In The Dark About Trailer Lights

By Mark Corke

Trailer lights get all but forgotten until you end up in trouble. With a little understanding, you won't be left in the dark.

Trailer lights have a much harder life than those on the tow vehicle; despite getting dunked in water, bounced around on the highway, and generally neglected, they're expected to work year after year with barely a thought. All trailers require lights, though regulations may differ from state to state. For lights to work properly, wiring and overall installation has to be top notch. Many trailer-light components are simply just not designed for the harsh marine environment, and although these same components may fare well in the RV market, it's only a matter of when, not if, they'll expire when mounted on a boat trailer.

Keeping The Lights Alight

All is not lost, however, and there are measures that you can take to keep your trailer lights working properly. Many standard lights have traditional incandescent bulbs, and while these may be perfectly functional, when the trailer is new, over time the seals around the removable lens covers perish, allowing water in. Backing the trailer down the boat ramp, the lights go underwater, and with the seals gone, as soon as water touches the hot bulb, the thermal shock will often cause bulbs to fail. Water also causes severe corrosion, and while a little dielectric grease or spraying with WD-40 might help, it's only a matter of time before rusting components, such as bulb holders and connections, stop the lights from working.

So what can you do to keep your lamps working properly? The first is to prevent your lights from getting wet in the first place. One option is to consider attaching lights to a removable lightbar that can be swiftly detached from the trailer before you back down the ramp. This works for me, and I've had my trailer for more than 10 years, and in all that time, I don't think I've replaced one bulb. A second option is to replace your standard lights with LEDs. Lights are now available in which the whole unit is epoxy encapsulated, or to use the technical term, potted, effectively sealing them for life. Because the price of LED technology is competitive against the cost of traditional bulbs, many trailer manufacturers are starting to install LEDs from the outset, which largely mitigates the problems associated with heat and water.

Wiring Woes

Sub-par trailer wiring is often a failure waiting to happen. Cables get chafed through, connections corrode, and the plug may be a less than stellar fit. Chances are that the wiring and plug will have four to seven wires, depending on the trailer. (See table below for how these are connected.) A four-way connector provides a ground circuit plus three functions (typically left turn/stop, right turn/stop, and taillights). A five-way connector includes all these but adds a blue wire for a brake controller or auxiliary function. A seven-way connector includes all these plus reverse.

Trailer Wiring Color Code ChartClick image to enlarge

Many trailer problems are due to a poor grounding connection, which is the white wire. If the ground is poor, lights may work intermittently or not at all. Ground connections to the trailer frame need to be bright and clean, free of paint and rust, and well secured. All wiring should be of the highest order, which means no wires that are simply twisted together as a "temporary" fix. As much care needs to go into the trailer lighting as on the boat's. That means that connections should be properly crimped (see "Making Perfect Crimped Joints" for how to make proper connections) and covered with heat shrink tubing to keep moisture out. 

— Published: Spring 2016


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