I was driving one day and my gauge went from full to empty and the fuel light came on. It is a 2000 dodge 3500 4x4. I was told it most likely was the sending unit. I replaced it with a new one from dodge but gauge still doesn't move off empty. I was then told it was the gauge itself before I bought a cluster it took one out of another dodge and his working cluster didn't let the fuel gauge work either. Mine worked in his truck so I know it isn't the gauge. I can use an ohm meter on the 2 wires at the harness plug on the backside of the front driverside wheel well and it tells me how full the tank is so I know it isn't in the frame rail harness. If I go to the passenger side neg battery cable and unplug the gound wire going to the harness along the fire wall it will work for a sec when I plug it back in then go back to empty. Anyone have any thoughts.

 

Sounds like maybe a ground is off or corroded.