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Chapter 6: Wiring/Electrical

Before you can legally tow your trailer on public roads, you have to have working trailer lights. In fact, this is more important than simply risking a ticket – if your trailer lights don’t work, you’re inviting an accident. Always check your trailer lights before you leave on any towing trip, no matter how short.

Most trailers come with lights and wiring installed for you. If your trailer’s lights and wiring are missing or beyond repair, it’s usually very easy to install replacements. Any reputable camping or trailer center will have kits that use standard wiring connections and standard color-coded wires.

Understanding Trailer Wiring

Trailer wiring harnesses use 4 to 7 wires to control basic lighting and brake functions. The simplest connectors for the smallest trailers use four wires on a flat plug to control tail lights, brake lights, and turn signals. Connectors with 5, 6, and 7 wires add (in order) backup lights, electric brake control, and auxiliary power.

Choosing a Connector

Assuming that your trailer wiring is in order, you also have to connect that trailer wiring to your tow vehicle. Most trucks, vans, SUVs and RVs made since the mid-1990s include factory-installed trailer lighting connectors and even ready-made plugs in the cabin to connect a trailer brake controller. If your vehicle is so equipped, setting up your trailer electronics will be easy. However, if your vehicle is not prepared for towing, you will need to install a wiring connector. With some planning and research, you can safely splice trailer wiring connectors into your vehicle’s wiring system, or take your vehicle to a professional camping or trailer shop and have the wiring installed there.

Figure 6-2: Trailer Wiring Connector Styles (click above image to enlarge)

Most lighting kits use the common 4-way flat connector with 3 male and 1 female end. The connector is designed this way so that you cannot connect the plug backwards. This connector is mirrored on the vehicle side. Be sure to connect the white wire from the plug to a good ground on your trailer chassis. Then you can connect the ground wires from each light to the chassis for a solid ground path.

If your trailer includes backup lights, you can still use a standard wiring kit, but most vehicles that are wired for towing do not include a 5-way flat connection. You may need an adapter, or to install extra wiring in your vehicle to support the extra light. 5-way flat and 4-way, 6-way, and 7-way round pin connectors are comparatively rare compared to the 4-way flat and 7-way RV blade style connectors.

If you have a truck, RV or SUV made since the mid-1990s, chances are it was made with a 4-way flat connector, a 7-way RV blade style connector, or possibly both. In general, if your trailer uses electric brakes, you should install the 7-way RV blade connection whether or not you plan to use backup lights or auxiliary power. If your trailer does not use electric brakes, chances are you’ll be fine with the 4-way flat connection.

If you do need to connect all 7 wires, the best solution is to use a sealed, weatherproof wiring box, available at any camping or trailer center, and bring your lighting wires, your brake wires, and your auxiliary wires (if any) together in the box. Then run a harness to your connector from the box. The box may be located anywhere on the trailer, but place it so that it’s safe from road hazards and from weather. As with any wiring, make sure your connections are solid and your wires are protected.

If you doubt your ability to do a good job on trailer wiring, take your trailer to a professional. A rewiring job doesn’t take long or cost that much.

Locating Factory-Provided Hookups on Your Vehicle

If your vehicle came from the factory with a towing or camping package, chances are that the factory also installed a 4-way flat or 7-way RV blade style car-side connector. The 7-way connectors are always in the vicinity of the center of your rear bumper, mounted on a tab or mounted into the bumper itself. Sometimes, 4-way flat connectors are tucked up under the rear of the vehicle. Your vehicle owner’s manual should tell you how to determine if a connector is already installed. Figures 6-3, 6-4, and 6-5 show you representative examples of these parts.

Figure 6-3: 7-way vehicle-side connector

Figure 6-4: 7-way RV blade style trailer-side connector

Figure 6-5: 4-way flat wiring with both connectors

If your vehicle does not have a connector installed, it may have wiring plugs in place to help you install trailer wiring. Figure 6-3 shows common locations for wiring plugs in different types of vehicles. You should talk to your dealer, read your owner’s manual, and check your vehicle thoroughly before you cut or tap into any wires on your vehicle.

(click image to enlarge)

Figure 6-6: Trailer Wiring Hookup Locations

Make sure you are absolutely certain that no provisions have been made for you before you think about tapping into your wiring. Even the best taps weaken your wires and cut through the factory-provided wire sheath. If you can avoid taps, you’re better off.

Installing Trailer Wiring Connections

If you have an older vehicle or one that was not designed for easy towing, you may need to tap into your existing vehicle wiring harness to install a towing connector. Luckily, most of the wiring you need connects to your vehicle’s tail lights. The exception is the wiring for a brake controller, but if your vehicle was not made with a towing package, you should take special care before attempting to tow a trailer large enough to require brakes and a controller.

You might need to remove your vehicle’s tail lights to access the wires you need, and those wires may not correspond to the trailer wiring color scheme shown above. The best practice is to obtain a schematic drawing of your vehicle’s wiring, but you can also determine the wiring plan for yourself by turning on each light (tail light, right turn signal, left turn signal, brakes) in turn and using a voltmeter to note which wires receive current.

Label your wires clearly as you learn which controls what function – use a tab of masking tape and a ball point pen – and leave the labels on the wires. You’ll thank yourself in a year or two when you come back to check your connections or fix a problem.

Figure 6-7: Wiring Taps

When you have labeled your wires, you can use taps like the ones shown in Figure 6-7 to add an additional wire to any circuit. Place your trailer connector wire in the tap, and then position the tap around your vehicle’s corresponding wire. Use a pair of pliers to fold over the clamp and the metal blade slides over the existing and the new wire. The blade cuts the plastic insulating sheath around the wires and makes a new connection. The clamp snaps into place and holds the tap on the wire. Run your new wire to your vehicle-side connector.

As you install the taps, remember that the brake lights on your trailer are a combination of both the right and left turn signal lights. You need to tap into just three wires to install a 4-way flat connector: right and left turn signals, and one tail light wire. Connect your ground wire on the vehicle side to a good ground on the vehicle’s body or chassis.

Bad ground connections are the most common cause of trailer wiring problems. A bad ground can show up as an overall lighting failure even when the voltmeter says you’ve got current. Worse, a bad ground can create an intermittent failure, causing your lights to flicker as you drive down the road. Pick an existing factory ground connection (where other ground wires are connected) or make sure your connection is solidly into the vehicle’s chassis.

If you need to install a brake controller, see Chapter 7 for information on those products.

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