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Right Brake Light on 4-Pole Trailer Connector Not Working on 2004 GMC Sierra

Question:

I have verified that I do not have voltage on the right brake light terminal in the truck provided plug. The left works find and the right turn signal works fine which tells me the problem is before the fuse panel in the truck! I am not sure where the wiring problem could be, the fuses for the trailer turn and brake light is good. The truck brake light works fine.

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Expert Reply:

If your 2004 GMC Sierra uses a 4-pole flat trailer connector like the Tow Ready # 118384 that you referenced the right turn and right brake light signals both will be carried on the green wire. Please refer to the linked photo.

The most common cause for one light function (the right turn light) to work while another carried on the same wire (the right brake light) does NOT work is a bad ground connection. Turn signals do not pull as much current as a brake light signal, since turn signals blink on and off while a brake light remains on as long as the pedal is pressed. If the trailer connector's white ground wire is not firmly affixed to a clean grease-free bare metal surface it may be able to pass enough current for a blinking turn signal but not for a steady brake light signal.

I recommend you trace the white ground wire back to the point where it attaches to the truck chassis and thoroughly sand and clean that spot, or you can relocate the ground wire to a new clean bare metal surface. Be sure to sand off any paint or primer. You can also try installing a ground jumper wire spliced into the existing white ground wire, that you can then attach to the chassis, or to the battery negative terminal, to establish a better ground. Since you have already checked your fuses and have confirmed they are all okay, then this is the most likely cause for the issue.

You can also try cleaning the contacts on the 4-pole in case there is a corrosion issue preventing full power from passing through. Also check your trailer connector in the same way. You can also unplug the round connector under the vehicle where the 4-pole connects and check those contacts for corrosion, dirt or moisture.

If you are unable to get the factory harness to work then you have the option of installing a new 4-pole wiring harness like Curt # C55515 which will plug in under the vehicle to the round U.S. car connector behind the rear bumper.

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