Grounding the instrument voltage regulator

On my 67 there was a BUNCH of stuff wrong with the cluster. That is why I keep preaching to check these END TO END as a SYSTEM. Visualize in your mind the entire path, from sender, through each harness connection, plug, connector, wire, pin, mating female connection, the PC board, etc.

On the cluster itself there is a number of factors, including loose broken harness connector pins, poor connections at the regulator, poor connections at the gauge studs, of course, bad regulator and or gauge units, and poor ground from the cluster / PC board(s) to the body of the vehicle.

On my 67, where the regulator plugs into the PC board, there are brass finger contacts which form the "socket" into which the regulator fits. None of these brass fingers were reliably making connection to the PC board. I had to solder jumpers from the brass fingers to the board traces.

As Red said, add a ground pigtail from the cluster. Bolt this to a solid ground, such as the column support brace.