7 pin(semi) plug to 7 blade utility trailer

No the pins do not match up.

You would need to install an electric brake controller and wire in a seporate receptacle for the RV style plug. The other option, if you are never going to pull the trailer with any other vehicle is to change the trailer cord end to a round 7 pin and wire your electric brakes to the center pin. This will pose a problem if your heavy trailer is ABS equipped and you plan to switch back and forth regularly. Even though you may feel your big truck has more than enough braking power to stop the lighter trailer, electric trailer brakes will still need to function when and if DOT goes to check them.

 

Weight carrying capacity of the axle/tires has nothing to do with the trailer brakes being connected to the towing vehicle and functioning.

What am I missing?

 

Nothing, I'm just guessing here, but I don't think he wants to buy, install, & wire an EBC to the truck; however, you're spot on, it is an OOS when inspected roadside, even worse if you have an accident, then the fun begins, roll the dice I guess...

 

You might also need a tail light converter for your truck. Most of the bumper pull and smaller trailers are combined systems, while class 8 trucks are separate systems. If you just match up the wires to the plug, your brake lights may not work.

 

Just wire the tail - turn - stop lamps to the tail and turn signal wires on the semi. They will work fine. Disreguard the brake light wire used for a semi.
If he swaps back and forth with trailer sizes just install two plugs on the truck.

 

Is this a one time thing or permanent?
If it's permanent then there is an over the counter solution for the brakes. Will still have to wire up a pigtail though.

https://www.etrailer.com/Trailer-Brake-Controller/Hayes/HA100400C.html

 

if you have no electric brakes hooked up on trailer, the truck will try to stop faster than trailer and possibly cause a Jack knife. Especially if on bad road surfaces or heavy rain and ice. Trailer starts swaying etc

 

Check out e-trailer. I believe they have a converter to go from 7 pin to 7 blade. But like stated above, having no brakes regardless of how light it is behind you is a major problem.