YOUR TRAILER'S LIGHT SYSTEM
By Tom Burden, Last updated: 9/29/2019
Trailer lighting keeps your trailer legal with your state’s Motor Vehicle department. More importantly, lights are critical for safety, letting other drivers know where your trailer is and what your intentions are. In darkness and poor visibility, a functioning set of trailer lights is your best insurance against collision (especially rear-enders) and severe damage to your boat. Don’t skimp on the quality of trailer lights. Consider them essential boating safety gear for being on the road.
How Your Trailer Lights Work
Trailer wiring consists of a plug connected to the tow vehicle’s lighting circuitry, a matching connector on the trailer, a wiring harness that runs the length of the trailer frame and a variety of stop, tail, turn signal and side marker lights around the trailer’s perimeter.
Multi-function lights may combine as many as seven functions into one compact fixture, including both lighting functions and reflectors. This makes mounting lights and wiring them substantially easier.
Shining In a Rough, Tough Environment
Trailer lights get the stuffing beat out of them on a regular basis due to:
Suffering thermal shock when warm or hot lights are suddenly submerged in cold water. Submerging the lights cause short circuits, corrosion of the connections and sockets, burned out filaments and cracks in plastic lenses. Corrosion is especially severe in saltwater.
Getting covered and encrusted with road grime, which works its corrosive mischief if it gets into the connections.
Enduring hours of damaging highway vibration that can weaken and break incandescent bulb filaments and loosen lens seals.
A drop in voltage, caused by high current draw from several lights, from the towing vehicle all the way to the taillights of the trailer.
Protruding light covers that attract hard objects such as fences, high curbs or lampposts while backing up.
Matching an Existing Set of Lights
If you’re stuck with lights that don’t illuminate, perhaps a quick fix is all that you need. Before buying a set of replacement lights, clean the mounting bolts on the existing lights with a wire brush. Many lights ground electrically to the trailer frame through the mounting hardware, and an improper or “bad” ground is a common cause for lights not working.
If you have chronic trailer light problems and your current lights look pretty beat up, we suggest (not the least bit self-servingly) that you replace the entire mess. A new light kit with wiring harness can cost as little as about $25, and can be installed in an hour or so. It may be difficult to run wires through a box section trailer frame, so you may want to pull a messenger through before yanking the old wires completely out. Cut the trailer plug from the old harness and tie the end of the new harness to the old. Then carefully pull the old harness out the rear of the trailer.
Wiring Color-codes and Converters
Basic color-coding is simple (as shown in the Wiring Harness Color & Function Chart). The white wire (paradoxically in our opinion) is the ground wire, and even though the trailer hitch acts as a ground, you should connect the white wire to the vehicle ground and the trailer frame. The brown wire is for taillights and runs to both the red lights in the back, and to the clearance and identification lights. The green wire (think starboard) is for the right turn indicator, and the yellow wire is for the left turn indicator.
Don’t use household “wire nuts” on your trailer. Make the connections to your lights with waterproof adhesive-lined butt connectors and keep corrosion out of the wiring. Use a quality wiring tool like the Ancor Stainless Steel Wire Cutter Stripper Crimper.
European, Japanese and some American vehicles use separate circuits for turning and for the brake lights. If your vehicle uses amber rear turn indicators, or uses a different area of its light fixture for turning and braking, you’ll need a five wire-to-four wire converter. These are relatively cheap and can be made a permanent part of the tow vehicle wiring harness.
Advantages of LED Trailer Lights
LEDs address many deficiencies of conventional trailer lights, which burn out from vibration and cold water, and use more electricity. LED advantages include:
Higher life expectancy: LED lights are rated at up to 100,000 hours of service life compared to 3,000 hours for incandescent lights, plus LED lights have no filaments to break due to vibration. This virtually guarantees that you will never have to replace a bulb again and the lights will probably last the life of your trailer (except for the occasional navigation error where you back your trailer into a fence...)
LEDs are impervious to submersion and road grime: LED lights are permanently sealed in a welded polycarbonate lens so there are no bulb bases to short or corrode.
No thermal shock: LED lights generate very little heat so that thermal shock due to immersion is not a problem.
Minimal voltage drop: LED lights draw 1/8th the current of comparable incandescent lamps so that the voltage drop is minimal.
Low profile: LED lights are very low profile so that there is less chance of collision damage to a protruding light.
LED lights activate instantly: Drivers behind you have an extra half-second to apply their brakes when you make a quick stop.