Pre-Testing Diagnosis
Normally, 5-15 seconds after starting a warm engine, the by-pass signal electronically operates a by-pass switch in the HEI module. The module's RPM-controlled timing signal will no longer be able to flow directly to the ignition coil. It is instead diverted to the ECM for modification by information from engine sensors.
If the reference or EST signals are interrupted due to open wires or a faulty ECM, the vehicle can still operate. The HEI module will provide a timing signal based on engine RPM. If the by-pass signal is lost, the ECM cannot control timing. The by-pass switch will permit direct flow of information to the ignition coil rather than to the ECM. See Figure.
Loss of the EST signal with the by-pass signal activated will stop the engine, as the HEI module will no longer be able to send signals directly to the ignition coil, but to the ECM. A lost EST signal interrupts 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 is activated.