take out the sender, put a temporary ground from its mounting flange to the cab, move the float arm manually thru its full swing and see what the gauge does - could be the sender has a problem, or the gauge needle may be sticking, or you need a resistor to adapt the 6V gauge to 12V - if you can make it go to full moving the float arm, you might need to bend the arm, or you might have put the float in wrong so it's hitting the side of the tank
Bill
