My pyrometer reads correctly, until I turn the headlights on. Then the needle slowly drop, finally coming to a rest on the stop well below the lowest number on the scale. Turn the lights off, needle jump back up to normal. The gauge has been in the truck for years, it just started doing this within the last couple weeks. Its a Autometer Ultra-lite gauge if that matters. Suggestions?

 

Try unhooking the power for the gauge light and see what happens

 

I had a similar problem a few years back and I called Autometer's tech. line. The tech. recommended a direct battery ground. I previously had an under dash chassis ground. This solved my problem, I hope it solves yours.
John

 

Real pyros don't need any power to work.

 

Well, I guess Autometer does not make a real pyro.
John