2003 379 led headlight issue

Sounds like a bad ground. Find a lot of issues in the buckets and @ the hood hinges.

 

Yes definitely sounds like a bad ground

 

I had a similar issue where the right side would go completely out when the brights were activated (I have the old double round headlights). I found on accident that if I wiggled the marker light relay the right side would come back on. I replaced the marker light relay and the problem went away.

Someone had mentioned to me that the relay that was in there maybe wasn't rated properly for that circuit. I never bothered to check as the lights were working fine until now. So now I have to start figuring out why again.

Low beams fine on both sides, turn on high beams and right side completely goes out left side is fine. Swap lights from right to left and same thing so it isn't the lights themselves. I don't understand what the marker light circuit has to do with the right side headlights. One thing I noticed is the fuse box itself around the marker light spades was brown like it was over heated.

 

I bought some LEDs to put in my pete 386 they work well on low beam problem is they won't switch to high beam. I bought error cancellors hoping to correct it but still same issue put different set of led bulbs in same so just returned them until i figure out what the issue is why they will not switch to high beam. Has anyone else had this problem with their led bulbs.

 

Peterbilt 386 came from factory with 9007 bulbs. Chances are your LED’s are 9004 the difference is ground and low beam position. So basically you’re ground out one and using the other. When you go to re pin 9007 to 9004 pay close attention to the diagram you’re looking at cause half of them on the internet are wrong. Trust me I fought this for along time.