Sender and gauge are not volts specific. The problem is most likely a ground or bad wire so look at those real well. If it worked on the bench and the float is good you have a connection issue. If you pull the seat back out you can access the sending unit from in the cab. Lift it out of the tank and see if it will work when hooked up and you move the float up and down. Use care, you are sitting over an open container with the tank in there. Let us know what you find.
