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Will my 55 ford F100 fuel gauge work off 12 volts without an under dash voltage regulator? Will be using a Mustange tank and sender, don't know if they changed the ohm's on the sending units.
Your 55 didn't even have a sending unit with a variable resistor if it is like my 53 f100.
They put 6v negative ground to the guage, and then coming out of the gauge they went to the sending unit. Inside the sending unit was a bi-metal piece much like a turnsignal flasher. It would heat and then cool off, heat and the cool off continually. The arm with the float would put mechanical pressure on this bi-metal assembly, and that would make the "flasher" speed change. If it flashed slower or faster, that would change the average voltage the fuel guage would get, and that would make the needle move on the scale.
I just put a under dash unit on my old 53. If you wanted to play with your guage I would take the dash apart and take it out. Then put a battery charger on one lead and experiment with your mustang sending unit putting the ground on the other terminal to the other lead of the battery charger.