When I put on the lights in my 82 Brat the gas gauge rises, so does my temp and voltage gauges. Oil pressure seems unaffected. When I put the blinker on, the voltage gauge clicks up and down slightly with pulsing of the blinker. Same thing happens if I turn on the fan for the heater, the temp gauge rises and etc.

 

I'm curious if this effects when the electric fan comes on to cool the radiator, but I'm not sure.

 

Suggestions on how to fix this problem would be appreciated. I don't need perfect gauges, but I want them to at least be consistent.

Tighten/clean the ground wire that comes out of the harness under the left side of the dash.Maybe run a dedicated ground from battery negative to there.

Another thing to check - try and see if you can tell whether turning on the lights makes the gauges read higher. If so locate the light relay and replace it making sure the connector is clean. Worked for me. Also pull the cluster and make sure the gauge mounting nuts and studs are clean and tight.