Jeep Ignition Coil test is one of those test you really need to know. It can save you lots of headache and money knowing how to do it. You don't need any fancy tools to test a Jeep ignition coil. All you need is a simple multi-meter and a 12-volt test light. So, with those two simple tools and about 15 minutes can answer a few vital diagnostic questions for your ailing Wrangler or Cherokee.
At the bottom of the page is a YouTube vid explaining how to test the ignition coil and associated harness plug.
First of all, where is the ignition coil located? On the YJ, XJ & some ZJs they are located on the lower front passenger side of the block. If you own a TJ (Wranglers after '97) or other Jeep with the coil-on-plug configuration there will be no coil or plug wires coming from the passenger side of the engine block.
The coil in this article will be the one for the YJ, XJ style ignition systems.
After locating the Jeep ignition coil, first thing I would do is to test spark coming from the plug wire post.
So, to test for spark. Have your Jeep in neutral (or park if it's an automatic). Be sure the emergency brake is full engaged and/or wheels chocked to prevent it rolling over your toes or worse. Pull the coil wire from the distributor and put an extra spark plug in the end you removed from the distributor. Ground the spark plug and have someone turn the key over like they are starting the engine. Keep your fingers/hand away from the metal; part of the plug. There is a high chance for getting a hard electrical shock, trust me ... it hurts!!
If you got a nice blue spark as you turn the engine over, no need to investigate farther. If the spark is orange or some similar color. Either the coil is weak and is going bad or you do not have proper voltage going into the coil.
If you have a weak spark coming from the spark plug, lets check the ignition input voltage first. On the plug there is a green and a white wire. The White wire is a 12 volt constant. Turn the key to ignition on/run (no need to try to start it). Test for voltage at the white wire.
There should be at least 12 volts reading here. If there is not, you either have PCM or wiring harness issues. If there is 12+ volts, you have checked this one off as good! :-)
Now, testing the Signal or green wire. This wire comes from your PCM so your coil knows when to shoot the mega voltage to the distributor then to the spark plugs. Use a small bare wire to back prob the plug at the green wire location. Insert the wire into the back of the plug until it bottoms out. This should give you electrical contact with the metal contact that connects to the ignition coil. Don't use a multi-meter on this. It will just drive you and the meter crazy. Get a cheap 12 volt test light to make contact to the bare wire stuck in the back of the coil wiring harness plug. Turn the engine over like you are starting it. The test light should pulse for each time the PCM sends signal. It will be rather fast like a strobe light.
If there is no electrical pulse (ie test light flash), double check that the wire has not lost contact with the inside of the plug. If you are sure it is making proper connection and you are getting no test light pulse/flash. You have with a PCM, wiring harness, Cam Position Sensor or Crank Position Sensor issue. I'd test the Cam Position and Crank position Sensors first because typically one of the two will be the issue.
Now, lets check the Primary & Secondary posts of the Jeep ignition coil. Remove the coil from the bracket by removing the two 10mm bolts. Or, you could remove the coil/bracket combination from the engine block, which ever you prefer. Having the ability to move the coil around to access the posts will make testing so much easier.
To measure the Primary posts, set your multi-meter to around 20 Ω's. Place the leads across the post where the wiring harness plug snaps onto the coil. You should get a measurement between .95 - 1.20 Ω's. This can vary a little but too much. If its way out, replace the coil.
To measure the Secondary posts, set your multi-meter to around 20K Ω's. Place one lead on the post where the spark plug wire goes. Touch one of the small pins where the wiring harness plugs in with the other meter lead. Then the other small pin. You should get a reading between 11.3M (11,300) - 15.3M (15,300) Ω's. Again this may vary by a little .. but not too far from that. If your reading are way out from that, replace the coil.
If your ignition coil test bad you can replace it with either a factory stock replacement from an auto parts store. Or, if you would like better performance and more fuel mileage use a high performance replacement like this MSD ignition coil. True, performance coils do cost more upfront. The money saved with better fuel mileage will pay you back for the added expense of the upgraded coil.