I just installed a set of electric gauges on my 70. Everything is hooked up but I have an issue with the temp gauge. For some reason when I flip the lights on the temp goes up by 20 degrees. I flip them off and it goes down 20 degrees. Not sure why this is. I hooked it up to the original wiring using the idiot light wires. I did use the hot wire from the ignition leading to the idiot light for the temp to hook up all three. This wire turns them on when I turn on the ignition. They all come on fine and only the temp gauge does this. Any suggestions?

Check all your grounds on the dash and the guages.

X2 on grounds.

Im not sure how to do that. I grounded it to another screw that is a ground screw for the dash. They seem to be grounded fine.

Check grounds AND alternator output

If you are maintaining less than 14v while driving----this can drop to less than 11 at night when the lights are on.

less than 14volts going to the gage can wreak havok on your nerves.

I have recently gone thru this on my 59 with digital gages---what a nightmare
thought i was overheating, but was not

Ill check that out. I know its an older alternator. The volt gauge says its pulling 14 volts. Didn't look when they lights are on though.