Troubleshooting Fairbanks-Morse FMJ Magneto

bmoran4 · Jul 27, 2022

I'm going through my collection of older Wisconsin engines that have FMJ or similar magnetos. I have some honestly "ran when parked" engines that I am attempting to bring back to operation.

The one in particular that I am focused on has everything except spark. The magneto looks like it has been refreshed not that long ago using a kit similar to what is available to the Farmall magneto rebuild kits with the red coil and whatnot. I get about 6 ohms of resistance between the spark contact and the single coil wire. The points are clean, gaped to ~.020, and they make and break contact as expected. Impact drive definitely impacts.

I understand that testing a condenser is nearly impossible with a cheap multi-meter. Anyone have a crossover part number that I could get from a Napa or of similar ease and expense? Anything else I should be looking at? Is there a FM Magneto service manual?

560Dennis · Jul 27, 2022

Manuals for them on eBay for around 20 bucks ,get one that covers your model and go from there .

tractorguy2 · Jul 29, 2022

fairbanks mags are known for having weak magnets. If you can't feel a good amount of resistance when turning backwards. The magnets are too weak.

bmoran4 · Jul 29, 2022

(quoted from post at 14:20:17 07/29/22) fairbanks mags are known for having weak magnets. If you can't feel a good amount of resistance when turning backwards. The magnets are too weak.

What is the fix? Is there some obscure magnatizer? Are the magnets replaceable? Is there some way of compensating for weak magnets (different coil, condenser, gap?)

teddy52food · Jul 30, 2022

I have the magnet charger and condenser tester if you need help. I work on them as a
hobby.(cheap)

bmoran4 · Aug 1, 2022

(quoted from post at 10:23:47 07/30/22) I have the magnet charger and condenser tester if you need help. I work on them as a
hobby.(cheap)

I would like to acquire the tools myself - any recommendations?

bmoran4 · Aug 4, 2022

(quoted from post at 22:40:13 07/27/22) Manuals for them on eBay for around 20 bucks ,get one that covers your model and go from there .

Found a big book 4 inches thick for the FM Mags (much more than $20, that's for sure) and it only had one page on behind the FM-H, J, K, O, OH separator tab - how to perform a rotation changeover with a kit. Other mentions throughout of the FM-J series show it to be obsoleted. Thankfully, this had me look again and it turns out I got confused between a magneto I had in a pile (the FM-J) and the one I am focused on getting to work (FMXD1B7S). I found the original part number for the condenser (AXMR2433), and found it crossed with a FM200, which crossed with a BWD FG6 which I was able to order a bunch for a very reasonable cost. We shall see if that is the culprit.

bmoran4 · Aug 8, 2022

Just out of curiosity, can something like a STENS MEGA FIRE be fitted to these magnetos?

Troubleshooting Fairbanks-Morse FMJ Magneto

bmoran4

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560Dennis

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tractorguy2

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bmoran4

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teddy52food

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