How to Test 12 Volt Ignition Coils

by Nathan Fisher

An automobile ignition coil acts as a transformer, having a primary and secondary bundle of wires that create and send an electrical charge to the spark plugs, causing them to fire. While ignition coils will generally last for the life of a vehicle, faulty spark plugs or spark plug wires can send irregular voltage surges through the ignition coil, causing the coil to burn out. You can test a 12-volt ignition coil for effectiveness by checking the resistance of its wire windings with a multimeter.

Disconnect your engine's main negative battery cable with a wrench.

Pull the main ignition coil wire, the one that leads to the distributor, off the coil. Disconnect the small grounding wire, attached to the side of the coil, with a wrench.

Turn on the multimeter and set it to the "ohms" function.

Insert one of the multimeter's probes into the center opening of the coil, contacting the metal terminal inside the coil. Touch the second probe of the meter to the ignition coil's grounding terminal. The meter should read 6,000 to 15,000 ohms. If it does not, the coil's secondary winding is faulty.

Remove meter's probe from the center terminal and touch it to the terminal bolt on the opposite side of the coil from the probe touching the grounding terminal. The meter should read between approximately 0.4 and 2 ohms. If it does not, the coil's primary winding is faulty.

Tip

  • check If the coil checks out test the main coil wire, that was removed from the distributor, for shorts. Set the multimeter to the "continuity" setting and insert one probe into each end of the wire. If the meter does not show any reading, or fluctuates, the wire needs to be replaced.

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