Fuel gauge diagnostics

[1972RedNeck]

1963 Chevy. Regulator died and over volted the temp and fuel gauge. Got the charging problem fixed. Bought a temp and fuel gauge from Truck and Car shop. Temp gauge works perfect.

Fuel gauge does not work. I have 12 volts at the gauge. To bypass the whoe sending circuit, I grounded the sense terminal at the gauge. Gauge should go to full when grounded but it just sits below empty and doesn't even wiggle.

They sent me another gauge to try - same thing. Again verified power. Again grounded sense terminal at the gauge. Still does nothing.

What am I doing wrong?