2006 T800 Headlights - high beams lighting up on low beams

Is low and high beam the same bulb on your truck?

 

Then it is a wire or connector that is making a bad connection between low beam and high.
Or the switch for high/low beam is bad.(doubtfull as you normally should have a relay between it and the actual lights)




Hope this helps.





If it had been a single bulb,then there was the possibility off a internal short in the bulb.

And if it was a ground issue,both high and low beam should light up the same.

 

From the head light to the turn signal switch they are two separate circuits. Either two exposed wires are barely touching, water in a shared connector or the turn signal/high beam switch could be bad.

Have you recently worked on the headlights?

 

Before you start tearing up your electrical system looking for a broken wire, check to see if the low beam bulbs are not burned out. On Kenworth trucks from 2005 - 2010 have a module that controls the headlamps and turn signals. If the module detects a burned out low beam it will turn on the high beam at reduced intensity as a safety precaution. Your symptoms sound to me like you got both low beam bulbs burned out and the truck is auto-illuminating the high beams.

Check it out before you get to aggressive with your diagnosis.

 

The low beam bulb has two filaments in it, one for low beam and one connected to the high bean circuit. When you see all four lamps illuminated in high beam, that is due to the secondary filament in the low beam bulb. Replace your low beams and I'll bet everything will work as expected.