Detecting, Diagnosing, and Repairing Ignition Coils

The ignition coil is an electrical transformer, or induction coil, in your vehicle’s ignition system that transfers voltage, or power, from the battery to ignite the fuel. It causes the spark plugs to spark. The ignition coil is connected to the vehicle’s distributor, which in turn has wires going to all of the spark plugs. It stores energy in an open magnetic circuit that surrounds it. This is the energy that is sent to the spark plugs. There is a primary and a secondary coil. The battery runs through the primary coil, and the secondary coil has many more wires running through it.

Signs of a Failing Ignition Coil

If your vehicle’s ignition coil is going bad, it will let you know. Spark plugs that are wearing out require more power to create a spark. This causes a higher power flow into the primary circuit, which can affect your battery. Your vehicle will backfire, have trouble starting, vibrate when you are idling, jerk at higher speeds, stall, and you may notice you are getting less gas mileage per gallon. This is because the vehicle is using more fuel due to a lower power transfer.

Testing the Ignition Coil

There are several ways to test your vehicle’s ignition coil. A quick and easy way is to let the vehicle run for at least 30 minutes. With the engine still running, tap the module with a screwdriver’s head. If the engine stops, the coil is most likely faulty.
Another way to test your ignition coil is to start by turning the ignition switch off. Disconnect ignition coil’s output wire and connect a spark plug to that wire and remove the ground wire. Connect one of the disconnected output wires to the ignition coil’s negative terminal. Switch to the on position. Touch the ground wire to the negative terminal and lightly tap. You should be seeing sparks every time you make contact. If you don’t, use an ohm meter to test the coil. The primary coil should be between 0.7 and 1.7 ohms.


Worn-out spark plugs or cracked coil cases can cause the ignition coil to cease working properly. Checking the vehicle’s high tension leads is a good way to troubleshoot as well. There should be a spark from every lead going to every spark plug.

Replacing the Ignition Coil

To replace the ignition coil, disconnect the battery. Remove the old coil. Put dielectric compound on the new coil’s rubber boots or bottom to prevent corrosion. Put in the new ignition coil, checking your owner’s manual for how much torque to use. Reconnect the battery and start the engine. All cylinders should be firing into the spark plugs from the new ignition coil.


The ignition coil is a very important part of the electrical firing system of your vehicle. Maintaining clean and well-working spark plugs can go a long way in the life of your ignition coil. Regular tuneups are always recommended every 3,000 miles.

All of the electrical components of the engine center around the ignition coil. Now you know the signs and symptoms of a possible failing ignition coil. You also have learned the troubleshooting steps you can take to see if it needs replaced. Always practice good maintenance with all parts of the vehicle to keep it running smoothly and safely and save you gas money.