Ignition System - HEI-Est: Diagnosis

If EST circuitry inside the ECM should fail, internal back-up circuitry will provide a timing signal based solely on engine RPM. Engine will continue to run, although less efficiently. If RPM reference to ECM or by-pass signal to ignition module is lost, by-pass switch will direct pick-up coil RPM information directly to ignition coil rather than to ECM. This will cause fuel injected vehicles to stall. Carbureted vehicles may continue to run, however, performance will be reduced due to the limited advance built into the ignition module.

Loss of EST signal from ECM when 5-volt by-pass signal is present will cause engine to stop because ignition 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.

Loss of HEI reference ground wire may cause ECM to monitor an incorrect engine speed signal. This could result in multiple firings of fuel injectors, incorrect RPM readings on a "Scan" tester, ignition cut-out or setting of a false Code 42.