Ignition System - 4-CYL: Description

The Delco-Remy HEI-EST system, a part of all General Motors Computer Command Control systems, is designed to provide optimum performance through electronic control of air/fuel ratios, spark timing, air management and idle speed.

The distributor has neither vacuum nor centrifugal advance mechanisms. See Fig 1 and Fig 2 . The distributor contains a 7-terminal HEI-EST control module, a timer core, pick-up coil, radio noise suppression capacitor, and on most models, an integrally mounted ignition coil, located under the distributor cap's coil cover. Distributors on vehicles equipped with 2.5L VIN 2 and 2.5L VIN R engines also contain a Hall Effect switch and shutter blades.

Fig 1: Disassembled View of HEI-EST Distributor Integral coil model shown.

Fig 2: HEI-EST Distributor with Hall Effect Switch External coil model shown.

The Hall Effect switch and shutter blades act as a second pick-up coil and timer core. When the engine is in the cranking mode, the pick-up coil and timer core are used to send RPM impulses to the ECM. When the engine is in the run mode, the Hall Effect switch and shutter blades perform this function.

On 2.5L VIN 2 and 2.5L VIN R engines, the shutter blades point upward since the ignition coil is mounted externally (not in the distributor cap). The number of shutter blades must equal the number of cylinders in the engine being used.

The distributor is connected to the EST system by means of a 4-wire connector, leading to the external electronic control module (ECM). The ECM (not the distributor HEI-EST module) receives voltage signals from a number of sensors. A typical system could be provided signals from oxygen, engine coolant temperature, throttle position, barometric pressure and manifold absolute pressure sensors, as well as the distributor pick-up coil and Hall Effect switch (if equipped).

Fig 3: Schematic of HEI-EST Ignition System Including Relationship to Computer Command Control Electronic Control Module

There are 4 basic components to the ESC system. They include detonation sensor, an HEI-EST distributor, an ESC controller, and the ECM. When detonation (engine knock) occurs, the detonation sensor sends an electrical signal to the ESC controller. The ESC controller amplifies this signal and transmits it to the ECM. ECM then retards the spark timing until it no longer receives a signal from the detonation sensor, through the ESC controller.

Fig 4: Schematic of Electronic Spark Control System