My 26 developed an issue this morning that is new to me.
When running on the battery the amp gauge reads normal, charging about 3 amps.
When I switch to the mag the amp gauge needle swings back and forth from charge to discharge.
The engine seems to run fine?
Sounds like a defective switch. The oscillating needle sounds like you are measuring the magneto's AC sine wave voltage. I wouldn't run it at all until that's figured out. I think I would check the wiring first, then remove and disassemble the switch itself to check its components.
Thanks Kevin, that is quite likely the case, as I have been having issues with this (replacement, likely Chinese) switch since I installed it.
I think I'll put the original unit back in service, which is pretty beat up but at least it will work.
Kevin and Dave:
Applying the magneto voltage to the ammeter would not likely cause the ammeter to move with the AC magneto voltage and certainly not at a slow rate that you could see. I think you more likely just have an intermittent connection somewhere in the ammeter wiring between the generator and battery positive post which includes about 5 wires that also go to the firewall mounted barrier strip and the back side of the ignition switch. If you did get the magneto crossed to either post of the ammeter then your magnets inside the motor would likely be weakened by discharge which would result in the car not running on magneto very well if at all.
Kevin. Try turning on the headlights and see what the amp meter does, it should show charge, It sounds to Me like the cut out may need to be adjusted. when You go to Mag there is no load on the battery or generator except for charging the battery which if fully charged could cause the cut out / regulator to cut in and out on the charge cycle but the amp meter should not be negative unless You have some other current draw. Was this problem observed at idle, the generator will not put out much charge at a slow idle, and the amp meter can bounce back and forth.