Spark Timing

The Electronic Spark Timing (EST) system used on 2.8L engines consists of the ECM and a 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 containing 4 circuits: Distributor reference circuit, by-pass circuit, EST circuit and ground circuit.

Whenever pick-up coil signals HEI module to open primary circuit, spark timing signals are sent 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, disregarding 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 this information, ECM calculates proper spark advance and supplies signal to HEI distributor through EST line.