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High Energy Ignition (HEI) System



Description & Operation

Diagnosis & Testing

Coil Ignition

Ignition Module












Description And Operation

The HEI system, used on 2.5L, 2.8L and 4.0L engines, is a pulse-triggered, transistorized controlled, inductive discharge ignition system. The entire HEI system (except for the ignition coil on fuel injected engines) is contained within the distributor cap.

The distributor, in addition to housing the mechanical and vacuum advance mechanisms, contains 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 an 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.



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 if your Jeep is equipped 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 Jeep, 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, the tests given in the following section will require only an ohmmeter and a voltmeter.

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Diagnosis and Testing


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:

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 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.

  1. 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.
  2. 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.
  3. Check for fuel system problems, or fouled spark plugs.
  4. 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 or (2) Engine runs, but runs rough or cuts out.

Engine Fails to Start

  1. 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.
  2. If battery voltage is not present, 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.
  3. 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 test the ignition coil.

Engine Runs, but Runs Roughly or Cuts Out

  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.
  2. If the plug wires are OK, remove the cap assembly, and check for moisture, cracks, chips, or carbon tracks, or any other high voltage leaks or failures.
  3. 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 test the ignition coil.




  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.
  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.

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Coil Ignition




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

The ignition coil is designed to operate without an external ballast resistor. Inspect the coil for arcing. Test the coil primary and secondary resistance.

Measure the primary resistance as follows:

  1. Connect an ohmmeter between the positive (+) and negative (-) terminals (the terminals which are connected to the engine wiring harness) on the coil.
  2. To test the secondary resistance, connect an ohmmeter between the positive (+) coil terminal and the high voltage cable terminal on the coil, then, measure the resistance between the positive (+) coil terminal and the coil case; the resistance for the case should exhibit infinite resistance.
  3. For coils manufactured by Diamond, the primary resistance at 70-80F (21-27C) should be 0.97-1.18 ohms and the secondary resistance should be 11,300-15,300 ohms.
  4. Coils built by the Toyodenso company should exhibit a primary resistance at 70-80F (21-27C) of 0.95-1.20 ohms and a secondary resistance of 11,300-13,300 ohms.
  5. Replace any coil with a new one if it does not meet the specifications.
  6. If the ignition coil is replaced with a new one due to a burned tower, carbon tracking, arcing at the tower, or damage to the terminal or boot on the coil end of the secondary cable, the cable must also be replaced with a new one. Arcing at the tower will carbonize the nipple which, if it is connected to a new coil, will cause the coil to fail.
  7. If a secondary cable shows any signs of damage, the cable should be replaced with a new cable and new terminal. Carbon tracking on the old cable can cause arcing and the failure of a new coil.

Removal & Installation


Carbureted Engines

  1. Disconnect the feed and module wire terminal connectors from the distributor cap.
  2. Remove the ignition set retainer.
  3. Remove the coil cover-to-distributor cap screws and coil cover.
  4. Remove the 4 coil-to-distributor cap screws.
  5. Using a blunt drift, press the coil wire spade terminals up out of distributor cap.
  6. Lift the coil up out of the distributor cap.
  7. Remove and clean the coil spring, rubber seal washer and coil cavity of the distributor cap.
  8. Coat the rubber seal with a dielectric lubricant furnished in the replacement ignition coil package.
  9. Reverse the above procedures to install.

Fuel Injected Engines

  1. Make sure that the ignition switch is in the OFF position.
  2. Tag and disconnect the coil wire and the connector on the side of the coil.
  3. Remove the nuts holding the coil and bracket assembly to the engine and lift out the coil. The coil may be riveted to the bracket, to remove it will require drilling the rivets and punching them out.
  4. Position the coil on the engine and tighten the nuts.
  5. Engage the coil wire and electrical connectors.

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Ignition Module

Removal & Installation


  1. Remove the distributor cap and rotor.
  2. Disconnect the harness connector and pick-up coil spade connectors from the module. Be careful not to damage the wires when removing the connector.
  3. Remove the two screws and module from the distributor housing.
  4. Coat the bottom of the new module with dielectric silicone lubricant. This is usually supplied with the new module. Reverse the above procedure to install.
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  6. Remove the high-tension wires from the distributor cap terminal towers, noting their positions to assure correct reassembly.
  7. Remove the primary lead from the terminal post at the side of the distributor.
  8. Disconnect the vacuum line if equipped.
  9. Remove the distributor cap retaining hooks or screws and remove the distributor cap.
  10. Note the position of the rotor in relation to the base. Scribe a mark on the base of the distributor and on the engine to facilitate reinstallation. Align the marks with the direction the metal tip of the rotor is pointing.
  11. Remove the bolt that hold the distributor to the engine.
  12. Lift the distributor assembly from the engine.



2.8L Engine


  1. Insert the distributor shaft and assembly into the engine. Line up the mark on the distributor and the one on the engine with the metal tip of the rotor. Make sure that the vacuum advance diaphragm is pointed in the same direction as it was pointed originally. This will be done automatically if the marks on the engine and the distributor are line up with the rotor.
  2. Install the distributor hold-down bolt and clamp. Leave the screw loose enough so that you can move the distributor with heavy hand pressure.
  3. Connect the primary wire to the distributor side of the coil. Install the distributor cap on the distributor housing. Secure the distributor cap with the spring clips or the screw type retainers, whichever is used.
  4. Install the spark plug wires. Make sure that the wires are pressed all of the way into the top of the distributor cap and firmly onto the spark plugs.

Design of the V6 engine requires a special form of distributor cam. The distributor may be serviced in the regular way and should cause no more problems than any other distributor, if the firing plan is thoroughly understood. The distributor cam is not ground to standard 6-cylinder indexing intervals. This particular form requires that the original pattern of spark plug wiring be used. The engine will not run in balance if No. 1 spark plug wire is inserted into No. 6 distributor cap tower, even though each wire in the firing sequence is advanced to the next distributor tower. There is a difference between the firing intervals of each succeeding cylinder through the 720 engine cycle.

  1. Set the ignition timing.


If the engine has been turned while the distributor has been removed, or if the marks were not drawn, it will be necessary to initially time the engine.

  1. If the engine has been rotated while the distributor was out, you'll have to first put the engine at Top Dead Center firing position on No. 1 cylinder. You can either remove the valve cover or No. 1 spark plug to determine engine position. Rotate the engine with a socket wrench on the nut at the center of the front pulley in the normal direction of rotation. Either feel for air being expelled forcefully through the spark plug hole or watch for the engine to rotate up to the Top Center mark without the valves moving (both valves will be closed). If the valves are moving as you approach TDC or there is no air being expelled through the plug hole, turn the engine another full turn until you get the appropriate indication as the engine approaches TDC position.
  2. Start the distributor into the engine with the matchmarks between the distributor body and the engine lined up. Turn the rotor slightly until the matchmarks on the bottom of the distributor body and the bottom of the distributor shaft near the gear are aligned. Then, insert the distributor all the way into the engine. If you have trouble getting the distributor and camshaft gears to mesh, turn the rotor back and forth very slightly until the distributor can be inserted easily. If the rotor is not now lined up with the position of No. 1 plug terminal, you'll have to pull the distributor back out slightly, shift the position of the rotor appropriately, and then reinstall it.
  3. Align the matchmarks between the distributor and engine. Install the distributor mounting bolt and tighten it finger-tight. Reconnect the vacuum advance line and distributor wiring connector, and reinstall the cap. Reconnect the negative battery cable. Adjust the ignition timing as described in Section 1. Then, tighten the distributor mounting bolt securely.

2.5L and 4.0L


  1. Clean the mounting area of the cylinder block and install a new distributor mounting gasket.

On some models there is a fork on the distributor housing. The slot in the fork aligns with the distributor hold-down bolt hole in the engine block. The distributor is correctly installed when the rotor is correctly positioned and the slot is aligned with the hold-down bolt hole. On these computer controlled distributors, initial ignition timing is not adjustable.

  1. Align the rotor tip with the scribe mark on the distributor housing during removal, then turn the rotor approximately 1/8 turn counterclockwise past the scribe mark.
  2. Slide the distributor shaft down into the engine. It may be necessary to move the rotor and shaft slightly to engage the distributor shaft with the oil pump slot. Align the scribe mark on the distributor housing with the mark on the cylinder block.

Ensure that the distributor is fully seated against the cylinder block. It may be necessary to slightly rotate (bump) the engine while applying light downward force to fully engage the distributor shaft with the oil pump drive gear shaft.

  1. Install the distributor shaft hold-down and bolt. Tighten the bolt to 17 ft. lbs. (23 Nm).
  2. Install distributor cap and ignition wires. Insure that the wires are routed correctly before attempting to start engine.


  1. Rotate the engine until the No.1 piston is at TDC compression.
  2. Using a flat-bladed screwdriver, in the distributor hole, rotate the oil pump gear so that the slot in the oil pump shaft is in the correct position (see illustration).
  3. With the distributor cap removed, install the distributor so that the rotor is positioned correctly (see illustration). Insure that the distributor is fully seated against the cylinder block. If not, remove the distributor and perform the entire procedure again.
  4. Tighten the hold-down bolt.
  5. Install distributor cap and ignition wires. Insure that the wires are routed correctly before attempting to start engine.

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