Ignition System: Description and Operation

The standard High Energy Ignition (HEI) system has a modified distributor module which is used in connection with EST. Two different terminal arrangements are used depending on the distributor.
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 EST system consists of the distributor module, ECM, and connecting wires.


These circuits perform the following functions:

^ Distributor reference Hi - terminal "C" This provides the ECM with rpm and crankshaft position information.
^ Reference ground Low - terminal "A" 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.
^ Bypass - terminal "B" At about 400 rpm, the ECM applies 5 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 42 and the engine will run at base timing, plus a small amount of advance controlled by the HEI module.
^ EST - terminal "D" This circuit 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 from that point. Therefore, if the base timing is set incorrectly, the engine 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.