Coils have many different applications, ranging from relays (electric switches) to chokes in electronic circuits. The coil (ignition coil) in your car creates high-voltage electricity for the spark plugs. Your ignition coil is actually a transformer, made up of two coils. One coil has fewer windings of wire than the other coil. The coil with fewer windings receives a short burst of 12 volts of DC current (VDC). This coil transfers the 12 VDC pulse to the second coil and steps up the voltage to the required level.
Turn on the ignition but don't start the car at this stage. Using a multimeter in the DC volt position, check for a 12 VDC supply coming to the coil's positive terminal. (This terminal should have a black wire leading to it.) If there is no voltage, consult the vehicle manual.
If there is voltage, turn off the ignition switch and continue.
Remove the high-voltage wire from the center of the distributor cap.
Using a spark plug wrench or socket wrench, remove one spark plug from the engine. (If you have a spare spark plug, you can skip this step.)
Attach one end of a jumper wire (test lead) to the exposed metal at the end of the high-voltage wire and clip the other end onto the spark plug terminal. Ground the spark plug onto the engine body.
Attempt to start the engine while you watch for a spark across the gap of the plug. You should hear a snapping sound at the plug as well. Only a few turns of the engine will be necessary. Sparking across the spark plug gap indicates the coil is okay. The absence of a spark would make the coil suspect.
Remove the jumper lead from the plug terminal and high-voltage wire. Reinstall the spark plug, making sure to connect the spark plug wire back to the spark plug. Leave the high-voltage wire off the distributor.
Remove the 12 VDC supply wire from the coil.
Set the multimeter to the lowest possible ohm position and connect one of the meter probes to the terminal where the black wire was attached to the ignition coil. Set the other meter probe to where a green wire attaches to the coil. The resistance reading should be in the range of 0.3 to 1.7 ohms.
A reading less than the lower reading could indicate a short, and no reading indicates an open coil. The low reading is not always a guarantee that the coil is faulty, but if the coil is open the ignition coil should be replaced.
Set the multimeter to a higher reading (10k ohm range) and attach one meter probe to the green wire terminal and attach the other one to the end of the high-voltage lead. A reading from 5,000 to 13,000 ohms would be normal, depending on the car. Any reading lower than the lower reading should be suspect, meaning there is a short somewhere in the coil. No reading would indicate a broken coil. In either case, the ignition coil should be replaced.
