How Trailer Wiring Works

Trailer Lighting
Even small trailers like this one for towing motorcycles need to be wired to your vehicle.
Even small trailers like this one for towing motorcycles need to be wired to your vehicle.
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If you're going to tow a trailer, you need your trailer lights up and running. Most trailers have three circuits that you'll connect to your vehicle. One operates the taillights, one runs the left brake light and the last one runs the right brake light. When it's time to turn, your brake lights will flash to let everyone know which way you're going. Obviously, trailer lighting is essential to safe towing.

You also need a ground wire. This provides a conducting path which is independent of the normal current-carrying path -- a fancy way of saying that it prevents the system from shorting out due to an electrical surge. So this means your trailer wiring plug will need at least four contacts. The good news is that the standard connector for trailers uses four pins, plus the ground. These plugs are typically flat, with the pins set in a row. Easy enough, right?

Some of the other common styles of connectors use five, six or seven pins. These plugs are usually round and plug in much like an electrical socket would. But if you only need the brake lights and taillights hooked up, why would you ever need more than four pins? It's because larger trailers will occasionally have a separate circuit for running lights on the sides and front of the trailer. Your extra pins will operate these lights. Some trailers, like ones that carry cars or horses may have interior lights powered by the tow vehicle's battery. This can account for the need for another circuit as well.

If your vehicle is pre-rigged with a four-pin plug and your trailer is ready to go with a four-pin plug, all you need to do is plug it in and test it out by doing the following:

  • Turn on your vehicle lights and make sure the trailer taillights come on.
  • Tap your brakes to make sure the trailer's brake lights work.
  • Test each turn indicator and make sure the trailer complies.

That's all there is to it in the best case scenario. Another situation might find you with a seven-pin connector on your tow vehicle and a four-pin connector on your trailer. This is when you need to adapt -- literally. Go to your local auto parts store and ask the specialist which adapter you need, if you don't feel comfortable picking it out yourself. Chances are it will be as easy as locating something with a label that reads "Seven-pin to four-pin trailer wiring adapter."

­Read on to find out about buying trailer wiring kits whe­n your vehicle isn't pre-rigged.

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