jerbob, I may not be following what you are asking but, reversing the wires won't reduce 24 volts to 12. On a 24 volt positive ground system, with two batteries in series, you should have 12 volts at the negative terminal of the battery that has the positive post connected to ground. You can check that with a voltmeter. You would have to run a separate wire from there to supply your light switch with 12 volts. You could put a resistor in the 24 volt supply to the lights and reduce it to 12 volts. It would seem it would be easier to get and install the right 24 volt bulbs and reduce the chances shorting something out. Years ago I remember seeing screws driven into the lead connections between cells on the old 12 volt tar topped batteries so people could run 6 volt accessories from them or resistors installed for the same purpose. I also remember seeing somethings go up in smoke from these taps as well. If you decide to do something like this please put a fuse in your wire near the battery.

I agree with Steve. An old mechanical ammeter should work on 6, 12, or 24 volts as long as it's range can cover the charge rate. How many 6 volt tractors and other vehicles have been converted to 12 volts and the same ammeter used by just reversing the leads? They work until the current over loads them, which might be a long time. The likely reason a manufacturer will say an ammeter is for a certain voltage is the lighting kit/bulb they supply with it, as lighting is more commonly supplied in the gauges than it used to be. Stewart Warner gave an option of 12 or 24 volt lighting kits for their ammeters.

The seller, hwaccumulator, said they only work with 12 volt. I put a 35 amp one wire alternator on it, I think it should work.

Garlic Pete, thanks for the info. I was curious if that is the case. I can find many that say 12 volts, I’ll try one and let you know. My multimeter is showing around 27+ volts when charging. Both my HD11s have ammeters, not voltage meters. I believe they came that way because my dad bought the tractors new and he said he never replaced them.

I think, but am not sure if this is correct, that the ammeter doesn't care what voltage is applied to it.  It just measures the electrons that pass by.

https://www.grainger.com

Edited by steve(ill) - 19 Feb 2018 at 12:15pm

Anyone know of a source for a 24 volt ammeter for an HD11? All I can find are one wire digital ones or $150 aircraft gauges.