I was having the same problem with my fuel sending unit from Classic Chevy Truck parts. It would work outside the tank holding the wires to it, moving the float arm up the gauge went from E to F. Full was 30 ohms. Put it back in and it would not move off of empty. Remove signal center wire and would go to full. Just would not work in the tank. I ordered a new on and it checked good, 0 to 30 ohms and E to F by raising the float arm, just like the old one. My mechanic would not give up, must have removed it 15 times. Finally figured out that the center wire stud was making intermittent contact. He could wiggle it and the needle would defect up to full and back down. Never did figure out what caused it. Put the new same style sending unit and it works perfect.
