How to check a chain saw coil

Written by g.k. bayne
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How to check a chain saw coil
Chain saw coils produce a spark to ignite the fuel. (Jupiterimages/ Images)

Chain saw coils are part of the ignition system for the small engine. The coil is a high-voltage transformer that sends electrical power to the spark plug. Every revolution of the chain saw's engine creates a powerful spark to ignite the gasoline in the engine's combustion chamber. Over time, the coil may become damaged from excessive heat due to extended operation of the chain saw. Checking the chain saw coil is a troubleshooting process to perform when the saw will not start.

Skill level:

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Things you need

  • Fuel container
  • Spark-plug socket wrench tool
  • New spark plug

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  1. 1

    Remove all gasoline from the chain saw before undertaking the test. Pour the fuel into an approved container.

  2. 2

    Access the spark plug. Consult the owner's manual for the location of the spark plug, as it differs by manufacturer. In some cases, you will find the spark plug under an external housing.

  3. 3

    Pull the thick, rubber sparkplug wire from the top of the spark plug. Fit the socket wrench tool over the top of the spark plug. Turn the wrench counterclockwise. Pull the spark plug from the combustion chamber hole.

  4. 4

    Replace the spark plug with a brand new one. A bad spark plug will mimic a bad coil, so replacing the old one with a new one eliminates the plug from the problem.

  5. 5

    Insert the top end of the spark plug back into the thick rubber connector wire. Lay the lower end of the spark plug against the metal lip of the combustion chamber hole. The lower metal rim of the spark plug must be in contact with the metal combustion chamber (but not inside the chamber).

  6. 6

    Pull very slowly on the chain saw's starting cord. Watch the end of the spark plug that touches the metal combustion chamber. If a small blue spark appears at the end of the spark plug, the chain saw coil is good. If no spark appears after several pulling attempts, the coil is bad.

Tips and warnings

  • All chain saw manufacturers include a spark plug tool. In many cases, the opposite end of the spark plug socket wrench is a flathead screwdriver.
  • Do not expose the spark plug's spark to fresh fuel. The spark plug could ignite the fuel and cause a fire or burn you.

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