not to be a smart ass but something should not test right, you need a ohm meter, disconnect the fuel sending unit, measure from the power side to the ground there should be resistance( which could be many readings depending on manf.) what you want to see is resistance though not a open circuit. sending units usally are the part that fails. If that passes ,with the key on at the gauge make sure it has 12 volts if it has a stand alone gauge if it has a display panel , you will need to research what it should read.
