Can I bench test the coil on the J4 magneto with a multimeter?

[ThunderCloud]

[John *.?-!.* cub owner]

You can take resistance readings, but they are real low, problem is a multi meter only uses 1.5 volts for low resistance readings, but coils runs at as much as 40,000 volts. A few shorted turns in a winding will affect output, but would take a lab quality meter to read. If a winding is completely open it would show, but a few shorted turns would not.

[inairam]

you could try to spin the mag drive with a drill or small motor and check the spark.

[Rick Spivey]

Ue your resistance setting, check from the nub on the coil to the wire end that connects to the points, and then from the same nub to the tang that grounds under the bolt that holds the coil in the housing. In both cases, your reading should be between 9500 and 10500 ohms. An open circuit will show as zero, and anything much over 11,000 has always been a problematic coil, in my experience.

[inairam]

Rick are your instructions for a mag or a coil?

[JohnMihevic]

If you do the above tests and find a bad coil then you know you need a new one. However, if the above tests pass, it is still possible you could have a bad coil. At 20,000 - 40,000 volts, windings can still break down under the stress of high voltage which a multimeter won't show. Also, the windings can start breaking down under higher heat conditions, when the engine has warmed up or is operating under high load.

John M.

[Rick Spivey]

Rick are your instructions for a mag or a coil?

This is a quick, but not foolproof, test on a coil from a magneto. I don't know the values for the battery ignition coil. And, as John M. rightly points out, good readings on this test DON'T guarantee the coil is not bad. It is just a first test, because bad readings here DO guarantee a new coil is needed, at minimum.