Electronic Spark Timing (EST)

The EST type HEI distributor receives all spark timing information from the ECM when the engine is running. The ECM providee spark plug firing pulses based upon the various engine operating parameters. The electronic components for the EST system are integral with ECM. The Electronic Spark Timing system consists of the ECM and a modified HEI distributor which uses a seven terminal HEI module. The HEI distributor receives input signals from the ECM and delivers output signals to the ECM through four circuits: distributor reference circuit, bypass circuit, EST circuit and ground circuit. When the pickup coil signals the HEI module to open the primary circuit, it also sends the spark timing signals to the ECM through the reference circuit. During engine cranking when HEI bypass line voltage is zero, the HEI module operates in bypass mode. At this point, the HEI module is supplying spark advance according to a fixed value and disregards spark advance signals from the ECM. When the engine is running, voltage on the HEI bypass circuit is increased to five volts and this signals the HEI module to accept spark timing signals from the ECM. The ECM monitors engine speed through the HEI reference circuit and engine operating conditions through various sensors. Using these parameters, the ECM calculates the correct spark advance and sends this signal to the HEI module via the EST circuit.