Ignition System - HEI/Est/ESC: Diagnosis: HEI-Est-ESC

The ECM can retard timing directly, but the external detonation module must signal the ECM through a single-wire communications circuit. The ignition module leaves nine volts on the circuit to communicate no detonation, and grounds it to communicate detonation. The ECM responds appropriately.

If a retarded timing-type driveability problem is occurring, be sure to unplug this circuit to see if the problem goes away. If so, either the detonation module is falsely triggering a detonation condition, or the circuit is shorted to ground.

Beware of clearance in the valve train, or a loose trailer on a hitch, as they may create sufficient vibration to be considered as detonation.