Trailer lights. weird behaviors. Trailer brake/signal lights dont work when head lights on on Truck

When I connect my trailer to the truck trailer harness and turn on the engine and press the brake pedal or turn on the signal light all works fine on the trailer. but if I tun on the Head Lights on my truck then press on the brakes or turn on the signals, it doesnt work. At first I thought there was something wrong with the trailer wiring but then I tried putting on trailer lights from harbor frieght and I get exactly the same behaviour. Everything works fine when the headlights are off but brake and signal lights dont work when I turn them on.

Any help is appreciated. I have a camping trip this wednesday. I have a 2010 double cab tacoma.

 

Ground wire from the trailer plug on the truck to the frame of the truck. Faulty ground in the harness will cause a bunch of issues.

 

I checked and the trailer plug in my truck has a plastic cover and all the wires coming out of it is wrapped in a plastic tube. I followed to some extend but couldnt see where it grounded

 

Maybe just run an external ground wire between the truck and trailer. Plug in your trailer and check the lights. If everything works then you know it’s a grounding issue.

 

As @L J said, run a jumper to verify ground issue. If verified, it is relatively easy to fix
On a 4 or 7 pin, the ground wire should be white. You will want to trace it on the trailer side and on the truck side. On mine, the wire was just attached with a self tapping screw to a bracket. After a while, it gave me lots of weird shit when hooked up. Remove the wire from where it is attached, grind/sand/wire brush the area to bare metal. Clean the ring terminal of any dirt/rust/paint and reinstall it to the bare metal, then paint over it.

 

Thanks guys. I know the issue is on the truck side because I used one of those portable trailer lights from harbor freight and I got exactly the same behavior. If I understand correctly, I should connect a wire from the metal of the truck to the ground port of the trailer light harness. Is that correct. I also have a volt meter and was looking on you tube on how to test it.

does any one know which ports out of the 4 pin plug represents what light/signal? From what I read, White is ground. brown is tail lights, green is right turn and brake and yellow is left turn and brake. is that correct.

 

Bad grounds account for about 90% of trailer wiring issues. Green/yellow are your turns and brakes. Brown is running lights. White is ground.

To fix a bad ground on the vehicle side, assuming the socket is ok, you can either trace to where the socket itself is already grounded and clean the connection. Or you can just run a clean/fresh ground and bypass the factory. The factory grounds are usually pretty close to the outlet.

On the yay side of things, you might also have bad grounds in the tail lights themselves. Pretty much anywhere voltage can leak can cause issues. But the simple stuff usually works.

 

Did you figure out your problem? I have the same issue. Brake and blinkers work correctly if the headlight is not on. Once I turn the headlights on, both lights are on, but the blinker makes both sides blink and the brake flashes, but doesn't get any brighter.

I will start with the ground, but my connector has quite a bit of corrosion in it. I'm thinking I may need to replace it. I tested it tonight and found this issue. I need to figure it out Monday as I leave for CA Tuesday morning pulling a motorcycle trailer. I guess I should have checked it a few days ago.

I have the Hopkins wiring harness, which doesn't have a separate ground wire, so I'm not sure how to track down a grounding issue with it. I will probably replace the whole Hopkins wiring harness if I can find the right one. If not I will buy whatever I can find and splice in the new converter and see if that is the issue.