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Troubleshooting Trailer Tail Lights That Do Not Work After Installing Junction Box and New Lights

Question:

I have a 2006 Silverado 2500HD with OEM tow package and a 92 utility trailer dual axel and electric brakes. I have been having issues with the tail/marker lights working, I just added a junction box and LED lights and am still having the same issue. My trailer has a 7 way female plug but is basically a 4 way with the electric brakes in the same harness. Is there a special way this need to be wired in and do you have a diagram for the correct way it is to be hooked up Thanks.

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Expert Reply:

Whether the trailer wiring is the 4-pole flat format like # A35W42B or the 7-way format like # H20046 the tail and clearance light signals for the trailer are carried on one circuit, which means one specific wire and one specific pin. In 4-pole wiring that signal is carried on the brown wire, which is next to the white ground wire. In 7-way trailer wiring the running light signal is carried on the brown wire that connects to the pin at the 1-o-clock position as you look at the plug's contacts (see photo please).

You can refer to the linked article on trailer wiring for more photos and information to help with how these various wiring formats are configured.

I suggest you do an easy test on the trailer's running light circuit. You can use your vehicle's 12V battery or a spare if like me you have one laying around the garage. Just be sure the battery is well charged.

Disconnect the trailer from the vehicle. Connect the battery negative terminal to the ground pin on the trailer plug and connect the positive terminal to the pin for the running light circuit. If the lights all work then you know the issue is somewhere in the vehicle wiring.

If the trailer lights still do not work then check each light for a good ground. Weak grounds account for the majority of the lighting issues people find with their trailers. If the light uses its metal mounting stud to make the ground connection then make sure the mounting point is sanded clean so that there is no paint, primer, rust or dirt interfering with the connection. If the light uses a separate ground wire make sure this wire's ring terminal is firmly attached to a clean bare metal spot on the trailer frame. Again, this spot may need to be sanded clean to expose bare metal.

Once you get your connections made and the lights work you can apply dielectric grease like # 11755 to protect those connections from moisture that can lead to corrosion and issues.

In your junction box be sure to use ring terminals on all of the wires, items such as # DW05702-1. Bare wire will not give you a reliable connection inside the junction box.

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