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Troubleshooting 4-Pole Flat Wiring on Home Made Trailer

Question:

I am wiring a homemade trailer and I spliced the brown wire to have a brown for each side but my right side lights dont work at all. What could it be?

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Expert Reply:

Running dual brown running light wires when re-wiring a trailer is a convenient option that allows you to avoid having to run a running light jumper wire between the two taillight assemblies. We offer these wishbone style harnesses, such as # A35W42B.

First, I suggest you test the vehicle's 4-pole to ensure that the signals for your trailer are present on the vehicle connector. You can use a circuit tester like # PTW2993 for this purpose. This will tell you if the issue is in the vehicle or in the trailer. You can also try testing the trailer lights by applying power directly from a 12-volt battery, without going through the vehicle connector at all.

Also check the condition of both connectors, vehicle and trailer, looking for any signs of corrosion. Even very slight greenish or whitish discoloration on the contact surfaces can indicate corrosion that can interfere with signals.

If the trailer's left-side lighting functions all work properly (stop, turn and running lights) then this tends to eliminate a problem with the ground wire. However, depending on the light fixtures installed, these may require a ground through their mounting studs. If the lights you are using are grounded in this way (not with a separate ground wire) then you can check that the right-side lights' studs are able to make bare metal-to-metal contact with the trailer frame. If they each have a separate ground wire, make sure each one is affixed to a clean bare-metal surface on the trailer frame.

If it is only the right-side running lights that don't work, then focus on the brown wire to ensure it has not been nicked or pinched by suspension components or had its insulation cut by any sharp edges on the trailer frame.

I've linked an article and video that may help your troubleshooting efforts.

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