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How to check a low tension magneto

  • Thread starter Rusty Herrstrom
  • Start date

Rusty Herrstrom

I am helping my father restore a fairbanks morse 1 1/2 h.p. headless. I am trying to determin if the magneto is working properly. I removed the ignitor and grounded the ignitor , spun the engine over and there was no spark. Is there a better way of checking the mag?

Thanks, Russ

Keith Smigle

I check Associated mags with a 12 volt test light to see if ti will light up the light.

~Keith in Delaware, oHIo

Craig Anderson

One thing at a time. Take a wire and ground the output post of the magneto to itself and turn the armature and see if it builds a field (resistance) twice every revolution. If you feel a fair amount of resistance you know the magneto is working. Check the ignitor for grounding in the insulated post. The 12V test is good--we use a regular light bulb and a test cord. And be ABSOLUTELY POSITIVE the points are closing near the point of ignition. Good luck!

Andrew Mackey

remove mag from engine, if it is the impulse type, set up igniter feed wire so that it makes anf breaks ground contact at peak magnetic flux (usually when mag springs are at rest) you should see a flash. if mag is a rotary, spin the mag, and drag the igniter feed across the groung of the mag, you should see 2 flashes per revolution. On both mags, (on the impulse remove the springs) groung the igniter feed wire and try to spin the mag. It should nearly stop at the point of maximum magnetic flux, you should feel major resistance. Compare to wire not grounded. If the mag does not lock up with the feed wire grounded, either the magnets are real weak, or the windings are bad. Andrew

Mike Edwards

I agree with Keith. I have a 3hp 1917 FMZ throttler that I wasn't sure if the mag was functioning. I just used a cheap 12v light probe I got from the local auto parts store to see if any current was still being produced. Hooked the leads up,spun the mag and it lit the probe up on the impulse cycle. The engine's been running fine for 2 years now.

However, I'd still like to figure out someday how to get readings on my Fluke meter from these old mags for reference voltages for future projects.


Rob Skinner

>I'd still like to figure out someday how to get >readings on my Fluke meter from these old mags >for reference voltages for future projects.

G'day Mike, First thing to remember, NEVER NEVER NEVER put your meter on the secondary side of a high tension magneto. A high tension mag will be putting out a few tens of thousands of volts, which will not sit well with a run-of-the-mill meter.

Low tension mags are another thing. You CAN test them. But when spinning the magneto by hand, the voltage changes are so rapid that they're difficult to see. You might be able to see something with an analog meter. With a digital meter, you won't be able to see the voltage readings.

BUT, a lot of digital meters have a memory function. Set up the meter for memory, and hook up the meter leads. Spin the magneto over rapidly several times by hand.

Now review the memory. A typical magneto might give you a high reading of +6vdc and a low reading of -6vdc. In this example, what we're seeing is a high output when the armature sweeps through the magnetic field. Then, 180 degrees later when the armature sweeps through the magnetic field in it's other orientation, we see a peak negative voltage.

That's just an example. Your results will vary depending upon the mag you're working on.

Bob (ND)

Have you tried getting a peak-to-peak voltage reading of a low tension mag on an oscilloscope?



A 'scope is probably the best way to test a low tension magneto as long as you have a good mag to test as a reference.

There will be a low voltage "bump" (as opposed to a "pulse") created when the mag is rotated by hand. If the mag is on the engine and connected to a properly timed ignitor, the scope will show a very short high voltage (hundreds of volts) pulse at the instant the ignitor points open due to the rapid collapse of the magnetic field within the magneto.

Take a look at my web page. I did a study of the Webster low tension magneto on an Aermotor 8-cycle. Just go to the link below and, near the bottom is the "TECHNO-GEEK PAGE".

It's more than you really wanted to know about a Webster.

Take care - Elden

Aermotor And Webster Magneto Page