GM Grand Am/Achieva/Calais/Skylark/Somerset 1985-98

General Information


The High Energy Ignition (HEI) System is used on the 2.0L (VIN M) and 1985-86 2.5L (VIN U) engines and controls fuel combustion by providing a spark to ignite the air/fuel mixture at the appropriate time. This system consists of a modified module, which is used in conjunction with the Electronic Spark Timing (EST) function of the Electronic Control Module (ECM).

The HEI system features a longer spark duration which is essential in firing lean and EGR-diluted air/fuel mixtures. The condenser (capacitor) located within the HEI distributor is provided for noise (static) suppression purposes only and is not a regularly replaced ignition system component. Dwell is controlled by the ECM and cannot be adjusted.

The HEI distributor is equipped to aid in spark timing changes necessary for optimum emissions, fuel economy and performance. All spark timing changes in the HEI (EST) distributors are performed electronically by the ECM, which monitors information from the various engine sensors, computes the desired spark timing and signals the distributor to change the timing accordingly. No vacuum or centrifugal advance is used with this distributor.

The Electronic Spark Control (ESC) system is used to control spark knock and enable maximum spark advance to improve driveability and fuel economy. This system consists of a knock sensor and an ESC module (generally part of Mem-Cal). The ECM monitors the ESC signal to determine when engine detonation occurs.

The HEI distributor uses a magnetic pickup assembly, located inside the distributor containing a permanent magnet, a pole piece with internal teeth and a pickup coil. When the teeth of the rotating timer core and pole piece align, an induced voltage in the pickup coil signals the electronic module to open the coil primary circuit. As the primary current decreases, a high voltage is induced in the secondary windings of the ignition coil, directing a spark through the rotor and high voltage leads to fire the appropriate spark plug. The dwell period is automatically controlled by the ECM and increases with engine rpm.

To control ignition timing, the ECM receives information about the following conditions:

Engine speed (rpm)
Crankshaft position
Engine load (manifold pressure or vacuum)
Atmospheric (barometric) pressure
Engine temperature
Exhaust Gas Recirculation (EGR)

The ESC system is designed to retard spark timing 8-10° to reduce spark knock in the engine. When the knock sensor detects spark knock in the engine, it sends an AC voltage signal to the ECM, which increases with the severity of the knock. The ECM then signals the ESC circuit to adjust timing to reduce spark knock.

To control EST, the HEI module uses 4 connecting terminals. These terminals provide the following:

Distributor reference circuit
Reference ground circuit
Bypass circuit
EST circuit


Pickup Coil

The pickup coil is a device which generates an alternating current signal to determine crankshaft position.

HEI Module

The HEI module is a switching device which operates the primary circuit of the ignition coil.

Ignition Secondary

The ignition secondary consists of the ignition coil, rotor, distributor cap, plugs wires and spark plug. These components supply the high voltage to fire the spark plugs.

Electronic Spark Timing (EST)

The EST system consists of the distributor module, ECM and connecting wires. This system includes the following circuits:

Distributor reference circuit-provides the ECM with rpm and crankshaft position information.
Bypass signal-above 500 rpm, the ECM applies 5 volts to this circuit to switch spark timing control from the HEI module to the ECM.
EST signal-the ECM uses this circuit to trigger the HEI module, after bypass voltage is applied to the HEI module.
Reference ground circuit-this wire is grounded through the module and insures that the ground circuit has no voltage drop between the ignition module and the ECM which could affect performance.