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.
IMPORTANT: Before you start, it's important that you check all 6 spark plug wires for spark. If any one of those spark plug wires sparks, then the ignition coil is good.
OK, this what you need to do:
- 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
Attach the your spark tester (see photo above) to the end that connects to the center of the distributor cap.
- 3
Connect the spark tester to the battery negative (-) terminal using a battery jump start cable.
- 4
When everything is set, have a helper crank up the engine while you observe the spark tester from a safe distance.
- 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 is the correct and expected test result.
This test result tells you that the ignition coil and its high tension wire are good. You can conclude that the distributor cap and rotor are bad (and behind the no-start no-spark problem) if you have:
- Confirmed that none of the spark plug wires are sparking.
- Confirmed, in this test section, that the ignition coil's high tension wire is sparking.
Replace the distributor cap and rotor. If the spark plug wires are as old as the distributor cap and rotor -replace them too.
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 above 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
Remove the high tension wire from from your Jeep's ignition coil.
- 2
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 above (the orange arrow points to the vacuum hose securing the spark tester to the coil's tower). - 3
Connect the spark tester to the battery negative (-) terminal using a battery jump start cable.
- 4
When ready have your helper crank the engine as you eyeball the spark tester.
- 5
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 is the correct and expected test result.
You can correctly conclude that the ignition coil's high tension wire is FRIED and is the cause of your no-start condition if you have:
- Confirmed that none of the 6 spark plug wires are sparking (TEST 1).
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.
