Testing a spark plug ignition coil

Materials Needed

• Basic set of hand tools (for removal of ignition coil)
• Digital multimeter

Step 1: Research the specifications. Find out what the correct resistance reading of the ignition coils should be for your vehicle.

These specifications can usually be found in the factory service manual for the vehicle, and are usually specified as a range, measured in “ohms” (symbol: Ω).

Step 2: Locate the ignition coil, or coils, on your vehicle. These are usually located right on the engine, either bolted directly to the spark plugs or mounted remotely somewhere on top of the engine.

If necessary, remove any plastic covers that may be covering the ignition coils.

Step 3: Disconnect the wiring harness for the ignition coils. Remove them using your hand tools.

Ignition coils are usually very simple to remove, often times only held in by one or two bolts.

Step 4: Test the ignition coil’s primary ignition circuit. Ignition coils have two circuits that need to be checked: the primary and secondary ignition circuit.

Connect the positive and negative leads of your multimeter to the positive and negative terminals of your ignition coil. On some coils, the terminals will be explicitly marked positive and negative; others will simply have the two pins or terminals that are located at the connector.

Most ignition coils should have a primary resistance falling somewhere between 0.4 and 2 ohms; however, refer to your manufacturer's specifications for the correct reading. If a reading of zero is displayed, that signifies that the ignition coil has shorted internally in the primary windings and needs to be replaced. A reading over the specification signals that the ignition coil is open, which would also indicate a need to replace the coil.

Step 5: Test the secondary circuit of the ignition coil. Connect your multimeter to the positive terminal or pin of your coil, and to the high output terminal that goes to the spark plug.

Most ignition coils should have a secondary resistance falling somewhere between 6,000 to 10,000 ohms;however, refer to manufacturer specifications for the correct range. If a reading of zero is displayed, that signifies that the coil has short circuited and needs to be replaced, while an excessive reading signifies that the coil is open, and also needs to be replaced.

Step 6: Repeat the procedure as needed. Follow Step 4 and Step 5 for testing each individual coil if your vehicle has more than one. Ensure that all of the ignition coils are operating correctly.

Step 7: Reinstall the ignition coils, after replacing them as needed. Once all of the ignition coils have been tested, replace any faulty coils, and reinstall the rest of them on the vehicle.

Make sure to reconnect all of the electrical connectors, and reinstall any engine covers or other trim pieces that may have been removed.

Ignition coils are a very important part of the ignition system, and can dramatically affect the engine’s performance when not functioning correctly. A failed coil can cause all sorts of problems, and testing them will ensure that your vehicle stays running in top condition. If you have any difficulty completing the steps to test your ignition coils, a certified mobile mechanic from YourMechanic will be happy to come out and have a look at them.