How to Troubleshoot Magneto Coils

Place the magneto on a worktable. Check its resistance by setting the dial on a digital multimeter to the ohm symbol, which resembles an upside down horseshoe. Plug the red probe into the resistance terminal of the multimeter.

Clip the multimeter’s red and black probes onto the two wires of the magneto’s primary coil. The resistance should be 500 ohms or less. If the resistance is 20,000 ohms or higher, the primary coil is open and must be replaced.

Remove one of the two probes and clip it onto the end of the spark plug wire. It does not matter which probe you use. Leave the other probe attached to the primary coil. The meter should read "infinite" or "open circuit.” Any reading lower than "infinite" indicates that the insulation has broken down and there is a short between the primary coil and the secondary coil.

Remove the probe attached to the primary coil. Clip this wire onto the magneto itself to measure the resistance across the secondary coil. The resistance of this coil should be no more than 20,000 ohms. A resistance level higher than 20,000 ohms means there is an open wire in the secondary coil.