Farmall Cub

I am running an externel coil on my 48. It has a magneto ignition. I would like to test the internal coil on the mag but am unsure of the procedure. I saw an earlyer post that explained that the rotor is saposed to be timed by aligning marks. I have had mine apart but didnt know about the marks. I will check and see if I reassembled the rotor corectly in the mag. History of tractor is , it didnt run when I got it. Had to put new piston rings in it and throw on an external coil to get it to run. Not sure if it had spark to begin with or not. But I did dismantal the mag while I was checking for spark and diconecting mag lead ,so i could hook up external coil. Runs good with external coil but would perfer to keep it mag original.

If you had the mag apart,--take it off the tractor again and hold it in a vise.---snap the impulse to see if there good or not,---Stick a plug wire in one hole of cap,-- snap impulse around 4 rotations to try finding the right plug wire, or take cap off and line impulse up with a wire and work back and forth snapping over that wire!---this gives a starting point to check from! thanks; sonny

playing with coils with no spark plug on the high voltage side can destroy a good coil. The high voltage tries to jump internally. I may have not worded it correctly but that is what happens.

Do NOT snap a magneto with no place for the spark to go. That is what Bill was saying above. It can destroy a good coil. There is no perfect test, but at least one check can be made with an ohm meter. Check from the primary wire that hooks to the kill switch, up to the nub on the coil; you should get approximately 9500-10000 ohms resistance.Then make same check from nub to the tab that sits under the hold down screw, same value. If you get 0, or a really high number, coil is definitely bad. Getting the correct resistance is not, however, a guarantee of a good coil, just an indicator that maybe it is good. You can check for spark, just use a proper tester from the lead that goes to the center tower, to a proper ground.

If your magneto has been running with the internal coil installed , but an external one wired up for any length of time, there is a 99% probability that the internal one is bad. If it wasn't when the change over was made it probably is by now. The one thing that might have saved it is that without the points and condenser connected to the internal coil it would not have built up full voltage. Since it runs with external coil, the points and condenser are good, and the rotor is timed right. Only thing left is the impulse (snaps when cranked by hand), and the coil.

For quality assurance, how did they check them on the asembly line before they sent them to IH?

assuming they did (big assumption), to actually be sure they worked would have required a machine to duplicate magneto operation by spinning magnet past with points and condenser operating. a lighter test would have been to do a resistance test, or even a waveform test with a signal generator.

viewtopic.php?f=1&t=47147

Above a discussion on bench testing a magneto from 2009.

64/67lo-boy wrote:For quality assurance, how did they check them on the assembly line before they sent them to IH?

Search the internet using "bench testing magneto".