I just rewired trailer lights from scratch.
The right turn signal and brake work fine.
The left turn signal on my vehicle operates BOTH left and right turn signals very dimly for some reason.
The left brake light doesn’t work at all.
Tail lights don’t light at all when I turn on the headlights.
My ground wire is attached to brightly sanded metal on the trailer tongue. All connections are secure. All wiring is brand new.
So what am I missing here?
How about the light-assembly grounds? Are they grounded through the bolt or is there a wire that grounds the Lights to the trailer?
If it’s the newer style lights where it gets the ground through the bolt make sure where it contacts the trailer that its cleaned up real well.
I always run ground wires from each light back to the plug so I don’t have ground issues.
That’s a very good possibility. I didn’t think of that. I will try that first and post again if I’m still having issues.
Some wiring rigs I’ve seen use the trailer hitch for ground also, not a good idea, run a ground wire to the vechicle wiring if you don’t have one..imho
Al
Verify the lights are being fed properly from the vehicle. I have seen guys struggle with their trailer when it is a truck problem. A cheap plug in style tester with small LEDs at the truck will eliminate this as a possibility.
Otherwise, Trailers and bad grounds are like peas and carrots 
At the plug attached to the trailer wire harness insert a small bolt into the ground terminal, the only female of the 4 plugs. Hook up the negative lead of a 12 volt source (a spare battery) to that bolt. Then one by one, and carefully, connect the positive lead from the battery to the other 3 male leads. When connected to the lead by the yellow wire the left brake/turn signal should light. Green wire, right brake/turn signal light. Brown wire, side markers and rear tail lights. Remember the stop/signal light must be noticibly brighter than the tail light. If they do not work this way you have to trace out your wiring or look for open connections. But until all this works as described the truck wiring is moot.
Bring it over
Took me all of 5 minutes to fix Al’s trailer lights. It’s a ground somewhere
get away from hard wiring into your truck go and pick up the plug in type wiring harnesses… it fixed my trailer
light problems 
Quote:
I just rewired trailer lights from scratch.
The right turn signal and brake work fine.
The left turn signal on my vehicle operates BOTH left and right turn signals very dimly for some reason.
The left brake light doesn’t work at all.
Tail lights don’t light at all when I turn on the headlights.
My ground wire is attached to brightly sanded metal on the trailer tongue. All connections are secure. All wiring is brand new.
I think you might have some wires crossed at the light assemblies. The brown wire is for the tail lights, the yellow and green wires are turn signal/brakes. Also note that there are two filaments in the bulb. One lights up brighter than the other. The bright one is used for brakes/turn signal and the dim one tail lights. If the left turn signal is lighting up BOTH sides, then maybe you have that wire crossed with the tail lights.
The light assemblies I’ve worked with were all setup with two wires and two brass bolts. One of the wires is used for the turn signal and the other wire is used for tail lights, and “ground” is done wih the mounting bolts. Make sure you clean all the paint and crud off around those bolts so that the nut pulls down onto clean shiny metal.
You’re pretty close to having this working right, make sure to let us know what you find out.
Rootski
Update:
One of the tail lights was inadequately grounded. I fixed that, so now the blinkers and the brake lights work. Thanks for pointing me in the right direction there! 
Still nothing when I turn on the headlights. I’m thinking it’s got to be either a fuse in the tow vehicle or a wiring problem in the tow vehicle causing it. Gotta run by the hardware store tomorrow and pick up a volt meter to check it.
This would be a whole lot easier if I hadn’t let my wife take my Tahoe to the airport instead of her van 
(so I can change the oil in her van while she’s gone 
). I *know* the trailer wiring’s good on my SUV.
No marker lights or tail lights screams fuse. I’ve had problems with a short in my trailer lights in the past, so I could change the fuse under the hood with my eyes closed. Check your owners manual for which fuse controls the trailer running lights.
X2
Yup, the fuse for the trailer tail lights in the van was smoked. Between that and fixing the grounds on each tail light, all my problems are solved.
Thanks for the help, guys!
Great news, now I don’t have to work on it at work 
C’mon Ron, you need to do something on break besides read your fishing porn mags!
You must be logged in to reply to this topic.
