How to Test Ignition Coils

by Christian Killian

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From coils and coil packs to electronic ignitions and computers, the ignition system can be a mystery. How do you know whether a coil is causing the problems that you are experiencing in your car? Testing the coil is the first step in troubleshooting the system and will tell you without question if the coil is functioning as it was designed. The procedure is the same no matter what type of car you drive--the only difference is the location and type of coil you have to work with.

Step 1

Use a wrench to loosen the bolt on the negative battery cable and set the cable aside in a spot that will not allow it to touch the battery while you are working.

Step 2

Locate the coil on your vehicle. Remove the center wire that runs from the coil to the distributor, then remove any wires connected to the positive and negative terminals on the coil with a wrench. On older-style coils these will be posts on the top of the coil. Newer systems using coil packs that require you to unplug the wiring harness connector and use the pins in the connector for your points of contact.

Step 3

Place the first test lead from your ohmmeter on the negative terminal and the other on the positive terminal. Read the resistance across the terminals from the meter. You are looking for a range between 0.70 and 1.7 ohms. Anything outside that range indicates a defective coil.

Step 4

Move one of the test leads from the side terminal to the high-tension terminal in the center of the coil. It doesn't matter which lead is on which terminal, because you are reading resistance, not amps or volts. Note the reading from the ohmmeter--you are looking for a reading ranging from 7,500 to 11,000. Anything outside that range indicates a defective coil.

Step 5

Reinstall the coil wire on the high-tension terminal and the other wires on the side terminals. Install the retaining nuts on the side terminals and tighten them with a wrench.