Having several trailers and often ending up helping on others I bought a 7-way flat blade plug and socket. I made them up with 18" wires (colored to match the Traditional wiring) and put a label with the function on each wire, so the wire colors really don't matter. These test connectors make it easier to wire and trouble shoot alone. Wiring or checking a trailer, I plug the trailer cord into the test socket. Hook the ground wire of the socket to a battery (vehicle not required) negative post with a jumper and use another jumper (with a 20-amp fuse in it) from the positive post to the function I want to check the light on. And that function will have power on it if I need to find a bad wire in that circuit. The plug works to check that the vehicle socket is wired, and functions are working correctly. Using the ground wire out of the plug while testing the vehicle functions, confirms the ground to the vehicle is intact.
You can buy ready-made test plugs but having leads allows doing other checks. Using a separate battery eliminates running the vehicle battery down, unless it is kept running. Labelling the leads eliminates having to look back at what function a certain color wire is supposed to be. It cost a few bucks and some time, but I figure I had my money back in the time saving on the first truck and trailer's wiring I straightened out. I also have them for 7 round pin connectors. They do make test boxes to do this as well, but they cost even more. YMMV, works for me.
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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