If the sender is grounded then as said it has to be the sender assy itself (float/windings etc). I would fill the tank all the way up & if the gauge ain't pegged (Full) then it does not have a good ground because when the tank is full the rheostat windings are submerged & that is why the gauge is pegged with a full tank & it stays there for awhile before it starts to drop as the fuel level has to get below the windings so there's in effect no more dead short & it starts reading thru the actual resistance of the windings