If you ground the sender wire removed from the sender to ground (chassis, frame etc) you should peg the gauge (doing so for more than a few moments can fry the gauge)
If you use the test wire from sender body to ground (chassis, frame etc) and the guage moves (assuming you have enough fuel in the tank) then that indicates you don't have a good ground from tank to ground (chassis, frame etc)
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You do have fuel in the tank? like 1/2 tank?
