Trailer–Vehicle Connector Wiring
https://continuouswave.com/whaler/reference/7poleTrailerWiring.html
The motivation for this project was some forward thinking that I might implement a new brake system on the boat trailer. The first possible change would be switching to disc brakes from the free-backing drum brakes, which would necessitate adding the signal for the towing vehicle's back up lamps on the vehicle. This added signal to the trailer is generally required when the brakes are operated by a surge coupler; a surge coupler applies braking effort when the trailer is being push backwards, and disc brakes are quite happy to work in the reverse direction, while the existing free-back drum brakes are designed intentionally to not work when the trailer is being moved backwards. This method requires five circuits, and if often implement of a connector called a Flat-5, which adds the vehicle back-up lamps signal to the connector. The signal operates a solenoid that stops the pressure from the brake system from being applied to the brake hoses.
The second step in the plan was to allow for a change to a non-surge coupler and to control the trailer brakes electrically from a brake control in the towing vehicle. The hydraulic disc brakes would remain, but would be controlled by a electrically-operated hydraulic pump on the trailer. For that to work, two more circuits are needed; first, a 12-Volt supply from the towing vehicle is needed to keep the 12-Volt battery on the trailer charged so that it can power the electric motor that creates the hydraulic brake fluid pressure. A second signal is needed to actually run the the motor, allowing the amount of braking power to be controlled. (Note that a surge coupler essentially automatically regulates the brake pressure as the amount of braking is always in proportion to the amount of compression of the surge coupler having a moveable trailer hitch coupler link.) Now the electrical connector is up to 7-poles, and that is what I decided to install in my upgrade of the trailer and truck wiring.
As delivered from the factory, my 1995 GMC Suburban did not have a 7-pole connector at the rear bumper, but it did have all the necessary wiring circuits provided there. Four of the circuits were already being used for the Flat-4 connector, and the other three were found wrapped up and concealed in the frame of the truck. Many modern truck are now providing the 7-pole connector as a standard part of any "towing" package option, or even as standard equipment on many trucks. To fit the 7-pole connector on my truck, I had to buy a suitable receptacle, a mounting bracket, dig out the hidden wiring, and connect the wiring to the connector.
Figuring out the exact color scheme of the GMC truck and the appropriate pin numbers on a 7-pole trailer connector took quite a bit of research. I was lucky in that I had a complete copy of the eight or nine volume factory service manual for the GMC truck, and I could read the automotive style wiring diagrams (with some difficulty) and figure out the wire insulation colors that were used. I wrote the article as a way of archiving the data I found (so I could find it again 16-years later) and possibly helping others. (See below for a funny storage about that outcome.)
In addition to the GMC Suburban towing vehicle, I also had to wire the trailer for the 7-pole connector. I added an aluminum box to the trailer and ran the pre-made rubber-insulation cable from there. Looking back on all this work, I see it was a bit ambitious, and, here I am, 16-years later, and I never got around to making any change to the trailer brakes; I am continuing to use a surge coupler and free-backing drum brakes, inspite of all my "forward thinking" back in 2008. That is how life goes sometimes.
For the past 16-years the new trailer wiring and the revised GMC truck wiring have worked flawlessly, but in Spring 2024 I realized that the bracket that holds the 7-pole receptacle on the bumper of the truck was badly rusted, and the connector was being held in place by only a small sliver of rusty steel.
Because the wiring and the connector in the truck were working just fine, I decided not to tear into them and replace them, so I decided to just replace the rusted and weakened bracket. A new bracket was only $5, and it looked like a simple job.
The first step was getting the bracket off the bumper. I (re-)discovered that I had wisely used all stainless-steel machine screws to mount the bracket, I had drilled and tapped the bumper, and even added stainless-steel elastic-stop nuts as a back-up and to prevent vibration from loosening the machine screws. The only complication was the back side of the chromed bumper had accumulated a layer of rust, and that made getting an 7/32-inch open-end wrench on the elastic stop nuts a bit difficult.
The next complication was the connector mounting bracket fits behind the connector flange, and the connector and wiring bundle are more or less captive in the bracket. The solution was going to be simple: just cut the existing bracket and bend it out of the way; a minute with a Dremel tool and a cut-off disc attachment and the rusted steel was cut open and bent out of the way.
To my very happy surprise, the holes in the new connector mounting bracket aligned perfectly with the holes I drilled in the bumper and the hole layout on the old new bracket also fit perfectly with the old connector. Who knew there was such standardization in 7-pole connectors and their mounting brackets?
The final step was to take the Dremel tool cut-off disc to the new mounting bracket, and remove enough of the bottom part of the bracket that it could slip over the wire bundle. Of course, that exposed some bare steel, so I had to paint that part of the bracket with some Rust-Oleum flat black primer and paint. This added a day's delay in the refurbishment. The reason for doing this was to avoid having to disassemble the existing connector, remove all the wires and their connections to the contacts, then run the wiring though the hole in the bracket, remake all the connections (which would mean carefully following the wire-insulation-color to pin-assignment mapping necessary to get it wired right, then reassembling the connector. The actual connector was working fine, and I did not want to create a new problem by having to tear it apart, redo all the wire-to-contact connections, and put the connector back together. I figured that if was working fine, why invite problems by taking it apart.
- Fig. 1. Modified mounting bracket. A portion of the bracket surrounding the connector hole has been removed to allow the bracket to slip over the existing connector and cables.
- MountingBracketModified.jpg (89.48 KiB) Viewed 13553 times
The next morning I mounted the newly-cut and newly-painted bracket onto the bumper, slid the wire bundle through the gap, and fastened the connector to the bracket. There I again re-used the stainless-steel machine screws and stainless steel elastic stop nuts from 2008, which were all essentially completely free of rust. I do not think the cutting away of part of the bracket where it surrounds the connector has compromised its strength or durability. In manufacturing the bracket, it was probably simpler and better to leave material all around the large circular cut-out during fabrication of the large diameter hole.
- Fig. 2. The new bracket installed with old connector mounted to it. Note the 16-year-old stainless steel machine screws are in great condition. The 16-year-old trailer cable also looks in good condition.
- refurbishedMountOnBumper.jpeg (60.28 KiB) Viewed 13562 times
The total cost of the refurbishment was $5 for the new bracket. The bracket was made by CURT and was their part 58510.
In retrospect, I was surprised the old bracket became so rusted. Its appearance showed it was probably made with plain steel that was dipped in a rubberizing compound. Eventually some water got though the rubber coating and started the steel rusting, and 16-years later the entire bracket was badly thinned rusted steel with a strange rubber jacket surround that was mostly separated from the steel and only loosely held in place.
As part of my installation 16-years ago, I had located the 10-AWG Orange conductor in the engine compartment that was to be used to carry the 12-Volt battery to the trailer connector. (This was also mentioned in my article, as otherwise I would have never remember where to look.) It now occurred to me that there was a possibility that this exposed 12-Volt source might have had some leakage current to ground that promoted some galvanic corrosion of the steel in the mounting bracket (and perhaps even in the bumper) as seen 16-years later. Just in case that had been happening, I got under the hood and found the fuse box in the GMC Suburban, located the 10-AWG orange wire and removed the 30-Ampere fuse that was providing the 12-Volts to that circuit. I made a note about where this fuse came from in the circuit box, and put the fuse and the note in a small plastic bag, and then into the center console storage hatch that is already full of odds and ends related to the vehicle and the trailer. I will have to wait a few more years to see if the new bracket shows any sign of corrosion.
Funny Anecdote
A year or two after I wrote the article (linked above) about trailer connectors, a guy who was working at our plant as a temporary hire was driving an even older GMC Suburban. One day he approaches me and tells me he was trying to find information about wiring a 7-pole connector on his GMC Suburban, and he used GOOGLE to search for information. Then with great surprise, he relates that the first search result he got pointed to the article I wrote, which seemed to astound him that a guy he was working with was the source of such good information on such a specialized topic.