How Trailer Wiring Works

Trailer Plugs and Sockets

When it comes to trailer wiring and harness plugs, there are several to choose from in a wide variety of sizes and configurations. The standard trailer plug and socket design is a color coded system using four wires. In this case, the brown wire is used for the tail lights, the license plate light, and the side marker lights. The yellow wire gives power to the left-hand brake light and the left-hand turn signal. There's a green wire that controls the right-hand brake and light and the right-hand turn signal. The last wire of a standard four-wire system is white -- it's used as the ground wire. Most boat trailers and small utility trailers that don't have their own brakes use this kind of harness plug, often referred to as a "flat-four."

When researching what kind of trailer plug and socket system to go with, you may hear them referred to as having a certain amount of "poles." This term is synonymous with the number of wires. Following are some of the other wiring systems you may want to consider, depending on your needs:

Five-wire systems are the same as the four-wire system, right down to the wire color and corresponding function. The one difference is the additional wire. The fifth wire for these units is blue and is typically used to control things like hydraulic disc brakes or additional auxiliary outlets. This can include interior trailer lights or extra side lights.

Six-wire systems are also the same as the four-wire models but they have, you guessed it, two extra wires. In this case there's the additional blue wire that would serve the same purpose as the one for the five-wire system. There's also a red wire that acts as a 12-volt feed. This can operate everything from a cigarette lighter to a car alarm system.

The seven-wire system corresponds exactly to the six-wire systems plus one bonus pole that's generally used to power interior and exterior lighting. You'll often use a seven-wire setup for a camper trailer, RV or cargo trailer.

As with any kind of auto wiring, trailer plugs and sockets can be a little confusing for the novice trailer tower. Before you go and plunk down some hard-earned cash for a wiring harness, talk to an expert. Your mechanic or your local auto parts store employee may be able to give you the advice you need. It also may be a good idea to get some information from some wiring manufacturer Web sites before you start asking questions.

For more information on towing your boat and whatever else you'd like to link to your vehicle, visit the links below.

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More Great Links


  • "How do I troubleshoot trailer wiring problems?" 2008.
  • "How Plug-In Simple! Trailer Wiring Works" 2008.
  • "How to wire a car for trailer lights.", 2008.
  • "Trailer Wiring Diagrams." 2008.
  • "Trailer Wiring for the Do-It-Yourselfer." 2008.
  • Allen, Mike. "Saturday Mechanic: Wiring Your Trailer Hitch." February 2004.

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