How to Test Ignition Coil in a Vehicle

A hand securing an ignition coil in place inside an engine.
What You'll Need
Safety goggles
Hair ties (if needed)
Insulated pliers
Service manual
Spark plug socket
An assistant

In a vehicle, an ignition coil converts the 12 Volts of power from the battery to the thousands of volts that are needed to fire the spark plugs. This part is essentially an induction coil and, simply put, a high voltage transformer. While ignition coils are generally very sturdy and reliable, over time, due to heat, vibrations, and faulty insulation, they can become damaged.

There are two ways to test the ignition coil of your vehicle: the spark plug test and the bench test. While the spark plug test is effective, the bench test is more thorough. Why? In the first, you are relying on the spark to determine the condition of your coil; in fact, only in a no-spark condition would you be certain that your coil is faulty. In the second, you are relying on resistance readings and data to test the condition of the ignition coil. Here, even if there is only slight damage to your coil, your readings will indicate such.

Read on for proper instructions on how to conduct these tests so you can be sure if your coil needs to be replaced.

Step 1 - Take Precautions

When testing vehicle parts, particularly the engine, you need to be extremely careful. Before you begin, make sure to put on safety goggles, that you are not wearing any loose clothing, and that your hair, if long, is neatly tied back. Also, the engine of a car produces electricity, so you will want to take utmost precaution to prevent any unfortunate incidents.

Step 2 - Remove the Spark Plug/Windings

For the spark plug test, start by removing the wire from the plug. It is advisable to refer to your vehicle’s service manual to ensure that you are removing the right wires. Then, with the help of a spark plug socket, remove the spark plug.

If you're conducting the bench test, refer to your service manual and remove the coil to take it to bench.

Step 3 - Check for Sparks (Spark Plug Test)

After removing the spark plug, reattach the plug wire. Now, hold the plug wire using insulated pliers, and let the threaded bare end of the spark plug touch a grounding surface (any exposed metal area such as a bolt). Ask your assistant to start the ignition in the meantime. With the key turned, you should be able to see a bright blue spark at the end of the spark plug. If you do, your coil is fine. If you don’t, your coil needs replacing. However, even a poorly working coil can give a small spark; thus, to be thorough, you need the bench test.

Step 4 - Check Primary and Secondary Windings (Bench Test)

Refer to your service manual for the correct resistance readings applicable to your vehicle and model. Typically, for most automotive coils, a reading of 0.75 to 0.81 Ohms for the primary winding and 10,000 to 11,000 ohms for the secondary winding is correct. To check resistance, attach the multimeter/ohmmeter leads to the two outside poles on the primary winding. On the secondary winding, attach one lead to either of the side poles and the other to the central high tension terminal. If the readings are even slightly outside the resistance indicated in your service manual, get your ignition coil replaced.