Chrysler Repair: fuel gauge on 91 dodge, resistance meter, fuel gauge

Question
i have a 1991 dodge dakota of which the fuel gauge does not work. it is always stuck on full. fuel pump was replaced but i dont know if it was doing it prior to that.

Answer
Hi David,
I don't have the wiring diagrams for thr truck lime, but assuming that the electrical design and color codes are identical to the car line I would suggest that you go to the fuel tank from underneath the vehicle where you may well find a round flange with several wires connected to it on the top front or rear edge side of the tank. That is typically a combination fuel pump and fuel level unit located inside the tank.  The dark blue wire is the one from the fuel gauge at the dash to the float in the tank that moves down as you use the gas and thus increase the resistance of the unit inside the tank which causes the needle at the gauge to drop. I would remove the dark blue wire at connector and then measure the resistance at the disconnect pin on round flange compared the a shiney metal surface on the frame of the trunck (scrape off some dirt and get the bare metal, or follow the black wire at the flange to the place where it attaches to the body as a point of comparison for the resistance meter's other lead). It should read somewhere between 5 and 100 ohms dependind upon how much fuel is in the tank (less ohms the fuller the tank). Then you could drive the vehicle some more and see if that resistance value changes or not.
The other check would be to see what the resistance is between the dark blue wire's tip when disconnected and ground (with the ignition switch in the off position). If it reads near 0 ohms, then that means that the wire has lost some of its insulation at a point near the body and the conductor inside is touching body and providing a short circuit from the gauge to ground which will cause the gauge to read full scale. Then you need to follow that wire as far as you can to find out where it is grounded, and you can go to the other end at the dash and find that same color wire and check for a possible ground from the gauge to as far as you can trace it.
So either the wire is grounded or the fuel float unit (called the sender) is stuck at the top of the tank or is shorted to ground. All those are the possible reasons for a fullscale all the time reading and you just have to sort it out by means of an ohmmeter.
Roland