Dash lights go off when turning on headlights

[Dhatley10]

New guy here, got a 91 c1500 and when I turn on my headlights all of my exterior lights work I just lose all dash lights except my hazard lights stay illuminated and the high beam light on the dash but nothing else lights up and the guages will go crazy. After about 30 minutes most of the time the dash lights will come back on on their own and everything is fine. I’ve changed the headlight switch and dimmer with no changes. I’ve heard possibly a bad ground but I don’t know where to start trying to trace a bad ground.

Thanks for any help, I enjoy looking around the site

 

[Drunkcanuk]

Silly question, are you sure that the dimmer is not turned down? I'm sure you tried, but gotta ask.
20 yrs ago coming home from work, night shift, all my lights went out. Got it towed back to town. The shop said it was a "bad alternator", and "oh ya, a broken wire that went through the firewall". 99% chance I didn't need a new alternator but the truck was fixed.
So long long story short, and a place to start looking, check the wires going through the firewall.

 

[Dhatley10]

Yea I’ve played with the dimmer several times. Everything works great until I turn my headlights on then the dash goes to crap, but 90% of the time after I’m driving for 20-30 minutes it will start working like normal with the lights on.

 

[Donald Mitchell]

Look for rusted grounds under the dash.

 

[Dhatley10]

Well I went out and looked at it today, turn the lights on and dash goes crazy but if I turn the dimmer all the way down with the lights on the gauges go back to normal but it’s daylight so can’t tell if my dash lights are working

 

[Obsmallet]

Did you ever find a fix for this? My 89 c1500 is doin the exact same thing.

 

[GrimsterGMC]

Did you ever find a fix for this? My 89 c1500 is doin the exact same thing.

I was having the exact same issue. Turns out the trailer light connector on the bumper was full of dirt and when it got wet it would create a low current short between the terminals. It would always happen when I was driving to work in the morning while dark and the truck had been covered in condensation, or it had been raining. Then about an hour later, when the sun had come up, it would come right as the condensation had dried off and no longer shorted. I hope this helps.