2006 T800 Headlights - high beams lighting up on low beams

Ok. Now that you have illuminated the bulb it is time to check the next level of complexity. First, and I should have already recommended this, check all the fuses. There are multiple fuses for left and right bulbs. After that you need to check the correct operation of the turn stalk. Attempt to enable the high beams. If the switch is working correctly the high beams will go into full illumination or at least flicker. If nothing happens then your problem could be the turn stalk. Next thing to check is the module located behind the dash near the streering column. If none of these devices fix the issue then it is likely a wiring problem. I doubt it because left and right beam circuits are separate and for your symptoms to be a wiring issue it means both would have broken at the same time without the left and right high beam circuits being damaged. Regardless, check the condition of the hood harness connector located driver side hood pivot for corrosion or damage.

 

I had a similar problem on a Peterbilt, ended up being in the bobtail lights. I don't know why? Try unplugging them.

 

90% you have a bad ground. Low beam bulb has 3 prongs,1for dim ,1 for bright and the third is shared by hi & low for the ground.The ground wire then splices with ground for the high beam bulb (two prong)before leaving hood.Lots of time its the connector by where hood hinges.Your truck just has single relay for low and hi so loss of ground on either left or right can cause all 4 to burn dim and yes your dims wont lite up 100%. As emerg fix I have used a short wire twisted around radiator ground strap and tied in with headlight ground but usually you can unplug and clean connector and that will solve the problem .I'm a true believer of coating all elec connectors with di=electric grease so you can skip this kind of problem. If a dual filament bulb shorts internally it can feed back similarly but most of time results in 3 burning dim and 1 out completely. t8,t6,w9 all wire same but others can have a constant hot and dimer switches ground.Any elec problem I always check my connector plugs first.A/C working fine and quits low pressure connector at dryer, hope this can help you roll on

 

Dont forget they have same headlights like on t600 and w900.

 

the headlight harness has a connector plug near the rear of the trans, where the major group loop under frame rail. Its a 10 or 12 pin connection that has two ground pins in it that can go bad if extra lights are added to the front. these ground wires run up to that ground stud on the firewall above washer bottle area. check these connections, like others said, when others circuits come on dim its usually a back ground issue.

 

I only had issues with my foglights.

 

I have an issue
On my 2013 t800 the headlights start to give me issues
High beam is on the left side and low beam on the right side only one beam turn on on the right side and both are new
Any suggestions I havent touch any electrical

 

In case someone is looking for a fix for that problem, I found on a truck that I work on that the turn stalk was the problem.

 

I have the same problem all 4 come on low beam and weak power. Like half high beam power. I changed the switch and still same.