Troubleshooting Trailer Lights with Non-Functioning Right Brake Light

Question:

Ok I have marker lights right and left turn but I only have brake light on left side. Its a four wire system.

asked by: Gary C

Expert Reply:

In standard 4-pole trailer wiring that uses a connector like # A35W42B the marker/clearance/taillight function is carried on the brown wire and the green and yellow wires carry the combined stop/turn signals, green for the right (passenger) side and yellow for the left.

You can test your vehicle's 4-pole wiring harness for the correct signals if you have a circuit tester like # PTW2993. Have a helper press the brake pedal while you apply the tester to the pin for the green wire. You should detect a brake signal on the green wire when the pedal is pressed and when the right turn signal is engaged.

You can also test the trailer on its own by disconnecting it from the vehicle and applying 12V battery power directly to the trailer connector's pin for its green wire. This allows you to check the trailer light function without involving the vehicle wiring.

These tests help isolate the issue to either the vehicle wiring harness or to the trailer and its lights.

Also check for a bad bulb or a loose ground in the one light. Please refer to the two linked articles on wiring for additional help. Please feel free to tell me the results of your testing.