2000 W900L no brake lights on trailer but I have brake lights on tractor!!

You have a break in one of the lines going to your alternator. Take them off and make sure there are not a lot of strands of wire broken and make sure you have a good full connection. That will fix the problem.

 

can this be explained a little more in depth?? cause i'm really not grasping the concept of how the alternator will cause the trailer stop lamps and trailer stop lamps only to not work.

 

Check the wiring at the back of the plug on the connector on the truck for the light cord.

 

i'm not sure what all has been checked, but here's the run down of a fix.. get a multimeter (has to be a multimeter because it will measure amps).

Start by pulling the hand valve down to activate your brake lights, bungie it if you have to. Then go to your pigtail going into the trailer.. stick the black probe into the top socket (12o'clock position), and the red probe in the bottom 6o'clock position)..

If you read anything less then 12vdc, then you have an issue. A test light will light with any voltage, but most lights (had this issue with my taillights a couple of months ago) will only work with the proper voltage.

If you have no power at the end of your pigtail, disconnect it from your truck and probe the prongs there.. This will tell you if the issue is with your pigtail or not.

If your not getting any voltage there then find a fuse, if the fuse is good, then you need to find the relay... That's pretty much it.. The pigtail for all of the trailer lights run as one chunky solid cable from the front of the cab to the back of the cab.. It's not in a loom like the wires for the truck chassis.

I also have 20ft of jumper wire that i use to bypass large sections of my truck to narrow down the possible location of a break in a wire.

 

It has to do with the fact that a Kenworth has a separate circuit for the trailer lamps. Had this exact thing happen with my 2000 KW W900L and after chasing gremlins for a long time (trailer relays, turn switch, fuses, ect.) I found that some of the strands in one of the alternator wires were broken. After fixing and crimping a new end on, everything worked fine.

 

hey sorry but, I am having the same issue. I have a KW T660 tho where is this wire with a fuse located at? I am wondering if this is a common KW thing.

 

I had a 99 w900 didn't have trailer brake lights but did have truck brake lights.i knew about the fuses on the firewall but not fire to them. Checked relay and had fire coming in on the signal side but not on the constant power side. Traced the wire to the alternator and it was corroded! Abs wire to the relay also corroded, hopefully I've helped someone because 6 hours of my life are gone now

 

Sometimes it can be as simple as a loose/faulty plug which is the first thing I would look for then the next would be there could be a short sometimes in one of the wires on the trailer.