another fuel gauge issue

Mecanical limieters create a pulsing voltage all around 2.x to 3.x volts which averages around 5 to 6 volts. A good analog meter will show the average while a digital meter will flash numbers like crazy.
A solid state reulator provide a constant 5 volts seen on any volt meter.
Please note; The instrument panel will not work properly while dangling from the harness connectors. It needs a chassis ground.