1987 240 Temperature Gauge rises when I turn on the ligh

[Billreichart]

I just received as a gift, a 1987 Volvo 240 Sedan, and I've been driving it for one week. The previous owner took great care of it, but today as I needed to turn on the lights for the first time, something weird happened - The temperature gauge was dangerously rising. When I turned the lights off, it would start to decrease...and when I don't have the lights on, the gauge is right in the middle, where it's always been.

I've never thought that turning on lights would or could affect the temperature gauge....please advise because I am stymied..thanks

[lummert]

Search for temperature gauge compensation board. Maybe that's the problem.