GM Firebird 1967-1981 Repair Guide

Description and Operation


See Figures 1 and 2

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Fig. Fig. 1: Wire terminal identification for typical HEI distributors except for 1975-77 6-cylinder (which utilize external coils) and 1981 EST HEI distributors

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Fig. Fig. 2: The 1975-77 6-cylinder engines utilized an HEI distributor which made use of an external ignition coil all other HEI distributors use an integral coil

The General Motors HEI system is a pulse-triggered, transistor-controlled, inductive discharge ignition system. Except on inline 6-cylinder models through 1977, the entire HEI system is contained within the distributor cap. Inline 6-cylinder engines through 1977 have an external coil. Otherwise, the systems are the same.

The distributor, in addition to housing the mechanical and vacuum advance mechanisms, contains the ignition coil (except on some inline 6-cylinder engines), the electronic control module, and the magnetic triggering device. The magnetic pick-up assembly contains a permanent magnet, a pole piece with internal "teeth," and a pick-up coil (not to be confused with the ignition coil).

For 1981, an HEI distributor with Electronic Spark Timing (EST) is used (for more information on EST, refer to Driveability And Emissions Controls ). This system uses a one piece distributor with the ignition coil mounted in the distributor cap, similar to 1980 models.

All spark timing changes in the 1981 distributors are done electronically by the Electronic Control Module (ECM) which monitors information from various engine sensors, computes the desired spark timing, then signals the distributor to change the timing accordingly. No vacuum or mechanical advance systems are used whatsoever.

In the HEI system, as in other electronic ignition systems, the breaker points have been replaced with an electronic switch a transistor which is located within the control module. This switching transistor performs the same function points do in a conventional ignition system; it simply turns coil primary current ON and OFF at the correct time. Essentially then, electronic and conventional ignition systems operate on the same principle.

The module which houses the switching transistor is controlled (turned ON and OFF) by a magnetically generated impulse induced in the pick-up coil. When the teeth of the rotating timer align with the teeth of the pole piece, the induced voltage in the pick-up coil signals the electronic module to open the coil primary circuit. The primary current then decreases, and a high voltage is induced in the ignition coil secondary windings which is directed through the rotor and high voltage leads (spark plug wires) to fire the spark plugs.

In essence, the pick-up coil module system simply replaces the conventional breaker points and condenser. The condenser found within the distributor is for radio suppression purposes only and has nothing to do with the ignition process. The module automatically controls the dwell period, increasing it with increasing engine speed. Since dwell is automatically controlled, it cannot be adjusted. The module itself is non-adjustable and non-repairable and must be replaced if found defective.


See Figure 3

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Fig. Fig. 3: HEI Distributor Components

Before commencing with troubleshooting, it might be a good idea to take note of the following precautions:

Timing Light Use

Inductive pick-up timing lights are the best kind to use with HEI. Timing lights which connect between the spark plug and the spark plug wire occasionally (not always) give false readings.

Spark Plug Wires

The plug wires used with HEI systems are of a different construction than conventional wires. When replacing them, make sure you get the correct wires, since conventional wires won't carry the voltage. Also, handle them carefully to avoid cracking or splitting them and never pierce them.

Tachometer Use

Not all tachometers will operate or indicate correctly when used on a HEI system. While some tachometers may give a reading, this does not necessarily mean the reading is correct. In addition, some tachometers hook up differently from others. If you can't figure out whether or not your tachometer will work on your car, check with the tachometer manufacturer. Dwell readings, of course, have no significance at all.

HEI System Testers

Instruments designed specifically for testing HEI systems are available from several tool manufacturers. Some of these will even test the module itself. However, the tests given in the following section will require only an ohmmeter and a voltmeter.


There is a terminal marked TACH on the distributor cap. Connect one tachometer lead to this terminal and the other lead to a ground. On some tachometers, the leads must be connected to the TACH terminal and to the battery positive terminal.

Never ground the TACH terminal; serious module and ignition coil damage will result. If there is any doubt as to the correct tachometer hookup, check with the tachometer manufacturer.

1975-77 models with 6-cylinder engines utilize an HEI distributor with an external coil. For these particular vehicles, connect one tachometer lead to the TACH terminal on the ignition coil and the other one to a suitable ground.