i hate to bring up a somewhat old thread but im having a similar problem when I got the car "in pieces" my gauge was on 1/4 tank I never really paid attention to it until 9 months later when I got the car built. my gauge is on full all the time. I pulled everything apart in the car including the gauge cluster and see nothing wrong at all.
If I take the sending unit in hand and and manually move it it does not change the gauge at all, But even if I put the sending unit in the empty position and set it there for a few min "10" or so and watch the gauge it slowley rises all the way back to full. So I took a wire and grounded the fuel sending unit to see if maybe it was a bad ground and nothing changes.
If I unplug the sending unit it does nothing at all, So i took the sending unit apart and there are no broken wires or anything like that and it all looks to be good. Is there a way to test the sending unit for one "bench test somehow" and also what color is the wire/wires on the back of the gauge cluster for the fuel gauge? this way I can try to bypass everything and run 2 short wires from the sending unit straight to the gauge and see if it makes a difference?
thanks for the help