Suddenly lost magneto ignition?

[RadRacer203]

Drove my T to work today, and all was good on the way down, all good on the way back until I got to 5 miles from my house. The car sputtered once or twice and stopped. Pulled over, checked it out, and it started right back up on battery ignition. When I turn the switch to the mag it just dies, and even jiggling it, it doesn't change. I know the coil and timer are fine since it runs on battery, and I know my pickup on top of the trans is good because I swapped in a known good one when I stopped, to try that. I doubt the magneto itself is damaged, since I checked the oil and there's no metal, so what else could it be? The wire from the pickup to the key switch? The key switch itself? I'll start diagnosing once it cools down, but I'm not sure what to be looking for

[speedytinc]

Repop switch? Does the switch feel firm in the 3 positions or sloppy/loose?
Check magneto output. If dead, your switch could have introduced voltage & wiped the magnetic charge.

[DanTreace]

First to-do, check output from the mag post, running motor on battery to check.


One easy way is to provide a load, 12v stop/tail bulb in a socket, and wire clips to the mag post and to ground. At idle, the lamp should shine, at higher rpms, bright.

If good output, then the wire to switch or the switch to MAG is the issue.

[TWrenn]

Pull the mag post and check for fuzz or anything else that don't belong there

[RadRacer203]

The mag pickup on top of the trans is working fine, and I'm getting 9v at a very low idle, and it goes up to 14v very quickly. My ignition switch was a little iffy, but I cleamed it up and it's got good continuity from the MAG post to the COIL post, and same with BAT in that position. The red wire up to the ignition switch also has continuity

[RadRacer203]

Repop switch? Does the switch feel firm in the 3 positions or sloppy/loose?
Check magneto output. If dead, your switch could have introduced voltage & wiped the magnetic charge.

The switch was a little loose, someone was in it before and didn't put it back together tight, but no change. I wonder if it could still be the switch itself?

[RadRacer203]

If I'm thinking about this correctly, the only things that could be bad are the ignition switch or the wire from the mag to the ignition switch. I know everything after the switch is fine because it runs on battery, and it uses the same wire out of the switch for both battery and magneto, and I know the magneto is making good voltage. I'm thinking the best thing to do is swap the battery and magneto wires on the ignition switch, and see if it runs off the battery in the magneto position. If it does then I know the wire from the magneto to the switch is bad, if it doesn’t, I know the switch is bad.

[RajoRacer]

You best be very careful not to introduce ANY DC voltage into your magneto !!!

[Dan Hatch]

This is the tool you need.
https://www.mtfca.com/phpBB3/viewtopic.php?t=17869

[RadRacer203]

You best be very careful not to introduce ANY DC voltage into your magneto !!!

I mean just swap the wires on the switch to see if it'll run in the magneto position, make sure the switch is good. I could also run a new wire from the mag to the switch, but I think this is easier

[Mark Nunn]

Michael, my car does not have a magneto, so I only run on battery. My car would die unexpectedly from time to time. I swapped mag and bat wires on my switch, as you mentioned, and that solved my problem. I'm confident that your test will be enlightening. Good luck.

[RadRacer203]

Ok, I have no idea what it could be. My ignition switch is fine, and I ran a new wire from the magneto to the ignition switch. And it still won't even try to pop when it's on the mag

[Jerry VanOoteghem]

Try disconnecting the power wire from your coilbox. Then, run a piece of wire from the mag post directly to your coilbox power terminal, bypassing the switch entirely. Try starting the engine that way. (Obviously, you'll be trying to start on MAG only.)

[Jerry VanOoteghem]

The mag pickup on top of the trans is working fine, and I'm getting 9v at a very low idle, and it goes up to 14v very quickly.

I'd still check the mag post contact. You may have enough connection to run a voltmeter, but not enough to power a coil @1.3A.

[RadRacer203]

Looks like my issue was a nearly shorted wire from the mag to the ignition switch, and the magnets needed to be recharged. Ran a new wire up to the junction block, and zapped the magneto to recharge, and it runs good again