Electronic Spark Timing (EST)

The EST system consists of ECM and modified HEI distributor with 7 terminal HEI module. The EST distributor contains no vacuum or centrifugal advance. The HEI distributor communicates to ECM through a 4-terminal connector which contains 4 circuits: Distributor reference circuit, by-pass circuit, EST circuit and ground circuit.

Whenever pick-up coil signals HEI module to open primary circuit, it also sends spark timing signals to ECM through reference line. When voltage on HEI by-pass line is 0 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), HEI module accepts spark timing signal provided by ECM.

The ECM monitors engine speed through HEI reference line and monitors engine operating conditions through data sensors and switches. From these parameters, ECM calculates proper spark advance and supplies signal to HEI distributor through EST line. The EST system is used on all models except Federal Chevette and T1000 models without 5-speed manual transmission.