Troubleshooting Left Stop/Turn Signals on Trailer Towed by 2007 GMC Acadia  

Question:

Expert Reply:

You have already started the troubleshooting process to find the cause for your trailer's left-side turn and brake lights not working. Since another vehicle has no issue with this trailer this does tend to suggest that the issue is on the Acadia.

Since both 4-pole and 7-way trailer wiring setups combined the left stop and turn signals onto one wire, typically a yellow one, what I suggest is that you start a close inspection of this circuit/wire at the trailer connector itself. You can refer to the linked article on trailer wiring that shows you which pins carry the left stop/turn signals. Since the Acadia's own lights work this means you do not have a bad fuse.

Look for even slight greenish or whitish discoloration on the metal contact surface; this indicates oxidation/corrosion that can be enough to interfere with signal transmission. If you spot any such issue you can apply a spray-on electrical contact cleaner like Caig De-Oxit or a similar product made for electrical connections; this may remove enough of the corrosion to reactivate the circuit.
Even a chunk of foreign material, dirt or trapped water could interfere with the signal.

If the connector is okay then follow the wire forward from the trailer connector looking for signs of worn or damaged insulation, especially at spots where the wire has to make sharp bends and where it may contact the steel trailer frame. Wiring insulation can be damaged by long-term vibration and friction on the trailer frame.

If your wiring harness is original factory equipment then after 10 years it might just be toast. For a factory tow package replacement 4-pole use # 118384. For a factory 7-way replacement use # 30955. If your harness is an aftermarket 4-pole you can replace it with the easy-install plug-and-play # 118451. All of these use a standard round U.S. Car type connector to join the vehicle's wiring.

expert reply by: Adam R

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