There are two basic methods of testing ignition coils: coil on the vehicle and coil off of the vehicle. The most common method used by backyard mechanics involves shorting a spark plug to the engine or chassis and cranking the engine over. If there is spark, the coil is good. The most accurate method involves the use of a multimeter. By using this method, both the primary and the secondary windings can be tested. While the first method is a basic go or no-go test, it does not indicate the condition of the coil, whereas the second test does.
An ignition coil is made up of the primary and the secondary wire windings. The primary winding supplies the electrical charge from the vehicle's electrical system to the coil. The secondary winding takes the electrical charge from the coil to the distributor or to the spark plug. When testing ignition coils, it is possible to have some spark; however, the spark may not be sufficient to fire the spark plugs under compression conditions. This is why the multimeter test is superior to the shade-tree method of grounding a spark plug to the engine or chassis.
In order to check the condition of the primary winding, the terminals of the multimeter must be touched to both the positive and negative posts on the coil. The reading should be compared to the recommended readings found in the operations manual of the particular vehicle. Any variation at all from the recommended reading is a sign that the coil should be replaced. When testing the condition of the secondary winding, the terminals of the multimeter should be placed on one of the outer posts as well as the center post where the coil wire attaches. The reading should again be checked against the recommended reading found in the manual and the coil replaced if there is any variance.
The reason that testing ignition coils is important when diagnosing an ill-running vehicle engine is that a weak electrical charge emanating from the coil can cause an engine to run roughly, lack power, or get very poor fuel mileage. Often, a weak coil will pass the shade-tree method of testing, yet lack sufficient power in the spark to ignite the fuel mixture properly. By using a multimeter, the coils can be checked against a known number and any difference will be clearly shown.