DELCO-REMY Electronic Spark Timing (Est) Ignition System: Description

NOTE: The Delco-Remy High Energy Ignition System with Electronic Spark Timing (HEI-EST) is used with minor variations on practically all General Motors models. Some Chevrolet 229" VIN K engines are equipped with Electronic Module Retard (EMR). Some EST-equipped engines, such as the 3.81 VIN 3, 4.11 VIN 4, and 4.9L VIN W and T also utilize Electronic Spark Control (ESC) to control detonation. See Figure.

The Delco-Remy HEI-EST system, a part of most General Motors Computer Command Control Systems, is designed to provide optimum engine 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. It contains a 7-terminal HEI-EST control module, which is not interchangeable with other HEI systems. The distributor also contains a timer core, pick-up coil, radio noise suppression capacitor and, on most models, an internally mounted ignition coil, located under the distributor cap's coil cover.

Fig 1: Disassembled View of EST Distributor (Model with Integral Coil Shown)

Courtesy of GENERAL MOTORS COMPANY

NOTE: On Chevette models with the 1.6L engine and all models with the 2.5L engine, an externally-mounted ignition coil is used.

The distributor is connected to the EST system by means of a 4-wire terminal, leading to the external electronic control module (ECM). The ECM (not the distributor 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.

The ECM reads all voltage input signals and then provides output signals to the mixture control solenoid in the carburetor to adjust air/fuel ratio or to the distributor to control spark timing and dwell.

CAUTION: Few components are interchangeable between HEI and HEI-EST distributors. Be sure correct part is used, as similar appearance does not mean identical design or operation.

The HEI-EST module in the distributor has 7 terminals. The ignition coil battery terminal (positive) is connected to the "B+" or "+" terminal, the coil's "TACH" or negative terminal to the "C" terminal. See Figure. Terminals "N" and "P" are attached to the pick-up coil. HEI-EST module terminal "R" connects through the EST connector "B" to ECM terminal 10 (Distributor Reference Pulse Hi). Terminal "B" connects through connector "C" to ECM terminal 11 (Ignition Module By-pass). Terminal "E" connects through connector "A" to ECM terminal 12 (EST). ECM terminal 13 (Distributor Reference Pulse Lo) connects to connector "D" and then to HEI-EST module ground.

In addition to normal HEI-EST system, some engines use an additional electronic spark control (ESC) system to retard spark advance when detonation occurs. The retard mode is held for 20 seconds and then reverts back to the EST system.

There are 3 basic components of the ESC system.- A detonation sensor, an HEI-EST distributor, and an ESC Controller. See Figure. The sensor sends an electrical signal to the controller terminal "B" in event of detonation. Failure of the sensor would permit no retard. The controller modifies the ECM's EST signal, based on the sensor signal, and sends a command signal from terminal "J" to the HEI-EST module's terminal "E" to adjust spark timing. Constant monitoring of detonation occurs. The ESC controller has no memory function, and failure would result in no ignition, no retard or full retard.