Trailer Wiring
 

Question for trailering experts. My "new" '72 Sierra Grande has factory-installed trailer wiring. The rear plug has the 7 round pins and I need to connect it to a 4-pin trailer connector. No problem, it would seem - these adapters are sold everywhere (as in Tractor Supply). The problem comes in because one of the round pins in the truck's plug (the top pin) is much larger than the others and will not go into the adapters. What's up with that big pin? Power for the trailer brakes? Is there an adapter that will fit or do I need to replace the truck's plug assembly?

Re: Trailer Wiring
 

There are two styles of 7 pin plugs that I know of. One is a "blade" style that is commonly called an RV plug. It is what is we are used to seeing on utility and car trailers.

The other style is a round pin, commercial/utility plug. The best place to find an adapter for that style would be a truck stop or Truck-Pro. I don't know that i have ever seen a 7 round pin to flat four adapter. You may have to use two adapters. One to go from 7 round to 6 round and another to go from 6 round to 4 flat.

If you don't need that round pin plug, I would replace it with the more common 7 pin blade style.

Re: Trailer Wiring
 

Too bad your not closer. I have both style adapters in my store

Re: Trailer Wiring
 

I don't think I was clear in my post. The truck has a "standard" 7-pin male plug with round pins. I'm converting to a 4-pin female plug with round pins. These adapters can be bought anywhere for $10. The issue is that ONE pin in the truck's plug is oversize and doesn't squeeze into a standard adapter. I'm assuming this is the power feeder for brakes but that's not important.

Re: Trailer Wiring
 

I've answered my own question - etrailer.com shows the type of plug GMC was using in 1972. The oversize pin is the ground connection, usually a white wire. I should be able to locate an adapter using that information.