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, 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 no longer can flow directly to the ignition coil, but is diverted to the ECM for modification by information from 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.

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.