Temp Gauge reading changes when lights are on

[Oxyacetylene]

I have been wondering if the fan clutch solenoid is operating properly because the truck seemed to be running a little hot. For the last few months I have only taken it on relatively short drives and haven't had much chance to get the truck up to full temp and keep it there. Yesterday I drove it to work, it was 94 degrees outside. When I left work the temp came up and the fan clutch engaged at least a couple times during the drive. I had a stop to make before heading home. The truck sat for about an hour and a half before I cranked back up and drove home. By now it was getting late but not too dark. I turned the lights on, serv drive, panel bright. While driving home the temp went higher than it had earlier in the day but the fan never engaged. When I pulled in the driveway and turned the lights off, the temp gauge dropped. Now I am wondering if the temps are running normal and just the gauge reading high when the panel lights are on. Here are two pictures showing the temp with the lights on, and immediately after turning them off. Anyone else notice this on theirs?

 

[porkysplace]

Clean the grounds on your instrument panel and see if it helps on the gauge readings.

 

[Suprman]

The temp sender is on the engine above the alternator make sure its plugged in tight and zip tie up the wire going to it.

 

[MtnSnow]

Yep clean the grounds & make sure the sending unit wire at both ends it clean & tight.