Pre-Testing Diagnosis

If the reference or EST signals are interrupted due to open wires or a faulty ECM, the vehicle will still run. The HEI-EST module will provide a timing signal based on engine RPM.

If the by-pass signal is lost, the ECM cannot control spark timing, as the by-pass switch will permit direct flow of information to the ignition coil rather than to the ECM.

Normally, a few seconds (5-15 seconds) after starting a warm engine, the by-pass signal electronically operates a by-pass switch in the HEI-EST module. The HEI-EST module's RPM-controlled timing signal can no longer flow directly to the ignition coil, but is diverted first to the ECM for modification by information received from the engine sensors.

Loss of the EST signal with the by-pass signal "ON", however, will stop the engine, because the HEI-EST module is no longer sending signals directly to the ignition coil, but to the ECM, and any loss of the EST signal cuts all flow to the coil. If an attempt is made to restart the vehicle, the engine will run for a few seconds and then stop when the by-pass signal comes back on.

NOTE: Ignition coils with Red and White leads should be matched with pick-up coils having either a Clear, a Black, or a Blue connector. Ignition coils with Red and Yellow leads must be matched with pick-up coils with a Yellow connector.