How to Test a Magneto Ignition Coil

by Joe McElroy
Most small outboard motors use a magneto.

Most small outboard motors use a magneto.

Many small engines use a magneto to produce electrical current and spark. In such a system, a large, permanent magnet is embedded in the flywheel. As the flywheel turns, the magnet spins around a transformer with exposed copper wire windings, inducing a current which fires the spark plug and ignites the gasoline. The transformer is the magneto. Magnetos are most common in small outboard motors. They also are common in antique motorcycles.

Remove the flywheel shroud. Usually there are three bolts on the shroud, but some engines made in the mid-1970s have four bolts.

Locate the magneto. It is the only part that has exposed copper windings. It will connect directly to the spark plug through a rubber casing. Pull the rubber casing off the spark plug.

Set the multimeter into the ohms function, or just use an ohmmeter. Manually set the dial or button on the meter to the 40 k range. Do not use auto ranging, as it is unreliable with a magneto.

Touch one probe tip to the engine block and the other to the metal clip inside the rubber casing which normally attaches to the spark plug. Readings are given in thousands of ohms, or "k" ohms. A good reading will range from 3 k to 15 k. A higher reading indicates a poor connection.

Check the connection of the high voltage, or HV, wire to the coil and to the spark plug clip if you obtain a reading higher than 15 k. After making sure the connections are clean and unobstructed, retest.

Replace the magneto if the meter reads "OL." This indicates an internal short in the magneto.

Items you will need

  • Socket set
  • Ohmmeter or multimeter

Tip

  • For two-cylinder engines, you should test each cylinder to make sure you do not have a bad connection in the HV wires. If the magneto is generating the proper ohms, check to make sure the spark plug is working properly. A weak reading could indicate a magnet that has lost some of its strength. Check with your boat or small engine repair shop to see if the magnet can be recharged before replacing the magneto.

Warning

  • Wear rubber safety gloves when working with any sort of electrical wiring.

About the Author

Joe McElroy has been writing on politics and culture since 1983. His articles have appeared in a diverse array of publications, including the "Chicago Daily Observer" and "Immaculata" magazine. McElroy works occasionally as a strategic consultant to federal candidates. He majored in American history at Northwestern University.

Photo Credits

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