I have a similar problem! With another sender unit (new) and jump leads to the car harness I have found that with the sender unit held "upside down" the guage will move with the float arm and gives approximately the correct reading!So....the guage works ,the sender works out of the tank if held upside down,the guage in the instrument panel can not be incorrectly fitted,the sender unit can not be incorrectly fitted and the wiring seems to be ok.I have checked and cleaned the earth points......I am now out of ideas!!!!
