This is good reading as my mechanic friend is going through the same thing now. Fuel gauge is original and never removed from cluster., all other gauges work. New tank and sending unit, the old one was a rusted cruded up mess. Original wiring, sender is oriented in the correct direction, float floats in a bucket of water( never had fuel in the float) I have about 10 gallons of fuel in tank, gauge reads empty. Remove the center wire from sender and gauge goes to full. Sending unit out of tank with gauge wire attached to center stud and then ground the flange to tank, move float up and fuel gauge reads E to F as it is raised up. Sending unit back in tank and ran ground wire to neg on battery, still reads empty. Same problem as original poster. The 5 screws holding the fuel sender flange to the tank are new, old ones were still clutch head screws. I don't remember where I bought the sending unit, as it has been a few years when I was buying parts for a truck that had sat for 31 years, my fault. It would have come from a vender selling parts for these AD trucks. Will keep at it and get it solved, your replies have helped to try some things. 1950 3100 that is very original, just neglected from sitting for 31 years. It does start easily and runs good now.
