The sending unit varies the ground or negative to the gauge hence moving the needle. You need to ground your sending unit and using a volt meter measure the ohms as the float goes from empty to full. If I'm not mistaken you should see more resistance when at full creating more ground to the gauge, less resistance/ground shows empty on gauge. To test your gauge simply ground the terminal opposite the positve and it should go to full. Could be a bad coil in the gauge or a bad sending unit. Hope this helps.
