Est System
The ECM monitors information concerning crankshaft position, engine RPM, engine load, atmospheric conditions, engine temperature, and transmission gear position. This information is used by the ECM to compute desired spark timing which is relayed to the distributor, enabling appropriate changes to ignition timing. A back-up spark advance system is incorporated to signal ignition module in the event of ECM failure.
CAUTION:
Although similar in appearance, components of HEI/EST and HEI distributors are NOT interchangeable.
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. See Fig 1. 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 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.