Changed the cabs on my 83 f800 diesel with a cab from an 84 f800 gasser. Can't get the fuel gauge to read to full even when moving the sender unit manually to that position. In the empty position the gauge reads waaaayy under empty. Tried another sending unit from the other truck I took the cab off to see if the sending units had to be compatible for whatever reason but it made no difference. Any ideas? Right now I'm just using an old broom stick to dip the tank but I think I'd rather have a properly functioning fuel gauge