Posted by D Sherman on Thursday, April 26, 2012
By all means DO NOT FIX the well-known grounding problem since YOURS IS WORKING FINE!
Okay, if you want to bench-test it, do what I do. Take the gauge out of the truck. Find a potentiometer with the same resistance as the gas sender, or a little more. 120 ohms comes to mind, but I could be wrong. Measure the sender's resistance at each extreme position of the float to be sure what it's resistance should be. If you can't get a steady reading on your ohm-meter, solder the wire as previously described even though THE GROUND IS NOT THE PROBLEM. The reading will then be steady. Turn your bench power supply to 12.6 volts. Hook the ground wire of the gauge to the negative side of the power supply and the hot (center) wire of the gauge to the positive side. The needle should peg (I don't remember which direction). If nothing happens, perhaps that one coil of the gauge is burned out (common when somebody hooks 12 volts to a 6 volt gauge). Hook the pot from the "gauge" terminal to the negative side of the supply. As you turn the pot, the needle should move smoothly and steadily. If it sticks and jumps, you have dirt in the pivot and probably need a new gauge (next to impossible to clean out).
The basic key to electrical troubleshooting is to simplify the system as much as possible until the only "variable" is the device under test. Known good power supply plus known good variable resistance means only the gauge can be at fault.
If this is too complicated, keep swapping parts, guessing, and assuring yourself that it absolutely can't be such-and-such, until it works or you run out of money.
I once got a '65 D200 that had a gas gauge that didn't work. It had two take-out gas gauge senders in the tool box behind the cab. The problem was the aforementioned bad ground between the wiper arm and the bracket. All the senders were good, but someone had obviously swapped 2 others in before giving up entirely.