I have converted my chevelle over to a SS clone with a new ss gauge cluster set up from Ground up , Amps, fuel, Tach water temp gauges. All new wiring harness from M&D harnesses for the same set up along with new engine wiring harness and forward lamp harness, my problem is that the temp gauge without the car running slowly creeps up all the way to pegged again without the car running at all and cold. I was wondering if there is a difference between the old sending unit for the idiot light versus the gauge now in the car? Also it has a ballast resistor on the rear of the gauge along with some sort of insulator. Any ideas on how the ballast goes and the insulator would be helpful. Any ideas at all?

I think the sending units are different. When i changed out my dash i had to try about 3 different sending units before i got the right one.

a gauge uses a sending unit that varies in ohm value which is what causes the needle on the gauge to slowly move according to the temperature. a idiot light uses a switch that either has continuity or it is open circuit depending on the temperature and this is what turns the light on. so yes they are different your new gauges should have stated that you needed a specific ohm value sensing unit 30 degrees = ? ohms / 220 degrees = ? ohms or something like this.