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