Troubleshooting Trailer Wiring Running Light Circuit on 2005 Toyota Highlander

Updated 06/29/2015 | Published 06/25/2015

Products Featured in This Question

Factory Style 7-Way and 4-Way Flat Vehicle End Trailer Connector

(137)

$85.39

Loctite Bulb, Lamp, and Electrical Connection Dielectric Grease - 0.33-Oz Tube

(133)

$8.25

Hook Probe Circuit Tester

(65)

$13.35

Question:

Heres a strange problem. I have a camper trailer with a standard 6-pin connector. When I hooked up the trailer for the first time this season I could not get the tail/running lights to work. Everything else was fine. After a few minutes they all of a sudden started working without doing anything. This pattern has repeated 4 different times. Initially they dont work. After being plugged in for several minutes, they work. All connections look clean. I tested the trailer with wire and a 12 volt source, and it all works fine. Something is up with my cars wiring, but Im stumped. Tow vehicle is a 2005 Toyota Highlander. Any thoughts or suggestions? Thanks!

asked by: Phil S

Expert Reply:

Since you've tested your trailer with a direct 12V power source and confirmed that it is not the problem, I suggest you first focus on the item that is most often the cause for the kind of issue you describe: the trailer wiring harness ground wire. If the ground on the Highlander's wiring harness is not solidly affixed to a clean bare metal surface it may be able to pass enough current for on-and-off signals like brake and turn lights but may not be able to handle the steady current draw from running lights that remain on continuously. If it is at all loose it may move just enough to sometimes work and then stop.

Disconnect the ground wire from the frame and clean/sand that spot until you have bare metal, then re-connect the ground wire, Or if you prefer you can re-locate the ground wire's ring terminal to a new bare metal spot. If you drill be careful to avoid any other components.

Since your trailer has a 6-way connector you may be using an adapter to mate it to your Highlander's wiring. If so, make sure all the contact surfaces on the adapter are clean and free of any discoloration, which could indicate corrosion. Try cleaning the contact surfaces with fine sand paper and/or electrical contact cleaner. Use dielectric grease # LT37534 to protect them from water.

If you have a circuit tester like # PTW2993 you can test the vehicle wiring with the trailer disconnected. Have a helper activate each lighting function while you apply the tester to the appropriate pin on the vehicle's trailer. Please refer to the linked articles for help on which pins to test and to the linked video to see how to use the tester.

expert reply by: Adam R