Ignition Timing Control Systems (Gasoline): By-Pass

When the ECM receives an engine speed signal of approximately 400 RPM, it considers engine to be running and applies 5 volts to the ignition module on the by-pass wire. This causes ignition module to switch timing control over to the variable timing control circuit in the ECM. On some models, this by- pass wire contains a connector located between the 4-wire connector and the ECM. This is disconnected when adjusting base timing. On all models, an open or grounded by-pass circuit sets a related trouble code in ECM memory. The engine runs at base timing plus a small amount of advance built into the HEI module.