See Figures 1, 2, 3 and 4
The General Motors/Delco-Remy High Energy Ignition (HEI) system is breakerless, pulse-triggered, transistor-controlled, inductive discharge ignition system. It is used on all gasoline engine vehicles covered by this manual. The external ignition coil is normally mounted on the side of the engine, using a secondary circuit high tension wire to connect the coil to the cap. Interconnecting primary wiring is routed through the engine harness.
The distributor, in addition to housing the advance mechanisms, contains the electronic ignition module, and the magnetic pick-up assembly which contains a permanent magnet, a pole piece with internal teeth, and a pick-up coil (not to be confused with the ignition coil).
The HEI distributor is equipped to aid in spark timing changes, necessary for emissions, economy and performance. On some of the early model trucks covered by this manual, these timing changes are accomplished through vacuum and mechanical advance mechanisms. However, most of the models covered by this manual achieve timing changes through the Electronic Spark Timing (EST) control system. On these vehicles, timing changes are electronically controlled through the Engine Control Module (ECM).
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 ignition module. This switching transistor performs the same function the points did in a conventional ignition system; it simply turns coil primary current on and off at the correct time. Essentially, the 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 then directed through the rotor and high voltage leads (spark plug wires) to fire the spark plugs.
In essence then, 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. The HEI system features a longer spark duration which is instrumental in firing lean and Exhaust Gas Recirculation (EGR) diluted fuel/air mixtures. 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.
The distributor on the 1.9L engine uses vacuum and centrifugal advance and does not use EST.
The distributor on the 2.5L engine contains a Hall Effect Switch. It is mounted above the pick-up coil in the distributor and takes the place of the reference (R) terminal on the distributor module. The Hall Effect Switch provides a voltage signal to the ECM to tell it which cylinder will fire next.
The 2.8L and 4.3L engines are equipped with Electronic Spark Control (ESC). A knock sensor is mounted in the engine block. It is connected to the ESC module which is mounted on the cowl in the engine compartment. In response to engine knock, the sensor sends a signal to the ESC module. The module will then signal the ECM which will retard the spark timing in the distributor.
HEI SYSTEM PRECAUTIONS
Before proceeding with troubleshooting or HEI system service, please note the following precautions:Timing Light Use
Inductive pick-up timing lights are the best kind of use with the HEI system. Timing lights which connect between the spark plug and the spark plug wire occasionally (not always) give false readings due to the high voltage of the HEI system which more easily leads to arcing.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 point system 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.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, most of the tests given in the following section will require only ohmmeter and a voltmeter.