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Troubleshooting 4-Pole Trailer Wiring to Correct for Dim Lights on Left Side of Trailer

Question:

After having problems with the left side of my trailer lights the right side seems to work fine, where the left turn and tail light are very dim, I have exposed the wiring from the connector back towards the trailer lights. When checking the exposed wires with an ohmmeter, Im getting connectivity shorting when I touch any of the four wires together. This doesnt seem normal to me. Am I missing something here? Thank you for your attention to this.

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Expert Reply:

In a 4-pole wiring harness the brake and turn signals are combined into one wire, the yellow wire for the left (driver's) side and the green wire for the right (passenger's) side. Running lights, or taillights, are carried on the brown wire for both sides of the vehicle and trailer.

Since your lights do all work, just some of them illuminate dimly, chances are that the issue is localized at those specific lights. More often than not, the type of issue you describe is a result of a weak ground connection. Lights can ground via a separate ground wire, or they can be designed to use their mounting hardware (that touches the trailer's steel frame) to establish the ground. Either way rust, road grime or even paint on the trailer frame can interfere with that connection.

Ideally your ground wire (or grounding stud) will make bare metal to metal contact. If your lights use a white ground wire with a ring terminal, make sure this is securely affixed to a clean bare metal spot on the frame. In the same way, if your lights ground via the mounting stud make sure that stud is making direct metal-to-metal contact with the trailer frame. Either way you can try sanding off any paint or primer on that spot, and/or clean off any grease or rust, to ensure a good electrical connection.

You can also check the main white ground wire on the trailer's harness to ensure that it too is attached to a clean bare metal spot on the frame.

The linked article on troubleshooting 4-pole trailer wiring may be value.

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