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High Energy Ignition (HEI) System GM Chevy Mid-Size Cars 1964-1988 Repair Guide

See Figures 1 and 2

The General Motors/Delco-Remy High Energy Ignition (HEI) system is breakerless, pulse-triggered, transistor-controlled, inductive discharge ignition system. It was available as an option in 1974 and was standard all 1975 and later vehicles. With the exception of the inline 6-cylinder engine used through 1977, the entire HEI system is contained within the distributor cap. Inline 6-cylinder engines through 1977 are the only engines covered here to utilize an external coil. But with the exception of the external coil, the ignition systems are the same.

The distributor, in addition to housing the mechanical and vacuum advance mechanisms, contains the ignition coil (except on some inline six engines), 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).

For 1981 and later an HEI distributor with Electronic Spark Timing is used (for more information on EST, refer to Driveability & Emissions Controls). This system uses a one piece distributor cap, similar to 1980. All spark timing changes in the 1981 and later distributors are controlled electronically by the Electronic Control Module (ECM) which monitors information from various engine sensors, computes the desired spark timing and then signals the distributor to change the timing accordingly. No vacuum or mechanical advance systems are used whatsoever.

Fig. 1: Exploded view of a common HEI distributor assembly, (1981 and later models will have no vacuum advance unit)

Fig. 2: A partial exploded view of an HEI distributor assembly, equipped with EST (no vacuum advance unit)

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 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 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. 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 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.

Spark Plug Wires

See Figure 3

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.

Fig. 3: HEI distributors are equipped with a spark plug wiring retainer which allows removal of all wires from the distributor at the same time and without the need for labelling

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, or 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 ohmmeter and a voltmeter.

TROUBLESHOOTING THE HEI SYSTEM

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:

• Hard or no Starting

• Rough Idle

• Poor Fuel Economy

• Engine misses under load or while accelerating

If you suspect a problem in your 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 the vehicle battery is in a good state of charge. A defective or poorly charged battery will cause the various components of the ignition system to read incorrectly when they are being tested. Second, make sure all wiring connections are clean and tight, not only at the battery, but also at the distributor cap, ignition coil, and at the electronic control module.

Since the only change between electronic and conventional ignition systems is in the distributor component area, it is imperative to check the secondary ignition circuit first. If the secondary circuit checks out properly, then the engine condition is probably not the fault of the ignition system. To check the secondary ignition system, perform a simple spark test. Remove one of the plug wires and insert some sort of extension in the plug socket. An old spark plug with the ground electrode removed makes a good extension. Hold the wire so the extension is positioned about ¹/₄ in. away from the block, then 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 performed. Troubleshooting techniques fall into two categories, depending on the nature of the problem. The categories are (1) Engine cranks, but won't start or (2) Engine runs, but runs rough or cuts out. To begin with, let's consider the first case.

Engine Fails to Start

If the engine won't start, perform a spark test as described earlier. This will narrow the problem area down considerably. If no spark occurs, check for the presence of normal battery voltage of the battery (BAT) terminal in the distributor cap. The ignition switch must be in the ON position for this test. Either a voltmeter or a test light may be used for this test. Connect the test light wire to ground and probe end to the BAT terminal at the distributor. If the light comes on, you have voltage on 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 a 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 Rough or Cuts Out

See Figure 4

1. 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.

Fig. 4: SPARK PLUG WIRE RESISTANCE CHART-HEI IGNITION SYSTEM

2. If the plug wires are OK, remove the cap assembly and check for moisture, cracks, chips, carbon tracks, or any other high voltage leaks or failures. Replace the cap if any defects are found. 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 following.

DISTRIBUTOR COMPONENTS TESTING

See Figures 5, 6 and 7

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 (on internal-coil distributor caps) and the battery wire disconnected. If a tachometer is connected to the TACH terminal, disconnect it before making these tests.

1. Connect an ohmmeter between the TACH and BAT terminals in the distributor cap. The primary coil resistance should be less than 1&omega.

Fig. 5: Testing an external HEI coil, Test 1: uses the high scale setting and the reading should be very high or infinite. Test 2: uses the low setting and the reading should be very low or zero. Test 3: uses the high scale, the reading should NOT be infinite.

Fig. 6: Testing an internal HEI coil, Test 1: measures the primary coil resistance. Test 2: shows the secondary resistance connection.

2. To check the coil secondary resistance, connect an ohmmeter between the rotor button and BAT terminal. Note the reading. Connect the ohmmeter between the rotor button and the TACH terminal. Note the reading. The resistance in both cases should be between 6,000 and 30,000&omega. Be sure to test between the rotor button and both the BAT and TACH terminals.

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

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

4. 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. 7: Ohmmeter 1 shows testing the pickup coil. Ohmmeter 2 shows the testing the pickup coil continuity (1980 shown, most models similar)

5. Pick-up coil continuity is tested by connecting the ohmmeter (on low range) between the white and green leads. Normal resistance is between 650 and 850&omega on 1975-76 models or 500 and 1500&omega on 1977 and later models. 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 if the reading is outside the specified limits.

6. If no defects have been found at this time, 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.

HEI SYSTEM MAINTENANCE

Except for periodic checks of the spark plug wires, and an occasional check of the distributor cap for cracks (see Steps 1 and 2 under Engine Runs, But Runs Rough or Cuts Out for details), no maintenance is required on the HEI System. No periodic lubrication is necessary; engine oil lubricates the lower bushing, and an oil-filled reservoir lubricates the upper bushing.

COMPONENT REPLACEMENT

Integral Ignition Coil

See Figures 8, 9, 10, 11, 12, 13, 14, 15, 16 and 17

1. Disconnect the negative battery cable, then if necessary for access, remove the air cleaner assembly.

2. Remove the ignition wire set retainer along with the spark plug wires from the top of the distributor cap.

3. If necessary, disengage the feed and module wire terminal connectors from the distributor cap.

On some vehicles, interference with the cowl may make access to the cover and coil screws difficult. If necessary, the distributor cap may be removed from the vehicle to continue the service.

4. As applicable, remove the coil cover-to-distributor cap screws or unsnap the retainers and remove the coil cover.

5. Remove the coil-to-distributor cap retaining screws.

Fig. 8: The integral coil found in most HEI distributor assemblies is retained by 4 coil attaching screws

6. Using a pair of needle nose pliers, remove the coil wire spade terminals from the cap. If the cap was removed, you may also use a blunt drift to press the coil wire spade terminals up out of the distributor cap.

7. Lift the coil up out of the distributor cap.

Fig. 9: Check the condition of the rubber arc seal located under the coil

8. Remove and clean the coil spring, rubber seal washer and coil cavity of the distributor cap.

To install:

9. Coat the rubber seal with a dielectric compound furnished in the replacement ignition coil package.

10. Install the rubber arc seal washer and the coil spring, then position the coil into the distributor assembly. Make sure the wire spade terminals are properly positioned in the distributor cap.

11. Install and secure the coil retaining screws.

12. Install the coil cover to the distributor cap and secure using the fasteners.

13. If removed, engage the feed and module wire terminal connectors to the distributor cap.

14. Install the spark plug wires and ignition set retainer assembly to the top of the distributor cap.

15. Connect the negative battery cable.

Fig. 10: Remove the ignition wire set retainer and wires from the top of the distributor cap

Fig. 11: Loosen the coil cover retaining screws

Fig. 12: Remove the coil cover for access to the ignition coil assembly

Fig. 13: Remove the coil retaining screws

Fig. 14: Disengage the spade terminal wire connectors from the distributor cap

Fig. 15: Lift the coil from the distributor cap, then remove the rubber arc seal found under the coil

Fig. 16: Wipe the top of the distributor cap to remove any dirt, debris or moisture

Fig. 17: Coat the rubber arc seal with dielectric compound

Distributor Cap

See Figures 18, 19, 20 and 21

1. Disconnect the negative battery cable and if necessary for access, remove the air cleaner assembly.

2. Remove the retainer and spark plug wires from the cap.

3. Remove the feed and module wire terminal connectors from the distributor cap.

If wire removal is difficult with the cap installed, release the cap locktabs and reposition the cap for better access to the wire connectors.

4. Depress and release the 4 distributor cap-to-housing locktabs and lift off the cap assembly.

5. If necessary for component replacement, remove the integral coil from the distributor cap.

To install:

6. If removed, install the integral coil to the distributor cap.

7. Install the cap to the distributor assembly and secure using the housing retainers.

8. Engage the feed and module wire terminal connectors to the distributor cap assembly.

9. Install the spark plug wires and retainer to the cap.

10. Connect the negative battery cable.

Fig. 18: If the wires must be repositioned further away for access, tag and disconnect wires from the harness retainer

Fig. 19: Depress and turn the cap locktabs in order to release the cap from the distributor housing

Fig. 20: Use a screwdriver to carefully release the wiring connector tabs, then disengage the tabs from the cap

Fig. 21: Check the cap for corrosion or damage and clean or replace, as necessary

Rotor

See Figures 22, 23, 24 and 25

1. Disconnect the negative battery cable and if necessary for access, remove the air cleaner assembly.

2. Remove the distributor cap from the housing assembly.

Although most rotors can only be installed in 1 direction, it is still wise to note the rotors position before removal.

3. Remove the two rotor attaching screws and note the position of the rotor, then remove the rotor from the distributor.

To install:

4. Install the rotor facing in the direction noted earlier, then secure the rotor using the 2 attaching screws.

5. Install the distributor cap to the housing assembly.

6. Connect the negative battery cable.

Fig. 22: Release the locktabs and remove the distributor cap

Fig. 23: Check the rotor for moisture, corrosion or damage

Fig. 24: A moisture displacing lubricant may get a wet engine running again, at least enough to get you home

Fig. 25: Loosen the rotor retaining screws and remove the rotor from the distributor assembly

Pick-Up Coil

See Figures 26 and 27

1. Disconnect the negative battery cable.

2. Matchmark and remove the distributor assembly from the vehicle. Be sure to mark both the rotor-to-housing position and housing-to-engine position for assembly purposes. For details on distributor removal, refer to Engine & Engine Overhaul of this information.

3. Remove the rotor from the distributor, then mount the distributor assembly in a suitable soft-jawed vise. Be careful not to damage the distributor housing.

4. Mark the distributor shaft and gear so they may be reassembled in the same position, then drive out the roll pin from the base of the shaft.

Fig. 26: Drive out the roll pin from the base of the distributor shaft using a punch

5. Remove the gear and pull the shaft assembly from the distributor.

Fig. 27: Pick-up coil removal (note that 1981 and later models have no vacuum advance unit)

6. Remove the 3 attaching screws, then remove the magnetic shield, as applicable.

7. Remove the retaining ring, pick-up coil, magnet and pole piece.

To install:

8. Install the pole piece, magnet, pick-up coil and retaining ring.

9. As applicable, install the magnetic shield and the 3 attaching screws.

10. Install the shaft assembly and gear to the distributor.

11. Make sure the shaft and gear are aligned as noted during removal, then drive roll pin into position.

12. Install the rotor to the distributor assembly as noted during removed, then remove the assembly from the vise.

13. Install the distributor assembly to the vehicle aligning the marks made during removal. When the distributor is inserted into the block, then rotor must turn and face the original rotor-to-housing alignment mark in order to assure correct engine timing.

14. Connect the negative battery cable.

Vacuum Advance (1975-80)

1. Disconnect the negative battery cable.

2. Remove the distributor cap and rotor as previously described.

3. Disconnect the vacuum hose from the vacuum advance unit.

4. Remove the two vacuum advance retaining screws, pull the advance unit outward, rotate and disengage the operating rod from its tang.

To install:

5. Install the vacuum advance unit to the distributor assembly by engaging the operating rod to the tang, then rotating and pushing the advance unit inward.

6. Secure the advance unit using the retaining screws.

7. Connect the vacuum hose to the unit.

8. Install the rotor and the distributor cap to the assembly.

9. Connect the negative battery cable.

Module

See Figures 28, 29, 30 and 31

1. Disconnect the negative battery cable.

2. Remove the distributor cap and rotor as previously described.

3. Disengage the harness connector and pick-up coil spade connectors from the module while noting their positions for installation purposes. Be careful not to damage the wires when removing the connector.

If difficulty is encountered, it may be easier to separate the module from the distributor, then disengage the wiring.

4. Remove the two screws and module from the distributor housing.

To install:

5. Coat the bottom of the new module with dielectric compound supplied with the new module.

6. Install the ignition module to the distributor housing and secure using the retaining screws.

7. Carefully engage the harness connector and the pick-up coil space connectors to the module.

8. Install the rotor and the distributor cap to the distributor assembly.

9. Connect the negative battery cable.

Fig. 28: Remove the module retainers

Fig. 29: Gently pry the module upward to remove it

Fig. 30: Ignition module replacement, be sure to coat the mating surfaces with silicone lubricant

Fig. 31: View of the distributor housing with the ignition module/capacitor assembly removed

HEI SYSTEM TACHOMETER HOOKUP

For all vehicles with an internal coil distributor assembly, there should be a terminal marked TACH on the factory installed distributor cap. Connect one tachometer lead to this terminal and the other lead to a ground. On some tachometers, leads must be connected to the TACH terminal and to the battery positive terminal.

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

1975-77 models with an inline 6-cylinder engine 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 connect the other one to a suitable ground.