Hey Rick, I think you meant to say it reads beyond empty when grounded, no? That's the way I always tested a dash unit and wiring going to the sending unit /tank. Bill

Nope, that's the issue. It buries the gauge past full when I disconnect the sending wire (which is the way it should act) as well as when I ground the intermediate harness to sender lead using two different ground wires (both tested for continuity) and multiple ground locations to insure a good ground. Technically this cannot happen......but it is! Grounding the sending wire should give you an empty reading as you stated but it does not.

Call Mike at Instrument Specialties. He's a good guy and should have the answer.