Ignition Coil,Primary Winding.Secondary Winding

Why Bench Test Your Ignition Coil?

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. 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.

Testing the Primary Winding of Your Ignition Coil

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. 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 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

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. 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). Determine the resistance and check to see if it's within the 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.