General Information GM Full Size Vans 1987-1997 Repair Guide

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There are four general ignition systems used on these vehicles: High Energy Ignition (HEI), Electronic Spark Timing (EST), High Voltage Switch (HVS) and Distributor Ignition (DI) systems. 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 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. The Distributor Ignition (DI) system consists of the distributor, Hall effect switch (camshaft position sensor), ignition coil, secondary wires, spark plugs, knock sensor and crankshaft position sensor. The system is regulated by the Vehicle Control Module (VCM).

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

See Figures 1 through 13

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) and 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 through 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 contain 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.

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

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

Fig. 3: EST distributor with the coil mounted externally

Fig. 4: Electronic Spark Control components

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

Fig. 6: Distributor testing - HEI with "coil in cap" (continued)

Fig. 7: Distributor testing - HEI with "coil in cap" (continued)

Fig. 8: Distributor testing - HEI with remote coil

Fig. 9: Distributor testing - HEI with remote coil (continued)

Fig. 10: Ignition system check - 4.3L engine except with HVS

Fig. 11: Ignition system check - 4.3L engine except with HVS (continued)

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

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

IGNITION COIL

Carbureted Engines

1. Connect an ohmmeter between the TACH and BAT terminals in the distributor cap. The primary coil resistance should be less than one ohm (zero or nearly zero).

2. To check the coil secondary resistance, connect an ohmmeter between the rotor button and the BAT terminal. Then connect the ohmmeter between the ground terminal and the rotor button. The resistance in both cases should be between 6000 and 30,000 ohms.

3. Replace the coil only if the readings in Step 1 and 2 are infinite.

These resistance checks will not disclose shorted coil windings. This condition can be detected only with scope analysis or a suitably designed coil tester. If these instruments are unavailable, replace the coil with a known good coil as a final coil test.

Fuel Injected Engines

See Figure 14

1. Tag and disconnect the distributor lead and wiring from the coil.

2. Connect an ohmmeter as shown in Step 1 of the accompanying illustration. Place the ohmmeter on the high scale. The reading should be infinite.

Fig. 14: Fuel injected engine ignition coil test

3. Connect an ohmmeter as shown in Step 2 of the same illustration. Place the ohmmeter on the low scale. The reading should be very low or zero. If not, replace the coil.

4. Connect an ohmmeter as shown in Step 3 of the same illustration. Place the ohmmeter on the high scale. The meter should not read infinite. If it does, replace the coil.

5. Connect the distributor lead and wiring.

PICK-UP COIL

See Figures 15, 16 and 17

1. To test the pick-up coil, first disconnect the white and green module leads. Set the ohmmeter on the high scale and connect it between a ground and either the white or green lead. Any resistance measurement less than infinity requires replacement of the pick-up coil.

Fig. 15: The pick-up coil is located inside the distributor

Fig. 16: Testing the pick-up coil - coil-in-cap

Fig. 17: Testing the pick-up coil - separate coil

2. Pick-up coil continuity is tested by connecting the ohmmeter (on low range) between the white and green leads. Normal resistance is between 500 and 1500 ohms. Move the vacuum advance arm while performing this test. This will detect any break in coil continuity. Such a condition can cause intermittent misfiring. Replace the pick-up coil if the reading is outside the specified limits.

3. If no defects have been found at this time, and you still have a problem, then the module will have to be checked. If you do not have access to a module tester, the only possible alternative is a substitution test. If the module fails the substitution test, replace it.