I did some reading around and it looks like 5v is correct, could not find any schematics for the dash. But I did stumble on a couple of guys hammering on, I think the same problem you are having. Could be enough info there to help you along the way.

Trying to repair the sending unit windings will be a pain, you'll be better of just grabbing another unit. The senders are standard for instance Chevy is 0 - 90 ohm , looks like Dodge is going to be 240 empty ohm, 33 full ohm; this was for a 79, lower link.


Have a look;
Gas gauge or voltage limiter


Fuel sending unit :
Dodge D100 Parts > 1979 Dodge D100 AUTOMETER FUEL LEVEL SENDING UNIT240 empty ohm, 33 full ohm; Includes installation hardware; This fuel level sending unit fits the standard 5-bolt mounting flange; Adjustable for 5 in. to 24 in. deep tank; Does not

Good luck ask if I can help or be a sounding board, sounds like you have the skills for this.