How to Diagnose a Bad Ignition Coil
- 1). Disconnect the coil wire at the distributor cap. Insert an old spark plug into the coil wire and lay the spark plug on a metal engine component such as a valve cover. Crank the engine and observe the spark plug for operation. If you notice a hot blue spark, the ignition coil is working properly and requires no further testing. If you see no spark or a pale yellow spark, then continue the diagnosis.
- 2). Disconnect the negative battery cable with a socket and ratchet. Remove the wires at the ignition coil using a small combination wrench and note their positions. Inspect the coil for damage or oil leaks around the top. If any damage or leaks are found, replace the coil.
- 3). Connect the red ohmmeter lead to the positive (+) stud of the coil. Connect the black lead to the negative (-) stud of the coil. Turn the ohmmeter to the 0-2M ohm scale. The coil should read between .7 and 1.7 ohms across the terminals. Every coil will have a slightly different ohm reading, but if any coil shows zero or infinity as a reading, then the coil is bad and must be replaced.
- 4). Remove red meter lead from the primary stud and connect it to the center coil wire lead. Set the ohmmeter to the 2-20M ohm scale. The coil should read from 7,500 to 10,500 ohms. Every coil is slightly different, but if any coil reads zero or infinity on the ohmmeter, then the coil is bad and must be replaced.
- 5). Reconnect the high tension lead to the coil and distributor cap. Remove the old spark plug from the coil wire and reconnect the coil leads to the coil and distributor cap. Replace the coil wire leads in their original locations. Reconnect the negative battery terminal.
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