When you get the tank out, check the assembly with an ohmmeter, attach the ohmmeter to the possitive and neg. on the unit, then move the leaver, if the ohmmeter moves then you know that your unit is good, (Check the wiring doing to the unit) I have had bad wires before, or even bad plugs where it plugs onto the sending unit.
Hope this helps for a tip, i love electrical work, but what sucks is that i am colour blind so it takes me forever to do that work, but i love electrical
Use the proper tool for the job!!!!