How To Test A Vehicle's Ignition Coil

Posted on: 2 August 2017

Stalling, a louder engine, and a misfiring engine in your vehicle could be signs of a failing spark plug ignition coil. An ignition coil converts 12-volts into a high voltage that signals the spark plugs to start the engine. If the voltage gets too high, it can burn through the insulation and cause a short in the coil. Before you make repairs, test the ignition coil by following these tips:

Prepare to Test the Coil

To test the coil, you need:

  • work gloves
  • safety goggles
  • insulated pliers
  • wrench or ratchet set
  • spark plug socket
  • spark plug tester (optional)
  • service manual for your model
  • multimeter
  • an assistant

Park the vehicle in an area with plenty of light, turn off the motor, then let it cool for an hour. Raise the hood, prop it, and disconnect the negative battery cable.

Remove the Coil

Look for the ignition coils, which are commonly bolted to the engine or spark plugs or sometimes installed near the distributor, if there is one. Remove the coils with the correct socket or wrench; ensuring nothing drops into the hole. You may need to twist the coils slightly, so they can clear fuel injectors. Cover the hole with a shop rag.

Some coils are covered by plastic housing, which you should be able to pry off by hand. Use a spark plug socket to detach one wire from the spark plug.

Do the Spark Test

You may perform a spark test or a bench test. To do the spark plug test, grasp the spark plug with insulated pliers, and reconnect the wire. This test is easier with a spark plug testing kit, and it reduces the risk of damage to coils.

Let the coil dangle over a metal surface, such as the bolt. Have you assistant start the engine, and watch for a bright blue spark. If there is no spark, or you see an orange spark replace the coil. Proceed to the bench test, if you need further verification, or you get small blue sparks since it doesn't indicate the coils are in good condition.

Check with a Multimeter (Bench Test)

Consult your manual for the suggested resistance reading for the coils, which is commonly between 0.75 Ohms and 2.0 Ohms for primary coils and between 7,500 Ohms and 11,000 Ohms for secondary coils. Connect a probe to the outer poles on the primary coil, and check the reading. 

Attach a probe to the positive pole of the secondary coil, and leave a probe on one outer pole of the primary coil. Touch a probe to the output terminal on the spark plug. A 0 reading or a reading outside the suggested range on any pole indicates the coils are faulty.

Contact a company like AutoMedics for more information and assistance.