Hello, I have an 83 300 SD with a temperature gage that seemed to be malfunctioning.

Replaceing the cluster solved a problem i suspected of the temperature reading too high. However, still persisting is this problem of a 5 degree fluctation when a high-current device (i.e. A/C or high-beams) are switched on or off. I was thinking maybe this is related to a voltage regulator problem? All efforts for looking for a loose ground/loose connection have failed.

Any advice on what to do next?