See Figure 1
The General Motors HEI system is a pulse triggered, transistor controlled, inductive discharge ignition system. The entire HEI system is contained within the distributor cap.
The distributor, in addition to housing the mechanical and vacuum advance mechanisms, contains the ignition coil, 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).
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 the points did 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 then directed through the rotor and 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. 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.
HEI SYSTEM PRECAUTIONS
Before going on to 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 an 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 truck, check with the tachometer manufacturer. Dwell readings 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 test given in the following section will require only a multitester with volt and ohm scales.
TROUBLESHOOTING THE HEI SYSTEM
See Figures 2 and 3
The symptoms of a defective component within the HEI system are exactly the same as those you would encounter in a conventional system. Some of these symptoms are:
Engine Misses Under Load or While Accelerating
If you suspect a problem in the ignition system, there are certain preliminary checks which you should carry out before you begin to check the electronic portions of the system.
First, it is extremely important to make sure that the vehicle's battery is in good condition. A defective or poorly charged battery will cause the various components of the ignition system to read incorrectly when tested.
Second, make sure all of the wiring connections are clean and tight, not only at the battery, but also at the distributor cap, coil and module.
Since the major difference between electronic and point type ignition systems is in the distributor area, it is imperative to check the secondary ignition wires first. If the secondary system checks out okay, then the problem is probably not in the ignition system. To check the secondary system, perform a simple spark test. Remove on of the spark plug wires from the plug and insert a makeshift extension made of conductive metal, in the wire boot. Hold the wire and extension about 1 / 4 in. (6mm) away from the block and crank the engine. If a normal spark occurs, then the problem is most likely not in the ignition system. Check for fuel system problems, or fouled spark plugs.
If, however, there is no spark or a weak spark, then further ignition system testing will have to be done. Troubleshooting techniques fall into two categories, depending on the nature of the problem. The categories are (1) Engine cranks, but won't start, and (2) Engine runs, but runs rough or cuts out.Engine Fails to Start
If the engine won't start, perform a spark test as described earlier. If no spark occurs, check for the presence of normal battery voltage at the battery (BAT) terminal in the distributor cap. The ignition switch must be in the on position for this test. Either a multitester or a test light may be used for this test. Connect the test light wire to ground and the probe end to the BAT terminal at the distributor. If the light comes on, you have voltage to the distributor. If the light fails to come on, this indicates an open circuit in the ignition primary wiring leading to the distributor. In this case, you will have to check wiring continuity back to the ignition switch using test light. If there is battery voltage at the BAT terminal, but no spark at the plugs, then the problem lies within the distributor assembly. Go on to the distributor components test section.Engine Runs, but Runs Roughly or Cuts Out
- Make sure the plug wires are in good shape first. There should be no obvious cracks or breaks. You can check the plug wires with an ohmmeter, but do not pierce the wires with a probe. Check the chart for the correct plug wire resistance.
- If the plug wires are okay, remove the cap assembly, and check for moisture, cracks, chips, or carbon tracks, or any other high voltage leaks or failures. Replace the cap if you find any defects. Make sure the timer wheel rotates when the engine is cranked. If everything is all right so far, go on to the distributor components test section.
If the trouble has been narrowed down to the units within the distributor, the following tests can help pinpoint the defective component. An ohmmeter with both high and low ranges should be used. These tests are made with the cap assembly removed and the battery wire disconnected.
- 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).
- 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 6,000 and 30,000-.
- Replace the coil only if the readings in steps 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.
- 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.
- 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-. 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 specific limits.
- 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.
See Figures 4, 5, 6, 7, 8, 9, 10 and 11Integral Ignition Coil
- Disconnect the feed and module wire terminal connectors from the distributor cap.
- Remove the ignition set retainer.
- Remove the 4 coil cover-to-distributor cap screws and coil cover.
- Remove the 4 coil-to-distributor cap screws.
- Using a blunt drift, press the coil wire spade terminals up out of distributor cap.
- Lift the coil up out of the distributor cap.
- Remove and clean the coil spring, rubber seal washer and coil cavity of the distributor cap.
- Coat the rubber seal with a dielectric lubricant furnished in the replacement ignition coil package.
- Reverse the above procedures to install.
See Figure 12
- Remove the feed and module wire terminal connectors from the distributor cap.
- Remove the retainer and spark plug wires from the cap.
- Depress and release the 4 distributor cap-to-housing retainers and lift off the cap assembly.
- Remove the 4 coil cover screws and cover.
- Using a finger or a blunt drift, push the spade terminals up out of the distributor cap.
- Remove all 4 coil screws and lift the coil, coil spring, and rubber seal washer out of the cap coil cavity.
- Using a new distributor cap, reverse the above procedures to assembly, being sure to clean and lubricate the rubber seal washer with dielectric lubricant.
See Figure 13
- Disconnect the feed and module wire connectors from the distributor.
- Depress and release the 4 distributor cap to housing retainers and lift off the cap assembly.
- Remove the two rotor attaching screws and rotor.
- Reverse the above procedure to install.
- Remove the distributor cap and rotor as previously described.
- Disconnect the vacuum hose from the vacuum advance unit.
- Remove the two vacuum advance retaining screws, pull the advance unit outward, rotate, and disengage the operating rod from its tang.
- Reverse the above procedure to install.
See Figures 14 and 15
- Remove the distributor cap and rotor as previously described.
- Disconnect the harness connector and pick-up coil spade connectors from the module. Be careful not to damage the wires when removing the connector.
- Remove the two screws and module from the distributor housing.
- Coat the bottom of the new module with dielectric lubricant supplied with the new module. Reverse the above procedure to install.