"S" & "T" Series With EST & 2.5L Engine

The distributor module is connected to ECM by a 4-wire EST connector which performs the following functions:

• Terminal "A" of the 4-wire connector triggers the HEI module. The ECM advances or retards spark on this terminal based on calculations involving the reference signal from terminal "B" and other sensor input signals. If base timing is incorrectly set, entire advance curve will be incorrect.
• Terminal "B" of the 4-wire connector supplies RPM data and crankshaft position reference to the distributor. If wire is open or grounded on fuel injected models, engine will not run.
• Terminal "C" of the 4-wire connector is the by-pass circuit. At approximately 400 RPM, ECM applies 5 volts to this circuit, which switches spark timing control from HEI module to ECM. On some models, this by-pass wire contains a connector located between the 4-wire connector and the ECM. This is disconnected when checking or adjusting base timing. On all models, an open or grounded by-pass circuit will set a Code 42 in ECM memory. The engine will run at base timing plus a small amount of advance built into the HEI module.
• Terminal "D" of the 4-wire connector is the reference ground. It is grounded to distributor and ensures there is no voltage drop which could affect performance.