MTFCA Forum

It seems to me you sort of answered your own question when you said your T runs fine when the horn is disconnected, check it out for the starting point of your problem.

I think you have a short in the horn loop.

But the problem with the contact switch make me think.
Sometime I blow my magneto out on my 1926 touring with a short in the contact switch. The short connected the battery directly on the magneto.
The problem with the magneto started when the switch was hard to change from battery to magneto.
Here is what I did.
I bridged the contact switch with a three way switch.


Last week I had about the same problem with a 1914 touring. There the problem was a screw in the contact switch.

In both cars I had to recharge the magnets with the in the car recharge way.

Maybe this helps
Good luck
André
Belgium

I am working on a 20 for a friend. He had a problem with the switch. One of my cars has the same key number so I tried my key. It works fine and I can switch from batt to mag without any trouble. We bought a key with the same number at a swap meet and it was very hard to get it to turn the switch. Then we bought another key from Lang's which works perfectly.

One problem with keys is when we make a copy of a copy of a copy of a copy, the cutting of the key can be just a little bit off and each time it is recopied, it changes a little. Anyway, try another key, preferably an old stock key, if you can find it.

I think you have two problems. One is related to the switch and key, and the other is related to the horn. Something about the wiring to the horn is grounding out the magneto contact.
Norm

The wiring at the horn is usually very questionable. Most have a threaded screw from the inside with a couple of insulating washer. The inner one will crack and some times fail. I suggest you look carefully at the horn insulation.