Trailer–Vehicle Connector Wiring

by James W. Hebert

This article gives the color codes normally used with the common flat-4, flat-5 trailer connectors, and the pin designations and suggested color codes used with the 7-pole RV-style (six flat blades surrounding center round) trailer connectors. Many newer vehicles are pre-wired with an RV-style 7-pole connector. As a result, it is becoming increasingly common to find boat trailers wired (either directly or via adaptors) to use a 7-pole RV-style wiring arrangement.

Vehicle wiring color codes are shown for certain GM trucks. When authoritative reference information is located for other vehicle brands and model years, it will be added to the table. The information for the GMC c.1995 vehicles comes directly from the factory shop manual wiring diagrams published by General Motors. The actual wiring harness in my 1995 GMC Suburban matched the information shown in the manual, except that the battery lead insulation was orange instead of red. I was surprised by this discrepancy, as the factory service manual seemed otherwise in perfect agreement with my vehicle's actual wiring. This mismatch, however, shows how difficult it is to proceed with electrical modifications to a vehicle based only on color code. You often have to verify the circuit identification by other means of deduction. In my case, General Motors had attached an identification tape imprinted with the legend "Trailer Battery Feed and Auxiliary Circuit," so there was no doubt about this wire's intended function.

Trailer Wiring For GMC c.1995

ColorPin #FunctionColorAWGColorColor

NOTE: The precise colors employed in various pre-made RV-style connector harnesses may vary. Most important is that the association between pin number and function be strictly maintained. This will allow your connector to inter-operate with other vehicles and trailers. Be careful identifying the pin numbers on the connector, as they are not in order as you work around the circle of contacts.

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Installation Notes

In the process of using this information to install a 7-pole RV style connector on my 1995 GMC Suburban C1500, I discovered several obstacles. At the rear bumper there was a bundle of conductors pre-arranged in a harness, but only five wires were included. The battery feed (orange or red) and the brake circuit (blue) conductors were not in the bundle. On closer inspection, I found these two wires a foot farther back in the wiring harness, taped back and bundled on the chassis frame.

Finding the other end of these two wires was also a bit of a mystery to be solved. The orange conductor showed no indication of any voltage, so I deduced it was not connected to the battery distribution panel. I found the orange wire a foot away from the under-hood fuse and relay center, again taped back in the wiring harness. The blue conductor was there with it. The orange conductor had a ring terminal connector and was ready to be installed. Under the plastic removable cover of the under-hood fuse and relay center there are two 30-ampere fused circuits which feed two threaded studs (M8-1.25-pitch metric) marked "A" and "B." The vehicle schematic shows that the trailer feed should be connected to the "B" stud. If your nut and bolt drawer is like mine, you probably do not have any M8-1.25-pitch hex nuts in the box; a trip to the hardware store solved that. I also recommend getting a external star washer. Finding one in a metric size will be hard, but 5/16-inch will work. Remove the 30-A Maxi-fuses for both the A and B studs when you are connecting the orange wire to them. This will remove the voltage which could otherwise easily be shorted to ground by your wrench as you tighten the hex nut. There is not a lot of room to work around the auxiliary power stud.

The blue conductor is intended to be used with a trailer brake electric controller, but since these devices are usually in the passenger compartment, it will have to be routed through the firewall. I could not find any existing holes or grommets which looked useful for passing this wire, so I drilled my own. I located the hole just outboard and below the area of the firewall where the steering shaft comes through. The firewall body there is flat, can be easily accessed from both sides, and due to its shape and profile is easily visualized from either side. Just to be safe I first drilled a small exploratory hole. If a small hole proves to be poorly located it can always be filled with a little self-tapping screw. My hole location turned out to be fine. I enlarged the hole to 9/32-inch and installed a tight-fitting rubber grommet to protect the wire and to seal the hole.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

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Copyright © 2008 by James W. Hebert. Unauthorized reproduction prohibited!

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Author: James W. Hebert
This article first appeared May 17, 2008.