Strange electrical problem, any ideas

[nathandi7]

Hi guys
My 90 sierra is posessed i think. It goes fine but when I turn the lights or the heater on the temperature guage goes up, turn the lights off the guage goes straight back down. I dont think it is overheating i think it is something electronic. I did the el falcon alternator swap to fix it and improve the charging but it didnt make a difference. Id appreciat any ideas or advice from people if they have had the same problem.
cheers
Nathan

[PJ.zook]

It sounds like you have an earth problem. Check the passenger footwell for a wire that screws onto the lower dash support bar that sits above youre legs, thats the one that supplies earth to youre dash.

[zoook]

from a sparkies point of veiw, its a bad earth, the reason your temp gauge reads higher is because as you add load to the system, the increased resistance from the bad earth is leading to more current floating around the system that is looking for somewhere to go but it cant get to earth , check the earth at the motor and at the battery and if they are good add an earth between the batt and the motor and the body to ensure a zero potential across the whole system.this should ensure residual current is grounded and should fix your hassle,

[nathandi7]

I think you both hit th nail on the head. The inside dash earth was ok but the battery/body earth had about 10mm of missing wire under the insulation. I replaced it with the 125 amp wire I had left from the alternator conversion, havnt tested it yet but it shoul fix the problem.
Thanks heeps for the help

[lay80n]

Mine used to do exactly the same thing. Check the earths, my earth from the battery neg to the body had broken.

Layto....