Ignition System - HEI/Est/ESC: Diagnosis

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

Normally, 5-15 seconds after starting a warm engine, by-pass signal from ECM will operate a by-pass switch in HEI/EST module. The HEI/EST module's RPM-controlled timing signal will switch over and RPM signal will flow directly to the ECM for processing.

Loss of EST signal from ECM when 5-volt by-pass signal is present will cause engine to stop because HEI/EST module is no longer sending signals directly to ignition coil. Any loss of EST signal will stop all flow to coil. If vehicle is restarted, engine will run for a few seconds and stop when by-pass signal comes back on.