Stewart,
Some folks say that hooking 12 volts up to the gas gauge will kill it, while other folks have run theirs on 12 volts for years...don't ask me why it works this way.
The system works using a variable ground to move the pointer on the gauge between two coils mounted in the gauge. The variable ground is supplied by the movement of the float in the tank across a wound resistor wire.
You can get a inexpensive voltage reducer made just for things like the fuel gauge from many of the vendors.
I would install the reducer in the power line ahead of the gauge and proceed with a little test that should troubleshoot the gauge, wiring and sender.
With the reducer installed and the key ON, remove the wire at the tank sending unit. The gauge should read FULL. Take the same wire, and with the key still ON, ground the wire to a good ground on the cab or tank. The gauge should swing to EMPTY.
If this happens, the gauge is good and the wire back to the tank is good.
If it doesn't happen, the wire could be broken or grounded, or the gauge could be NFG, or the gauge might not be grounded in the dash assembly.
If the test does prove good and the gauge doesn't read properly when connected again, the sending unit is NFG.
If you are industrious enough to pull the sending unit, and you have a ohmmeter, you should get a variation of near 0 to 30 ohms when you swing the float stop to stop.
If the gauge doesn't respond correctly during the little test, it could be the wires from gauge to back of cab. To verify 100%, you would need to attach jumpers directly to the back of the gauge (sending unit terminal) and find a good clean ground for the test. If the gauge is toast, it would have to be replaced.
I know this is a mini-novel, but it isn't often that two-Stu's/Stew's get to BS! Good luck!
Stuart
