How To Test the Ignition Coil
(1991-1997 4.0L Jeep)

TEST 1: Checking the High Tension Wire

The ignition coil’s high tension wire has to work harder than the other 6 spark plug wires... since it has to feed each of the 6 cylinders’ spark to the distributor cap.

Due to this work load... it’s prone to failing sooner than any of the others. Specifically: it stops transmitting the spark the ignition coil creates to the distributor cap.

This is a very common problem, especially if the spark plug wires are over 3 years old.

So, the very first thing we’ll do, in troubleshooting the ignition coil, is see if there’s spark coming out of this high tension wire.

OK, this what you need to do:

  1. 1

    Disconnect the ignition coil’s high tension wire from the distributor cap but leave the end that connects to the ignition coil connected.

  2. 2

    Now, on the end that connects to the center of the distributor cap, attach the your spark tester (see photo in image viewer).

  3. 3

    Using a battery jump start cable, ground the spark tester to the battery negative (-) terminal.

  4. 4

    When everything is set, have a helper crank up the engine while you observe the spark tester from a safe distance.

  5. 5

    The spark tester will give you one of two results: spark or no spark.

OK, let’s take a look at what your test results mean:

CASE 1: You got spark: This test result automatically tells you that the ignition coil and the wire are good. If there’s spark, the ignition coil is not the cause of your Jeep’s no start no spark condition.

CASE 2: You got no spark: this test result doesn’t condemn the ignition coil or the high tension wire just yet.

The next step is to check for spark directly on your Jeep’s ignition coil’s tower. For this test go to TEST 2: Testing for Spark Directly on the Coil Tower.

TEST 2: Testing for Spark Directly on the Coil Tower

So far, you have checked and confirmed that your Jeep’s ignition coil’s high tension wire is not sparking (TEST 1)...

... The next step is to check for spark directly on the ignition coil’s tower (with the high tension wire removed).

The result of this test will let us know if the ignition coil’s high tension is bad (and thus the cause of the no spark condition) or if we need to move on to the next test step and continue diagnosing the ignition coil.

IMPORTANT: Remember, this is an on-car test of the ignition coil... so don’t remove it or unplug it from its harness connector. The photo in the image viewer shows it off of the vehicle just to make it easier to explain the test.

Alright, this is what you’ll need to do:

  1. 1

    Remove the high tension wire from from your Jeep’s ignition coil.

  2. 2

    Now, connect the your spark tester to the ignition coil’s tower using a small piece of vacuum hose.

    NOTE: This is important, see how I’ve done it in the photo in the image viewer (the orange arrow points to the vacuum hose securing the spark tester to the coil’s tower).

  3. 3

    Now ground the spark tester using a battery jump start cable directly on the battery negative (-) terminal.

  4. 4

    When ready have your helper crank the engine as you eyeball the spark tester, you’ll see one of two results: spark or no spark.

OK, let’s take a look at what your test results mean:

CASE 1: You got spark: This spark test result tells you that the high tension wire is FRIED and is the cause of your no start condition. Replace all of the spark plug wires as a set.

As mentioned at the beginning of this test section, having this particular high tension wire fail (and stop transmitting spark to the distributor cap) is a very common issue. You should also inspect the spark plugs and the distributor cap and rotor for excessive wear and tear (and replace as necessary).

CASE 2: You got no spark: This no spark test result eliminates the high tension wire and means you’re getting closer to the actual cause of the ignition coil’s no spark condition.

The next steps are to verify that the Powertrain Control Module (fuel injection computer) is feeding power and activating the ignition coil. For this test, go to TEST 3: Verifying the Ignition Coil is Getting Power.

Jeep Vehicles:

  • Cherokee 4.0L
    • 1991, 1992, 1993, 1994, 1995, 1996, 1997
  • Comanche 4.0L
    • 1991, 1992
  • Grand Cherokee 4.0L
    • 1993, 1994, 1995, 1996, 1997

Jeep Vehicles:

  • Grand Wagoneer 4.0L
    • 1993
  • Wrangler 4.0L
    • 1991, 1992, 1993, 1994, 1995, 1996, 1997

“My wife and I were happy for 20 years. Then we met.”
Rodney Dangerfield

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