How to check an ignition coil

Written by dan ferrell
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How to check an ignition coil
Your car's engine needs a healthy spark. (Photo courtesy of Sonett72 at Commons

Just as you need healthy foods to properly function during the day, your car's engine needs a healthy spark to properly function and take you places. That spark of life for your engine comes from the ignition coil. Although designs vary, its function remains the same. However, after years of use, the coil's internal windings tire, open or ground themselves and stop delivering the spark altogether. If your car misses frequently, stalls after warming up to operating temperature or refuses to start, you might have a bad coil. Follow these simple steps to check your ignition coil and replace it if necessary.

Skill level:

Things you need

  • Wrench set
  • Ratchet and socket set
  • Ohmmeter

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  1. 1

    Park your car in a safe place and open the hood.

  2. 2

    Disconnect the black, negative cable from the battery.

  3. 3

    Locate the coil or coil pack. Follow the spark plug wires to the distributor, then follow the wire coming out of the centre of the distributor cap which connects to the coil; if your car has a distributorless system, the spark plug wires will connect to two or three coil units, each with two plug wires attached to it.

  4. 4

    Unplug the wire coming from the distributor or spark plugs. If you have easy access to the coil for testing, leave the coil mounted in place and disconnect the electrical connector or wires; otherwise remove it using a wrench or ratchet and socket of the appropriate size.

  5. 5

    Unplug the electrical connectors from the coil and make sure their electrical contacts are free of dirt and corrosion. Clean the electrical connectors if necessary.

  6. 6

    Measure the coil primary resistance. Using an ohmmeter, connect the red lead to the coil's positive (+) electrical connector, and the ohmmeter's black lead to the coil's negative (-) electrical connector. An average resistance reading could be between 0.3 and 1.0 ohms.

  7. 7

    Measure the coil secondary resistance. Connect the ohmmeter's black lead to the coil's negative (-) electrical connector and the ohmmeter's red lead to the coil's terminal-where the distributor or spark plug wire attaches to. A typical resistance reading could be 8,000 to 11,500 ohms.

  8. 8

    Replace the coil if any of the readings for your particular coil is bellow or above the specifications indicated in your service manual.

  9. 9

    Reinstall the coil, electrical connector, wires and connect the negative cable to the battery terminal.

Tips and warnings

  • Consult the service manual for the correct primary and secondary resistance specifications on the coil installed in your vehicle. You can buy a service manual at most auto parts stores or consult one for free at most public libraries.

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