Troubleshooting Trailer Lights Not Working Correctly When plugged Into Vehicles 7 and 4 Pole Adapter

Question:

We are trying to wire up a new set of trailer lights. We wired according to the instructions, and our truck lights work as they should. But when we plug the trailer in to the truck, all the brake lights come on. When we try to turn on one blinker, all four come on. What are we doing wrong. It’s a 4 way plug from the trailer to a 7 to 4 adapter on the truck.

asked by: Patricia O

Expert Reply:

Trailer wiring is pretty straight forward. Green wire goes to the passenger side taillight, yellow goes to driver side taillight, white is ground, brown needs to connect to all lights on the trailer. The green and yellow wires are combined brake/turn signals for your taillights on your trailer. Brown is the running light signal for all the lights. If you have side marker lights that double as second turn signals for the front of the trailer you would just splice in the green and yellow wires as you run them back to the tail lights. Same goes for the brown wire. Double check to make sure the wires are connected correctly on each light.

With the trailer disconnected from the truck, test the functions of each colored wire going into the adapter on the truck to make sure it is wired correctly. Without knowing what adapter you have it's hard to narrow it down. Some 4 and 7 pole adapter plugs like Item # HM40975-11998 which have to be hardwired in, the functions for each colored wire are different from the standard yellow, green, brown, and white and could cause the functions to not work properly on the trailer.

We do offer test lights if you do not have one. I recommend circuit tester like Item # PTW2993. I have included links to a couple of videos. One is me wiring a trailer with side marker lights that double as turn signals and the other is the circuit tester being used to test a 4 pole plug on a vehicle.

expert reply by: Shane H