GM Astro/Safari 1985-1996 Repair Guide

General Information

Print

There are three general ignition systems used on these vehicles: High Energy Ignition (HEI), Electronic Spark Timing (EST) and High Voltage Switch (HVS). The HEI and EST systems are very similar to each other and differ more in nomenclature than anything else. These systems use distributors that contain pickups and electronic modules. The HVS system uses a distributor that contains nothing but a rotor and cap. All the HVS does is to distribute the spark to the appropriate cylinder; the engine control module (ECM) takes care of the rest of the timing parameters. The HVS distributor does contain a camshaft position sensor, but it is not used for timing determination.

HIGH ENERGY IGNITION (HEI) AND ELECTRONIC SPARK TIMING (EST) SYSTEMS



See Figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and 15

The HEI/EST system operates in basically the same manner as the conventional points type ignition system, with the exception of the type of switching device used. A toothed iron timer core is mounted on the distributor shaft which rotates inside of an electronic pole piece. The pole piece has internal teeth (corresponding to those on the timer core) which contains a permanent magnet and pick-up coil (not to be confused with the ignition coil). The pole piece senses the magnetic field of the timer core teeth and sends a signal to the ignition module which electronically controls the primary coil voltage. The ignition coil operates in basically the same manner as a conventional ignition coil (though the ignition coils DO NOT interchange).

Some distributors use a Hall Effect device to act as the switching device. This type of distributor uses a slotted vane that passes between a magnet and the Hall Effect device to signal when to initiate a spark. The Hall Effect device is a solid state sensor that acts as a magnetic activated switch. The slots in the vane effectively change the magnetic field set up by the magnet, causing the Hall Effect device to switch on and off. This signal is sent to the ignition module and processed in the same way that the above mentioned pick-up coil type distributor does.

The 4.3L engines up to 1995 use a knock sensor to retard the timing when knock is sensed. The knock sensor is located on the block and it sends a signal to the Electronic Spark Control (ESC) module located on a bracket at the back of the engine. The ESC module in turn sends a signal to the ECM to retard the timing.

The HEI/EST systems uses a capacitor within the distributor which is primarily used for radio interference suppression purposes.

None of the electrical components used in the HEI systems are adjustable. If a component is found to be defective, it must be replaced.



Click image to see an enlarged view

Fig. Fig. 1: EST distributor at home nestled at the back of the 4.3L engine



Click image to see an enlarged view

Fig. Fig. 2: HEI distributor with the coil mounted in the cap



Click image to see an enlarged view

Fig. Fig. 3: EST distributor with the coil mounted externally



Click image to see an enlarged view

Fig. Fig. 4: Electronic Spark Control components



Click image to see an enlarged view

Fig. Fig. 5: Distributor testing-HEI with "coil in cap''



Click image to see an enlarged view

Fig. Fig. 6: Distributor testing-HEI with "coil in cap,'' continued



Click image to see an enlarged view

Fig. Fig. 7: Distributor testing-HEI with "coil in cap,'' continued



Click image to see an enlarged view

Fig. Fig. 8: Distributor testing-HEI with remote coil



Click image to see an enlarged view

Fig. Fig. 9: Distributor testing-HEI with remoted coil, continued



Click image to see an enlarged view

Fig. Fig. 10: Ignition system check-2.5L engine



Click image to see an enlarged view

Fig. Fig. 11: Ignition system check-2.5L engine, continued



Click image to see an enlarged view

Fig. Fig. 12: Ignition system check-4.3L engine except with HVS



Click image to see an enlarged view

Fig. Fig. 13: Ignition system check-4.3L engine except with HVS, continued



Click image to see an enlarged view

Fig. Fig. 14: Ignition system check-4.3L engine except with HVS, continued



Click image to see an enlarged view

Fig. Fig. 15: Ignition system check-4.3L engine except with HVS, continued

HIGH VOLTAGE SWITCH (HVS) SYSTEM



See Figures 16 and 17



Click image to see an enlarged view

Fig. Fig. 16: HVS distributors are easily identified by their flat appearance and side mounted ignition wire towers



Click image to see an enlarged view

Fig. Fig. 17: HVS ignition systems have the ICM module mounted to the same bracket as the coil

The High Voltage Switch (HVS) system is the highest evolution in distributor based ignition systems. The only purpose the distributor has in the systems is to distribute the spark to the cylinders. It contains no ignition pickup coils, no ignition high voltage coils, no weights or springs, nor does it contain any ignition modules. It does contain a camshaft position sensor, but that sensor does not effect the operation of the vehicle. The camshaft position sensor is utilized to determine which cylinder is misfiring, under misfire conditions. The Powertrain Control Module (PCM) controls the ignition coil based on inputs from the various engine sensors. It sends a signal to the Ignition Control Module which uses the low voltage control signal (approximately 4 volts) to ground the ignition coil, thus completing the circuit. When the ICM switches off, the ignition coil fires. The ICM module is attached to the same bracket as the ignition coil.

PRECAUTIONS



Before troubleshooting the systems, 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. Timing lights which connect between the spark plug and the spark plug wire occasionally give false readings.

Some engines incorporate a magnetic timing probe terminal (at the damper pulley) for use of special electronic timing equipment. Refer to the manufacturer's instructions when using this equipment.

Spark Plug Wires

The plug wires are of a different construction than conventional wires. When replacing them, make sure to use the correct wires, since conventional wires won't carry the higher 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. While some tachometers may give a reading, this does not necessarily mean the reading is correct. In addition, some tachometers connect differently than others. If you can't figure out whether or not your tachometer will work on your vehicle, check with the tachometer manufacturer.

System Testers

Instruments designed specifically for testing the HEI system are available from several tool manufacturers. Some of these will even test the module.

 
label.common.footer.alt.autozoneLogo