Troubleshooting Trailer Tail Light Functions with 2010 Toyota FJ Cruiser

Published 09/28/2018

Products Featured in This Question

Hook Probe Circuit Tester

(70)

$12.41

Pollak 7-Way to RV Socket LED Circuit Tester - Vehicle End

(166)

$45.12

Question:

No trailer brake light on one side but turn signal is OK. Where would you look? Trailer uses 7pin. Just for my education, what should that light do if both brakes and turn signal are on? These do use the same wire, right

asked by: Laurence G

Helpful Expert Reply:

I recommend using a circuit tester like # PTW2993 to check the wire function at your 7-way. You should have both the stop and turn combined for the left side at the 9 o'clock pin position and the stop and turn for the right side combined at the 3 o'clock position. When you have the brake on, the turn signal should still function. You can use a 7-way tester like # PK12741 to test that function as well. I've attached a video to assist.

If you have the both functions for each at that location, then you have determined the issue is on the trailer side. If you don't have the functions there for both, then the issue is on the vehicle side and I recommend checking your fuses as outlined in your owners manual first. (For instance Fuse 3 and 28 are for trailer towing).

In either case, first check your ground connection to the frame. Your 7-way should be connected to your trailer frame with a white wire that should be bare metal to bare metal. On the trailer side, there should be a ground connection to your trailer frame from your 7-way wire connector. If you check the functions along the trailer prior to the tail lights and have the correct function, then you most likely need a better ground at the tail lights which often is through the mounting hardware. This should also be bare metal to bare metal.

The final thing that could be the issue is if you have dual filament bulbs, one of the filaments could be bad giving you just the one function.

expert reply by: Jason S