does not contain centrifugal advance weights, springs or a vacuum advance unit. Base timing is adjustable. If it is not correctly set, the entire spark advance curve will be off.

Spark knock (detonation) is controlled by an Electronic Spark Control (ESC) system that can retard ignition timing up to 20°. This allows the engine to use maximum spark advance to improve driveability and fuel economy.

The ignition system has a distributor module with four terminals for the EST system that are connected by the ECM.

To properly control ignition/combustion timing, the ECM needs to know the following:
- Crankshaft position
- Engine speed (rpm)
- Engine load (manifold pressure or vacuum)
- Atmospheric (barometric) pressure
- Engine coolant temperature

CIRCUIT DESCRIPTION
The EST system consists of the distributor module, an ECM, and connecting wires. The four terminals for EST are lettered in the module. The distributor four terminal connector is lettered A-B-C-D.

These circuits perform the following functions:

TERMINAL A - Referance Ground Lo
This wire is grounded in the ignition module and provides a reference ground from the ignition module to the ECM. The ECM compares reference ground with reference high voltage. If it is open, it may cause poor performance.

TERMINAL B - Bypass
At about 400 rpm, the ECM applies 5 volts to this circuit to switch spark timing control from the 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 built into the module.

TERMINAL C - Distributor Reference Hi
This provides the ECM with rpm and crankshaft position information.

TERMINAL D - EST
This circuit triggers the 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 from that point. Therefore, if the base timing is set incorrectly, the engine spark curve will be incorrect.