Bench Testing Your Ignition Coil

Why Bench Test Your Ignition Coil?

Resistance testing for coil performance.
Test your coil for resistance if you want a definitive answer. photo by Matt Wright, 2008

There are a number of on-the-car, shadetree tests for your ignition coil, but to definitively determine whether your coil is on the way out, the only proper coil test is with a multimeter. Why? Here's the rundown:
Inside an ignition coil are two coils of wire on top of each other. These coils are called windings. One winding is called 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 bad, clearly bad, as in it makes no spark at all. But if a coil is on the way out, but not dead yet, it can make a weak spark that can cause the car to run rough or wrong. This can not only make your car run poorly, but it can trigger the Check Engine lIght, which always equals potential spending of money down the road, or next week if it's time for your state inspection. By testing an ignition coil with a multimeter while it's disconnected, you'll be using data and numbers to determine the health of the coil rather than your eyeballs and dead reckoning.

We'll show you how to test both the primary and secondary ignition coil windings using a multimeter.

*You'll need the resistance specifications for your specific coil in order to perform this test. Consult your service manual for this information.

Testing the Primary Winding of Your Ignition Coil

Testing your ignition coil primary winding.
Test the primary winding on the outside contacts. photo by Matt Wright, 2008

The primary winding of your ignition coil is the first to receive voltage from the battery, so we'll follow its lead and go ahead 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 test. If it is even a little out of spec, the coil should be replaced.

Testing the Secondary Winding of Your Ignition Coil

Secondary ignition coil winding test.
Test the center and 12V poles for secondary winding resistance. photo by Matt Wright, 2008

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 12V pole 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 multi meter 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 investment to replace the coil.