I bought a 1925 Coupe the other week (my first model T!) and I can't get it to run on magneto. When I went to buy the car it start right up and drove fine on magneto, but I can't get anything now. She runs fine on battery, but dies as soon as I switch the key to magneto. I took the contact post out of the top of the transmission and cleaned it off and I checked the wires leading to and from the magneto, but still had no success. Any ideas? Thanks!
Maybe the magnets lost their charge - do the magneto test below while running the car on battery:
Check the ignition switch first, Ford original have the tumbler made of pot metal, and years later they can fail. Or might be contacts inside the switch need cleaning of any corrosion. Or might be contact screw securing the red wires at the terminal block on the firewall, creating a bad contact too.
Just take care to not put any battery voltage to the magneto post ever! That will for sure ruin the magnet charge and render the magneto DOA!
Did you do anything to the car between the time it ran on magneto and the time it didn't? If so, you might have inadvertently changed something in the circuit. If it just ran fine one day and did not run on mag the next time you tried to start it, the switch would be suspected. If it was running on mag at the time it quit, I would suspect something anywhere between the magneto itself and the coil box. Could be the magneto, the magneto plug, the wiring or the switch.
Under no circumstances should the battery be connected directly to the magneto. That includes accidentally through the switch itself. If it is connected, it will cause the magnets to discharge.
Start with the test noted by Mark. If you have low or no output from the magneto, you can trouble shoot the magneto itself. If you get a good output from the magneto, the wiring or switch is suspect.
Check your connections for a loose wire, or one that can move with vibration. About 2 weeks ago I had the exact same issue; turned out the post in the mag post was a bit loose, allowing the mag wire to ground to the block. Symptom was it ran great on mag for years, then all of a sudden it would die on mag but run fine on battery. Look for the simple stuff, including the switch itself.
If it ran pretty well on battery when you first started it then quit when switched to mag that's almost a sure sign its in the switch contacts. Try checking that or something around the mag post. Its probably something pretty simple.