Bench Testing Your Car's Ignition Coil

Secondary ignition coil winding test.
Test the center and 12V poles for secondary winding resistance.

Photo by Matt Wright

There are a number of ways to test your car's ignition coil without removing it, but the most definitive test is one that removes the coil and tests it with a multimeter. As one of the most important components in the ignition process, you want to know for sure if the problems you have starting your car are due to the coil or to something else. We'll show you how to test both the primary and secondary ignition coil windings using a multimeter.

The Makeup of the Coil

Ignition coils are made up of two coils of wire situated on top of each other. These coils are called windings. One winding is the primary winding, the other is the secondary. The primary winding gets the juice together to make a spark and the secondary sends it out the door to the distributor. Either one of these windings can go bad and cause your ignition coil to fail.

Sometimes an ignition coil is clearly bad because it makes no spark at all. But if a coil is only on its way out and not yet dead, it can make a weak spark that will make your car run poorly. It might also trigger the Check Engine light, which could mean a potential loss of money when you take it in to your service department for an inspection. By testing your disconnected ignition coil first, using a multimeter, you'll be using data and numbers to determine the health of the coil rather than relying on your eyeballs. You'll also know for sure if what you have is a bad coil or if you need to spend money on further troubleshooting.

Before Removing the Coil

To begin, you'll need the resistance specifications for your specific coil in order to perform this test. You should be able to find this information in your service and/or repair manual. You should also don a pair of safety glasses and old clothing that doesn't have any loose ends. If you have long hair, tie it back. Grab a socket set and ratchet and a set of wrenches and screwdrivers. Finally, disconnect your car's battery.

Remove the Coil

To locate the ignition coils on top of the engine, consult the repair manual for your car's make and model. Some coils must be disconnected from their electrical connections before removal. Others must be unbolted and then disconnected. Again, your manual will guide you through this process. Make sure to note which wires go to which connectors so you can properly reattach the coil, either a new one or the old one if it tests as good.

Test the Primary Winding

The primary winding of your ignition coil is the first to receive voltage from the battery, so we'll follow its lead and test the primary first. Find the resistance specifications for your car's primary coil winding in your repair manual. Then, using a multimeter, place the leads on the smaller, outside poles if you have a traditional round coil, or on the indicated poles if you have a newer enclosed unit. If the reading is within the range indicated as acceptable in your manual, your primary winding is ok and you can go on to the secondary winding test. If it is even a little out of spec, the coil should be replaced.

Test the Secondary Winding

The secondary winding of your ignition coil delivers the spark to the distributor to be sent to the spark plugs. We'll test this part of the ignition coil second, for obvious reasons. If it's bad, you'll get a weak spark or no spark at all.

To test the coil's secondary winding, attach the test probes to the outer 12-volt pole on the coil and the center pole (where the main wire goes to the distributor). The 12-volt pole is the spot where the power comes into your coil. It will be marked with a + sign, or it might only be indicated by a number. Your repair manual should tell you what number you're looking for to determine which pole is the 12-volt wiring terminal. Determine the resistance using your multimeter and check to see if it's within the acceptable range indicated in your repair manual. If it is, your coil is up to the task. If it's even slightly out of range, your coil should be replaced.

Remember to use your best judgment when evaluating. If both windings are at the very bottom of the acceptable range, and you're having ignition issues, it may be a good idea to replace the coil.