All Trailer Lights Come on When Brake Is Pressed and All Lights Blink with Turn Signal

Question:

All the light blink on the trailer when the turn signals are turned on. Also alll the lights light up when the brake is applied.

asked by: Tom L

Expert Reply:

Your problem is in the wiring in one of two places. It is either on the vehicle wiring harness or somewhere in the wiring on the trailer. The first thing to do would be to narrow it down to one of these two places, then we can pin-point the problem from there.

Disconnect the trailer from the vehicle so you can test the vehicle wiring to see if it is functioning properly. Using a circuit tester, like part # PTW2993, clip onto the exposed ground post, white wire, and check the other three contacts. Brown is taillights and should come on with head lights. Green is passenger side turn signal and brake, and yellow is driver side. If these functions all work properly, then the vehicle wiring is fine, and you can move on to the trailer. If not, then there is something wrong with the vehicle wiring and if you write back with the vehicle year, make and model and whether or not it is a factory installed harness and I could help you further diagnose the problem.

If all the functions one the vehicle connector work fine, then the trailer wiring is probably the root of the problem. The first thing to check is the ground wire, the white wire coming off the connector. A bad ground is a common problem on a trailer and will cause the lights to do all sorts of odd things. Make sure that the ground wire has good connection to the metal trailer frame. Even paint can sometimes prevent a good connection. Check the lights themselves and make sure they are grounded properly.

If you find that a ground is not your problem, then the trailer may be wired incorrectly. You should have 4 wires coming off the connector to the trailer white should be attached to the trailer frame, brown should split somewhere and run down both sides of the trailer to power running lights and taillights, yellow should run down the driver side for brake and turn lights and green down the passenger side for brake and turn lights.

I have included a link to a trailer wiring diagram and a video of a light kit install on a trailer so you can see how the wires need to be run.

expert reply by: Patrick B

Devere

4/17/2020

My left signal all the lights work my right signal nothing works

Chris R.

4/18/2020

Is it just the turn/brake signal on the right side that's not working, or are the running lights not working as well? My first instinct here is that there's a blown fuse under the tow vehicle's hood. Check for any fuse locations in the owner's manual related to those circuits. Let me know what you find and we'll keep troubleshooting (if needed).