MTFCA Forum

After a recent tour I now have very little power from the magneto. The car overheated after the fan belt came off. The car ran great on battery and magneto prior to this. Now several days later it runs great on battery but dies on mag. I have checked the wiring, coil box etc. Tried a magneto test with a bulb and only got about 3 volts at idle. Does anybody have any other ideas before I pull the engine to check the mag coil. I wonder if it could have gotten hot enough to melt a soldered connection in on of the field coils. The oil has been changed and no metal in it. I checked the lower engine and mains and rods are all good.

Likely not your mag coil but if the magnets got hot enough they would lose magnetism, meaning you'd need to recharge them. Worst case scenario would be when it got hot you lost the thrust shoulder on your 3rd main bearing which is what holds your magnet to coil distance - meaning you'll need to rebabbit at least the cap if not the bearing which... well... you know... Sorry!!!

Just how hot was hot?

Years ago I threw a belt on a tour on a day that was well into the 90's and without a motometer did not realize how hot the car was getting with the original radiator until I started loosing power. It got real hot to the point where the oil was so thinned out that I lost almost all of my braking capacity. After letting it cool down I limped home on a very weak magneto and charged the magnets in the car the next day. It has been more years than I care to remember since that happened and the car is still running strong on that recharge! I hope you are as lucky as I was but take heart as I doubt you could get one much hotter than mine.

Your first move should be to pull the magneto contact plug and take a good look around with a light and a mirror. There's a chance that you have a bit of wire or other debris causing a ground or a short, although usually if that's the case it stops the magneto entirely. Look especially closely on the driver's side of the contact button. With a reading of 3 volts, the trouble is probably located between 9 and 12 o'clock, so to speak, on the field coil, if the trouble is not in fact in the magnets.

It only takes 176° Fahrenheit for iron magnets to lose magnetism, so it follows that they became demagnetized when your engine overheated.
Remagnetizing your magneto magnets is possible, but it may not be feasible, due to the equipment required. Neodymium magnets are about 10 times stronger than iron magnets, and they could possibly be placed in such a way to restore the function of the magneto. They can also used to remagnetize the iron magnets to some extent.

I suspect the engine got above 176 degrees. I think attempting a magnet recharge sound better than pulling the engine however it may get to that at some point.

Don't dispair. It could have ended worse. Recharge process is not complicated, and once done, you'll have a new skill that you will undoubtedly share in the future.

I'd try an in car recharge first - 3x12v batteries in series, a compass, and jumper cables. IIRC the technique is in the official service manual... Not as good as an out of the car recharge, but it mmay give your mag an extended life...

I may be wildly wrong, but I cannot believe the Curie temperature of a Model T magneto magnet is anywhere near 176 F.
It has to be much higher than that.
Ron

When we had our motor rebuilt it was installed in the car. Ran great and was started on BAT. All worked well until changing over to MAG. Motor faltered. Looked everywhere and found no problem until the little block on the firewall. It is a 26 if makes any difference. Most all of the screws were loose and when tightened, the MAG was powerful and the am meter started to register again. You might try that area too. Maybe the terminals on the ignition switch.
Nick

I don't know the exact metallurgical make up of a Model T forged steel magnets, but the generally quoted Curie temperature for forged steel is 770 Celsius.

Wow,,, You can learn something everyday,...I must Google up IIRC and Curie temperature....N.

If the magnets lose magnetism at 176F then all Model T magnets should be de magnetized because the boiling point of water is 180F. And almost all Model T's have boiled at some time. Is it possible that when the belt came off that it might have hit a wire somewhere which is connected to the magneto? You could just have a loose connection somewhere. Anyway, try the light bulb to ground from the magneto post and measure the voltage across the bulb. You should have at least 6 volts at idle with the voltage increasing as the engine speeds up. If so, the mag is working and you should look elsewhere.
Norm

Boiling point of water 180 degrees? bobt

bobt wrote: ↑

Mon Jun 22, 2020 6:15 am

Boiling point of water 180 degrees? bobt

212°F at sea level, but the engine gets a lot hotter than that when the water boils out.

IIRC, the BP decreases about 1 F per 500 ft increase in elevation, so at 16000 ft elevation 180F would be about right. The Curie point remains the same however, jb.

I stand corrected. 220 degrees is correct. Many modern cars have 180 degree thermostat.
Norm

212 degrees. Not my best thinking day !
Norm

jab35 wrote: ↑

Mon Jun 22, 2020 3:40 pm

IIRC, the BP decreases about 1 F per 500 ft increase in elevation, so at 16000 ft elevation 180F would be about right. The Curie point remains the same however, jb.

Sixteen thousand feet? There are no roads that high, much less a Model T.

Mount Evens in Colorado falls short by less then 2000 feet but it's close!
https://www.dangerousroads.org/north-am ... n-usa.html

I think there may be some peaks in Alaska that would do. Probably no roads or T's up there though, drive careful everyone, jb.

charged magneto in the car and runs great again. Thanks for the help

Brent, that is great news and so good of you to post an epilogue. Thank you.