Ignition System: Description and Operation

Wiring Diagram For EST Circuit:

The Electronic Spark Timing (EST) system consists of an HEI module, Electronic Control Module (ECM), and connecting wires. The distributor has four wires that connect the HEI module to the ECM. These circuits perform the following functions:

1. Terminal "B" is the distributor reference, which provides the ECM with rpm and crankshaft position information.

2. Terminal "D" is the reference ground. This wire is grounded in the distributor and makes sure the ground circuit has no voltage drop which could affect performance. If it is open, it may cause poor performance.

3. Terminal "C" is the bypass circuit. At about 400 rpm the ECM applies 5.0 volts to this circuit to switch spark timing control from the HEI module to the ECM. An open or grounded bypass circuit will set a Code 432 and the engine will run at base timing, plus a small amount of advance controlled by the HEI module.

4. The EST terminal, terminal "A," triggers the HEI 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.

The ECM uses information from the pressure sensor (MAP) and coolant sensors in addition to rpm to calculate spark advance as follows.

Low pressure sensor (MAP) output voltage (high vacuum indication) = more spark advance.

Cold engine = more spark advance.

High pressure sensor (MAP) output voltage, (low vacuum indication) = less spark advance.

Hot engine = less spark advance.