How to Check the Spark on an Ignition Coil

Checking the coil spark could identify the cause of your car's poor performance.

A car's ignition coil transforms the 12-volt current from the battery into a much higher voltage to spark the spark plugs and start the engine. Modern cars have more than one coil and these are often contained together in a coil pack. A malfunctioning coil can cause difficulty in starting the engine or a misfiring and poorly performing engine. You can easily check if the coil is working properly by using an ohmmeter to test its resistance.

Things You'll Need

• Ohmmeter
• Socket set
• Car manual

Instructions

1. Disconnect the negative, or black, battery cable.

2. Locate the coil pack, using the car's manual to help. It should be located on, or be connected to, the main engine block. Disconnect the wire from the coil pack terminal and unplug the coil pack from the spark plugs.

3. Place the coil pack on a work surface with the connectors facing upward. Using the ohmmeter, test the coil's primary resistance by connecting the meter's red cable to the coil's positive connector and the black cable to the negative connector. The resistance reading should be between about 0.4 and 2 ohms.

4. Test the secondary resistance by connecting the red cable to the coil pack's terminal connector. Leave the black cable connected to the negative connector. The resistance reading should be between 6,000 and 15,000 ohms.

5. Replace your coil pack if either of the readings are outside the usual range.

Tips & Warnings

• Consult your car manual for the exact readings expected from a correctly functioning coil pack.

References

• AA1Car: Ignition Coils