How to Test an Ignition Coil

How to Test an Ignition Coil

The ignition coil is an electrical transformer that converts the battery's 12-volt output into a much higher voltage necessary to ignite the air/fuel mixture in each cylinder of your engine. An ignition coil, which resembles an oversize wire-wound transformer filled with an insulator, can produce up to 45,000 volts before it is fed into the distributor and spark plug. A properly working ignition coil will fire the spark plug in a timely manner, thus making it possible for your engine to run properly. A bad ignition coil can be one of the main causes of engine problems, including a hard-starting engine, sluggish acceleration and even backfiring, which is where unburned fuel escapes through the exhaust system.

An ignition coil consists of two primary components: the laminated iron core and the primary and secondary windings. The primary winding is made of a thick copper wire with very few turns, while the secondary winding is composed of very fine copper wires that have many more turns. When the engine control unit sends a signal to the ignition coil, the current in the primary winding flows for a short time, building up a magnetic field. As soon as the current stops flowing through the primary winding, this magnetic field abruptly degrades and generates a very high voltage in the secondary winding that is transmitted to the spark plug. The ignition coil also contains a diode that prevents the high-voltage output from damaging the spark plug.

In most cases, the problem with an ignition coil is due to internal insulation failure that results in cracks in the resin that encapsulates the coil's primary and secondary windings. This in turn allows 30,000 volts to leak down the coil's external insulation and damage the internal windings. If you notice your car's ignition coil displaying symptoms of internal insulation failure, the best option is to replace it.

If your vehicle still uses a traditional single-terminal "canister" ignition coil (which has been around in various forms since the 1920's), testing the primary and secondary windings with a multimeter is a good start. Follow your repair manual or online information to find out exactly which pins on the coil you should use for this test. Then, connect your multimeter's leads to the corresponding terminals and touch off the primary and secondary windings for a resistance reading.

Keep in mind that these tests are not foolproof, and that a variety of other issues can cause an ignition coil to fail. If your ignition coil is displaying intermittent problems that you can't track down with other tests, the simplest way to resolve them is to simply swap out the coil with another one in the same coil pack. If that doesn't fix the problem, you may have to swap the coils out in a neighboring cylinder bank to determine which ones are affected by the malfunction. In most cases, a new ignition coil should be relatively affordable and easy to install at home.

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