Troubleshooting Brake Lights Not Working on Trailer  

Question:

Expert Reply:

Troubleshooting vehicle and trailer wiring is easiest if you check the vehicle and the trailer individually when they are not connected to one another.

I suggest first testing the vehicle 4-pole wiring harness with a circuit tester like # PTW2992 or # PTW2993 to see if the brake light signals are present on the contacts. In trailer wiring the brake and turn light signals are combined into one circuit for each side of the vehicle/trailer so you'll only need to test two contacts on the vehicle to see if the appropriate signals are present.

Connect the tester ground to the ground pin on the 4-pole. Then have a helper apply the RIGHT turn signals and then the brake while you test the contact for the GREEN wire. The LEFT turn signal and brake light signals should be on the pin for the YELLOW wire. If the brake signal is not present on the vehicle check that your wiring harness connectors are fully seated to the mating ones on the vehicle; next check your owner's manual to identify any fuses related to trailer lighting and install or replace as needed. If the correct signals are there on the vehicle then odds are the problem is on the trailer.

You can test the trailer by directly applying battery power (from the vehicle battery or from a fully-charged spare if you have one). Connect the battery ground to the trailer connector ground pin (typically the one for the white wire) and then connect the battery positive to the pins for the green and yellow wires (one at a time) to see if the lights activate.

For both the vehicle and the trailer make sure the main ground wire ring terminal is tightly affixed to a clean bare metal spot on the vehicle and trailer frames. A bad ground on either the vehicle or trailer can cause all sorts of lighting issues.

expert reply by: Adam R

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