How to Check Auto Coils
- 1). Unplug the coil wire from the top of the distributor and push an old spark plug into the end of the coil wire. The spark plug may actually plug into the coil wire, but odds are it won't fit snugly. Wrap some electrical tape all around the plug (but not on the threads) and wrap it around the top of the coil wire until the two are firmly connected.
- 2). Lay the plug and wire on top of a rolled-up rag so it doesn't come into contact with anything metal. Turn your car's ignition to "On," but don't try to start the car. Grasp the tape-wrapped portion of the plug with a pair of insulated pliers or channel locks (the electrical tape may be enough to insulate you from the coil's charge, but don't count on it; a high-voltage shock may or may not kill you, but it's guaranteed to really hurt).
- 3). Tap the exposed metal of the spark plug on a metal part of your engine. The intake manifold may or may not provide a sufficient ground, but the alternator bracket, cylinder head or engine block almost certainly will. Upon tapping the plug, you should see a bright blue-white spark jump across the spark plug's electrodes. If the spark is thin and yellow and your don't hear the pronounced "snap" of the jumping electricity, then the coil isn't providing sufficient output. Don't leave the plug in contact with a ground for more than 1/2-second; repeatedly tapping it will give you a quantifiable result without burning up the coil.
- 4). Turn off the ignition. Remove the plug from the coil wire and reconnect everything, but remove the coil-end of the output wire. Break out your digital multimeter and set it to read ohms of resistance (abbreviated as "Ω", the Greek symbol for Omega). Place your multimeter's probe terminals on the power-in and ground terminals on your coil; you should get a reading of between 0.5 and 1.3 ohms, depending on the coil. Larger coils will read higher than smaller ones.
- 5). Leave the positive probe attached to the positive terminal and touch the negative probe to the output where the coil wire attached; you should get a reading of between 5,000 and 11,000 ohms, depending on the size of the coil. Any more than 1.5 ohms on the first measurement or 14,000 on the second almost always indicates excessive internal resistance, which means your coil is shot.
