Ignition System: Description and Operation

NOTE: For further Electronic Spark Timing (EST) and Electronic Spark Control (ESC) diagnosis, refer to COMPUTERIZED ENGINE CONTROLS.

Fig. 2 HEI-EST Distributor:

The High Energy Ignition System with Electronic Spark Timing (HEI-EST) is used on GM models.
All Models equipped with HEI-EST can be identified by the addition of a four-wire distributor connector and the lack of a distributor vacuum advance chamber.
The HEI-EST system consists of an electronic distributor, with an externally mounted coil. All spark timing changes in the HEI-EST system are performed electronically by the Electronic Control Module (ECM). The ECM monitors information from various engine sensors, determines the correct spark timing and signals the distributor to change timing as necessary. A secondary spark advance system is incorporated into the system to signal the ignition module in case of ECM failure. The HEI-EST system does not use vacuum or mechanical advance.


CAUTION: Never connect a wire directly between the Tach terminal of the distributor connector and ground as this will damage the electronic circuitry of the module. When using a timing light to adjust ignition timing, the connection should be made at the No. 1 spark plug. Forcing foreign objects through the boot at the No. 1 spark plug terminal will damage the boot and cause engine misfire.