66 Dart gas Gauge

Pawned said
If you have an original instrument cluster (I am referring to a early 70s model, not sure what year you have) the two screws coming out the back of the gauge get connected to the PCB when you tighten down the nuts. One is connected to the +12vdc and the other goes to the sending unit. There really should be no wires involved, just the traces on the PCB. The circuit is grounded at the sending unit. There is no need for a ground at the gauge.

The typical 2 post thermal gauge is connected to a voltage limiter, not 12 volts.
I don't recall where a 66 Dart has its inst' voltage limiter. If the fuel gauge is a three post gauge the voltage limiter is inside. The three post gauge 4 connections, 12 volts in, limited voltage out, sender, and has a slither of metal on its backside that sits against a small unpainted spot in back of the inst' housing. This is where the limiter gets its ground. In some cases loss of this ground was why the limiter and gauges stopped. Clean it up, snug it up and all work again.