Trailers need electrical power to operate their lights and sometimes other systems like electric trailer brakes. This power is supplied by the tow vehicle through a wiring harness.
A trailer wiring harness usually has a standardized plug that connects to a socket at the rear of the vehicle. This plug can have four wires, five, six or even seven, depending on the trailer type and number of electrical systems. There are also different configurations, such as 4-way flat versus 4-way round.
The wiring harness typically courses its way through the trailer frame where the various wires branch off to the taillight lamps, running lights, brakes as well as other electrical systems and trailer lighting.
Even though trailer wiring has become quite standardized, it can be confusing when you start to discover how many different varieties there are. 4-pin flat, 6-wire square, 7-way RV blade – what does it all mean?
First, the number in each configuration corresponds to the number of wires that make up the connection: 4-way has four wires, 5-way has five wires, and so on. These wires are color-coded and are used for specific functions, based on the color: green is for the right turn signal / brake light. Brown is for the taillights. White is the ground connection, etc.
Secondly, terms like way, pin, wire and prong are all synonyms. Simply put, they all mean the same thing. You can call it a 4-way, or you can call it a 4-wire, or you can call it a 4-pin – it’s the same configuration.
One caveat is that blade is more of a reference to the style of the individual contacts. Instead of a pin, each contact point is a flat blade-like piece of metal. On RV blade harnesses, the blades are arranged in a circle, which brings us to the final point.
Lastly, the terms flat, round and square are indicative of how the wires are arranged. In a flat configuration, the wire contacts are all laid out in a single, flat line. In round, the wires are in a circle. You get the idea.
When considering wire harness types and perhaps which one to buy as a replacement, the best course of action is to look at what you already have on your trailer. If it’s a 4-way flat, replace it with another 4-way flat. If it’s a 7-way RV blade, replace it with a 7-way RV blade.
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