LET THERE BE LIGHT
J. Tiger 12-02-06
Everyone who has towed has a trailer light story! Electrical problems are rampant on towed trailers, particularly those that are used hard. Begin by checking the lights for proper operation. If there's a malfunctioning or erratic bulb, the time to correct it is now — not on the side of the road on the way. Once all the lights (brake, turn signal and parking) are working correctly, check the connectors at the tow vehicle for corrosion, looseness, or broken, bare or exposed wires. For those in colder climates, check the connectors for corrosion carefully; winter takes a heavy toll on wiring connectors. Electrical tape-wrapped wiring harnesses retain water and corrode from the inside out. Coat all connectors with dielectric grease to keep out grime, protect from corrosion, and maintain a good connection. Check all light lenses carefully, as broken light covers invite moisture and corrosion. A trailer light circuit tester is a handy tool to carry with you, so that if there's a problem, you can check the tow vehicle wiring and 4-flat plug first.
Speaking of the tow vehicle, the plug and wiring leading from it to the trailer must match that of the trailer in order for the lights to work. All trailers have basically the same wiring system that uses four wires to get all the jobs done. The color code is as follows:
Green: Right turn signal and brake light
Yellow: Left turn signal and brake light
Brown: Running (parking) lights
White: Ground
Notice that the turn signals and brake lights operate using the same wire. That's because they use the same bulb in the trailer light housings. One bulb performs both brake and turn signal functions.
However, many of today's tow vehicles have taillight bulbs that only perform one of those two functions. In other words, they have one bulb for turn signals, and one bulb for brake lights. If your tow vehicle is like this, you will need a taillight converter with 4-flat plug Hidden Hitch part #31823. The converter simply combines the two-bulb system into one that works with a single-bulb system. The color coding of the wires is the same as noted above, except a fifth wire is added: a red one, to control the brake lights. So on the tow vehicle, the white is the ground; brown is connected to the running lights; green goes to the right turn signal, and yellow goes to the left turn signal. The red wire gets connected to the brake light signal (for both left and right sides).
BE CAREFUL if you have a newer tow vehicle and you are wiring it for towing! Many newer (usually more expensive) vehicles have very sensitive wiring systems and need a special powered taillight converter. Powered converters have another, thicker wire that gets connected directly to the tow vehicle's battery with a fuse installed to protect the converter. This extra wire is connected to reduce the additional load that the trailer lights place on the tow vehicle's wiring system, so that failures don't happen.
If you have doubts about hooking up your tow vehicle wiring, consult a good hitch installer for his advice!
Remember what your Dad probably (hopefully!) taught you about trailer wiring; if there's a problem, look at the ground circuit first. Don't rely, as many do, on the hitch ball and coupler connection for the trailer's ground. Wire a separate ground circuit and make sure it's connected cleanly and securely to the trailer frame. The ground is usually (and should be) a white wire and it must be connected to the frame of the trailer. At the coupler, the ground wire should be connected to one of the bolts that hold the coupler in place. In addition, that wire (as well as the rest of the wires leading out to the 4-flat plug that connects to your tow vehicle's plug) should be supported and protected so you won't rip, chafe or tear them while moving the trailer or towing it around corners. There should be enough wire to reach the tow vehicle plug and have enough slack for turning, but not so much slack that the wire drags on the ground.
If you have a new trailer (especially trailers that must be assembled at home) and the lights won't work, check the ground first. Often, new trailers have a lot of paint build-up on the steel frame that prohibits the electrical ground from working properly. The lights often use one of their mounting bolts as the ground "stud". If the ground stud can't contact the trailer frame directly because paint buildup is keeping them from touching, the ground (and therefore the lights) won't work. You may have to clean and scrape the paint from these areas to get a good ground contact.
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