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Troubleshooting Damaged Trailer Wiring Harness


1994 gmc sonoma. Wires got cut some not all there. Connected all wires together by color all but license plate light no wire anymore everything worked for 2 days. Now no more tail lights and brown wire get hot and smokes when connected and blows fuse. What have i done wrong? Please help. Thank you.


Expert Reply:

Sometimes wiring colors for trailer lighting functions will match up between the truck and trailer but color alone is not a reliable way to identify what a wire does. The best way to confirm the function of each wire on your truck and trailer is to use a circuit tester like # PTW2992 to test each one while a helper activates the truck's various lighting functions. The video linked at right shows how to use the tester.

It sounds like you may have connected multiple lighting circuits to your brown wire. This could lead to an overload of that circuit (often the running light circuit is carried on the brown wire) and that could be the cause for the hot smoking wire and blown fuses.

Start by applying your circuit tester at your truck's wiring, one circuit at a time, while your helper activates the turn signals, brakes and headlights. This process will tell you for sure what function each wire serves. Then apply 12V power to your trailer's connector pins, one circuit at a time, to see what each trailer wire does. You can use a spare 12V battery or the truck's battery to do this. Once you know the functions for all wires you can re-connect them using butt connectors like # DW05745-10.

If you prefer you can simply replace your 4-pole wiring harness with a new one. We offer the T-One Wiring Harness # HM41115 from Hopkins. This simply plugs in to your truck's existing wiring without need for cutting or splicing wires. A video on this harness is linked for you as well.

I also included two articles on trailer wiring and troubleshooting. These will give you additional help in testing and repairing your wiring.

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