the sender supplies the ground to the gauge thru it's variable resistance - power is supplied from the ignition switch to the gauge, not from the gauge to the sender .... the sender should be grounded by it's mounting to the tank - you can check the sender by removing it from the tank and manually working the float arm while you check the resistance from it's terminal to it's case, should vary from 0 to 30 [or maybe by 69 they're 90 ohm?] - you could also check before you remove it, from the terminal to the tank .... I'd suspect your gauge if it shows a constant reading, but it could be the sender
Bill
