Temp gauge mystery

I posted and then went backreading to see if you had been told b4. I found so many typing IVR. That is instrument voltage regulator. The one we would use this application is a solid state component that can take in 9 to 35 VDC and output 4.8 to 5.4 VDC constantly. The symptom you describe comes from yesterdays mechanical voltage limiter. I can't bad mouth yesterdays technology because it did work, very well in fact, for Chrysler, Ford, VW, etc... Nothing lasts forever.