No marker lights when headlights are on

@MNdriver: It happened regardless if I had a trailer or not. If it only happened with a trailer attached then that may hint at an overloaded relay or something. I'm not an electrician so I'm guessing on that one. Anyway, leftlanetruckin is right - just start unplugging lights one by one until things start working properly. Hopefully that's all you have to do to find the short [to ground]. I got lucky in that once my marker completely failed they were able to say "Hey, this might be the problem" because my rear lights (and dash) immediately started working again.

 

ok, so if it happens with / without a trailer, it gives you a place to start. Basically doing the same thing that leftlanetrucking is talking about.

Unplug the pigtail from the back of the cab too. Make sure it isn't that connector too.

And like you said then, process of elimination. And it sucks not having a wiring diagram to read things.

Start with the really simple things and work from there. Pigtail to trailer (both ends disconnected.) Tail lights. Any place with a SHARP bend in the wiring harness. places the harness has been repaired.

Even if you find a repair, open it up and look at it. If it's green or dark, cut it out and put weather tight fittings on it.

I'd personally start with the wiring harness on the hood. Especially around the hinges where the wires flex alot. But that's just me. Especially since this is an issue where you turn the headlights on and the markers go out.

You have short circuits, dead shorts, short to grounds.

Short circuits mean you are feeding into another circuit. If you have two positives feeding into it, you can actually simply turn lights off when you feed both sides. Think of your side marker and turn signals. You actually GROUND through a side maker your turn signal. When you have your markers on and your turn signal activated, that is why the side marker seems to blink opposite of the turn signal. During the day though when your markers are off, they both blink at the same time.

This can also be from wiring chafing and feeding into each other. Possible also when wiring work hardens, breaks and the insulation fails and you now have wiring exposed that is intermittent issues.

Dead shorts and shorts to ground tend to be the ones that blow fuses/circuit breakers.

 

nevermind... didnt see part where it happened regardless of being hooked to trailer. My last freightliner would do it when it was connected to the trailer only. However check the electrical cord end for your trailer hookup could be shorting out and could have blown the fuses. Happened to me twice when the lines fused together from salt corrosion this winter.

 

Wire 23 "all marker lights" is shorting to ground somewhere and popping your trailer tail fuse. Look for that fuse and see if the new one popps. Have seen a lot of times that the headlight/marker harness rubs against the hood and bumper, and shorts to ground. Also the harness that bends under the frame, left side under the air filter area

 

I figure that I should give another update. Marker Interrupt Switch. I forget where I read this, but a faulty interrupt switch will also cause the same/similar problem I was experiencing with my lights (read earlier posts). After fixing a faulty bulb socket (which was the source of my electrical short) everything was fine for a couple of months UNTIL I flipped the interrupt switch. Lol, I don't normally use this switch because it's awkwardly placed and it's much easier to flash my hazards a couple of times instead. Anyway, flipped the switch and lost dash/marker/clearance lights for a few minutes; still had headlights however. If I flipped the headlight switch down to the marker position then I'd have everything else except headlights. In any case, I usually only had to wait a few minutes until the interrupt switch reset itself and everything worked properly. I read that you can either clean or replace this switch. It's probably better to replace it if it keeps tripping due to the vibrations of the truck going down the road. It's also pretty easy to take apart the dash to access this switch (among other things).

Here's a video on dash removal https://www.youtube.com/watch?v=AJxrSFw2TFo Start at 4:35.

 

Having electrical problems running lights comes on for a few seconds then goes off but headlights works. Need some help

 

Check the tractor tail fuse it's #48 fuse on my 2005 Freightliner Columbia.

 

I had that same issues happen to me. I just happened to start checking my fuses first and found a blown 15 amp fuse. I replaced the fuse and it correct the problem simple fix maybe that's your problem.