The ignition coil amplifies the voltage coming from your vehicle battery to provide the spark necessary to ignite the fuel in your engine's combustion chambers. When an ignition coil fails, the voltage feeding the spark plugs drops. Your vehicle will begin to run rough or not refuse to start when the ignition coil fails. Checking the output of the ignition coil will ensure that the problem rests in the coil and not another component of your vehicle's ignition system.
Pull a spark plug wire from a spark plug. Attach a spark plug socket onto the end of a 3/8-inch ratchet handle. Turn the spark plug counterclockwise to remove it from the engine.
Remove the output wire from the top of the distributor cap. Plug the end of the removed spark plug into the loose end of the output wire.
Hook one end of an insulated copper wire to the threaded end of the spark plug. Turn on the key of your vehicle. Tap the loose end of the copper wire on the ground post of your vehicle's battery. Watch for sparks coming from the spark plug each time you tap the wire. Turn off the vehicle ignition.
Install the spark plug back in the vehicle engine. Connect the distributor wire to the distributor.
Hold a positive lead from an ohm meter to the black wire coming from the ignition coil. Place the end of the negative lead to the green wire on the coil. A number between .4 and .6 on the ohm meter display. Hold the positive lead on the coil output terminal. Place the negative lead to the negative terminal of the coil. A working coil will display numbers ranging from 5000 to 7200 ohms.
