How to Test Ignition Coil in a Vehicle

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. The ignition coil is essentially an induction coil, and simply put, it is 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: spark plug test and bench test. While the spark plug test is effective, the bench test is more thorough. Why? In the first test, 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 test, you are relying on resistance readings and data to test the condition of the ignition coil. Here, even if there is slight damage to your ignition coil, your readings will indicate that.

What You Need

Spark Plug Test:

• Spark plug socket
• Insulated pliers
• Safety goggles
• Service Manual

Also, do not forget to take the help of another person.

Bench Test:

• Multimeter/Ohmmeter
• Service Manual

Step 1 – Precautions

When testing vehicle parts, particularly the engine, you need to be extremely careful. Before you begin testing, make sure 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 so as to prevent any unfortunate incidents.

Step 2 – Remove the Spark Plug/Windings

Spark Plug Test: Remove the spark plug wire from the plug. It is advisable to refer to your vehicle’s service manual so as to ensure that you are removing the right wires. Next, with the help of a spark plug socket, remove the spark plug.

Bench Test: Refer to your service manual and remove the two windings (primary and secondary) that are inside your ignition coil.

Step 3 – Check for Sparks (Spark Plug Test)

After, removing the spark plug, attach the plug wire back to the spark plug. 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). Next, ask another person to start the ignition. 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 will 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. For 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.