Ignition System - HEI-Est: Diagnosis

If the reference or EST signals are interrupted due to an open circuit or a faulty ECM, the HEI (EST) module will provide a timing signal based on engine RPM. The engine will continue to run, although less efficiently. If the by-pass signal is lost, the by-pass switch will direct sensor information directly to the coil rather than through ECM.

Normally, 5-15 seconds after starting a warm engine, the by-pass signal will operate 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. Under these conditions, it is diverted to the ECM for processing.

Loss of the EST signal when the by-pass signal is on will cause the engine to stop because the HEI (EST) module is no longer sending signals directly to the ignition coil. Any loss of the EST signal will stop all flow to the coil. If vehicle is restarted, the engine will run for a few seconds and stop when the by-pass signal comes back on.