Troubleshooting 7-Way Trailer Wiring Harness Functions Not Matching Wire Colors

Question:

I have a 7 wire harness that includes a purple wire. Our trailer wiring has no purple wire, but the reverse lights worked with the old wiring harness that was on it. The hot is the thicker black wire, but when we tested it we had no power coming from it, even with the ground white wire hooked up. Also, we had no power to anything on the trailer after wiring everything else and connecting the black from the trailer to the black from the truck harness. We had everything connected correctly with green, yellow, brown, blue, and white. Then we found that when we connected the purple from the truck harness to the black from the trailer all the lights where working right, except the reverse light. When we put the truck in reverse all the lights went out. Then we found that the black wire from the trailer was hot even when we disconnected it from the truck. And we realized our trailer was hot too. So we’re trying to figure out why our black from the trailer and the trailer were hot, and why we only got power to the trailer when the purple was connected.

asked by: Lisa

Expert Reply:

Thanks for reaching out, Lisa!

Trailer wiring can be a bit confusing at times but the best piece of advice I can give is for you to ditch the colors of the wiring and focus more on what circuits they are connected to.

For example, the Hopkins 7-Way RV Style Connector with Molded Cable # H20046 uses the following functions for the following wires on the harness:

White Wire - Ground
Red Wire - Left Turn and Brake
Brown Wire - Right Turn and Brake
Green Wire - Tail Lights
Blue Wire - Brake Power
Black Wire - Aux.
Yellow Wire - Reverse Lights

I'm attaching photos of the standard pins for truck/trailer 7-Way connectors for you to reference and compare to what you have. You can use a circuit tester # PTW2993 to verify that your truck 7-Way functions match the diagram I'm sending in.