You will need to test the ignition coil if your vehicle does not start, misses frequently or stalls after warming up. In all these cases, the ignition coil either grounds itself or the internal windings become less effective due to age. There are two coils of wire, termed windings, within an ignition coil. One of these is positioned on top of the other. The primary winding receives voltage from the battery to create a spark, while the second winding transfers the spark to the distributer. Either winding can cause your ignition coil to fail and you will need to conduct ignition coil testing.
Remove one of the spark plug wires and then use a socket to remove the spark plug from its position.
Return the spark plug wire to the spark plug, which you need to hold with insulated pliers. Now touch the threaded portion of your spark plug to any exposed area of metal in the engine.
Ask an assistant to start the engine. Check to see if a bright blue spark jumps across the spark plug gap. Should this be the case, then your coil is functioning correctly.
Consult the service manual for the resistance specifications of your coil while using a multimeter/ohmmeter. This test is appropriate for almost all types of vehicles.
Disconnect the negative (black) cable from the battery. Locate the coil by following the wire from the spark plugs to the distributor. Continue to follow the wire that emerges from the distributor cap, as this connects to the coil. In the case of an engine that does not have a distributor, the spark plugs will connect directly with the coil.
Unplug the wiring from either the distributor or the spark plugs. Disconnect the electrical connector from the coil. In the case of a traditional round coil, position the leads of a multimeter on the outside poles of the primary coil. In the case of an enclosed unit, position the leads of the multimeter on the indicated poles. Check the readings you receive against those given in the service manual. This coil should give a reading of between .7 ohms and 1.7 ohms. Even small variations will dictate that the coil needs to be replaced. Should the primary coil be in order, continue to test the second winding.
Attach your test probes to the outer 12 V pole and to the center pole and once again check the reading that you receive against that in the service manual, which will between 7.5K ohms and 10.5K ohms. As in the case of the primary coil, even a small discrepancy will mean that the coil requires replacing. The coil resistance will vary depending on whether it is cold or hot, and the readings indicate a range appropriate to most vehicles.