Electronic Spark Timing (Est)

The EST system consists of ECM and modified HEI distributor with 7-terminal HEI module. The HEI distributor communicates to ECM through a 4-terminal connector which contains 5 circuits: distributor reference circuit, by-pass circuit, EST circuit, ground circuit and cam position circuit.

Whenever pickup coil signals HEI module to open primary circuit, it also sends spark timing signals to ECM through the reference line. When voltage on HEI by-pass line is zero volts (engine cranking), HEI module switches to by-pass circuit. In by-pass circuit, HEI module provides spark advance at base timing and disregards spark advance signal from ECM. When voltage on HEI by-pass circuit is 5 volts (engine running at about 400 RPM), HEI module accepts spark timing signal provided by ECM.

The ECM monitors engine speed through the HEI reference line and engine operating conditions through data sensors. From these parameters, ECM calculates proper spark advance and supplies signals to HEI distributor through the EST line.

The ECM will use the Hall Effect signal from the cam position circuit to determine when the No. 1 cylinder intake valve is opening. The ECM will then sequence the fuel injectors in cylinder firing order.