For the archives I’d like to add to the chorus for checking grounds. I just had issues with a ’56 restoration and this post helped me track it down.
I added a 16AWG ground wire from the ground stud on the firewall to the instrument cluster itself, and “cleaned” metal where it was attached and where each part of the gauges screwed to the housing. The gas gauge worked fine but the temp gauge pegged. I jumped a ground wire to the contact on the back of the gauge itself (with gauge out of the housing) and it worked fine. So, I re-cleaned all the connection spots with a die grinder wire brush and that did the trick. They were clean metal before (considering age), no rust, but the temp gauge required a lot better ground to work than the gas gauge did.
The ground path for the temp gauge goes from the instrument cluster housing to the individual (or pair, in this case) gauge back panel to a little solder contact on the back of the individual gauge's circuit board. All these contact points need to be good.
