The Trouble With Trailer Lights

"Don't you think we should test the trailer lights?"

It was a simple request and the only words his wife had spoken during the entire transaction, the culmination of a week's long-distance effort to advertise and sell my trailered boat. I had traded notarized documents for several thousands of dollars in cash to a first-time boat owner who had traveled with his wife and daughter from several states away to pick up the family's new boat.

The eager buyer had the trailer hitched to his late-model SUV, which had just been professionally fitted with a brand new hitch and wiring harness for the day-long tow back to his home waters. We had super-snugged the boat down to the trailer using transom tie-downs, an over-the gunwale strap, and double-checked the winch pin and bow-eye turnbuckle. Everything was a "go." I was ready to head out for my own weekend on the water and it was all over but the final handshake when his better half issued her logical, if tardy, inquiry.

With one test flick of the SUV's light controls, two families' weekend plans suffered setbacks. Try as we might, none of the trailer lights — which I had tested and found to be in perfect working order the day prior — would work. The four-prong flat fittings between trailer and tow vehicle were a match. His wiring rig was new, mine time-tested and recently proven. After checking grounds and bulbs and wiring on the trailer, I pulled my vehicle up and hooked into the system. This has always been a good way to begin troubleshooting trailer light problems — determine if the problem is with the trailer or with the truck.

All the lights worked.

I breathed a sigh of relief as the fold of cash settled a little deeper in the pocket of my cargo shorts, even as my buyer's eyes flashed a panicked look.

I calmed him down with words of encouragement and learned that he had employed a national trailer rental chain to install the hitch and wiring on his vehicle. We located a local franchise nearby that agreed to give it a look and he was on his way. An angst-filled hour later than anticipated, we parted ways, following our respective weekend plans.

I got the call later that day that all was well and the family was making good headway in the tow back home.

"It was a bad ground in the vehicle," he answered when I asked what the problem had been."Only took a second to reattach it." So ended yet another instance of TLGB, Trailer Lights Gone Bad, with the common culprit being, again, a faulty ground.

The most important part of troubleshooting a trailer lighting issue is giving the rig a simple, but thorough, visual inspection. That involves checking the vehicle-to-trailer connection and all ground connections, which are usually made with a white wire, to make sure they are clean and have direct, metal-to-metal contact with the trailer. With all of the grounds checked, look to see that the wiring on the trailer is physically intact, checking for any obvious breaks or nicks in the wiring, and repair as needed.

Next, examine all bulbs and light fixtures up close for discoloration or broken filaments. Look for any water that might be trapped inside the light fixtures. When in doubt, fix the connection or replace the bulb or fixture, and keep replacements on hand and in the vehicle.

Once on the road, check the lights each time you stop for fuel or to stretch your legs. Ditto your trailer's hubs, tires, and the straps that secure the boat to the trailer. If you find one light is out, it's likely a faulty bulb, so replace it. If all the lights stop working at once, you need to check the ground and the connection to the tow vehicle. If you still can't fix the lighting problem, it's time to get to a shop or call in the road assistance professionals. Day or night, you do not want to risk the dangers — to you and others — of towing an unlighted boat trailer.

Grounds For A New Connection

As in my recent case, the most common problem with trailer wiring is a faulty ground connection, either on the vehicle or at the trailer. To eliminate the possibility of a false or faulty ground from the tow vehicle, don't connect the trailer hitch to the tow vehicle ball, or hook up the safety chains or cables, which can provide false grounds to the trailer. The only connection between the trailer and tow vehicle should be the jack-and-plug connection. In addition to having no lights at all, symptoms of bad ground connections can include the turn signal lights on the wrong side of the trailer coming on, one side of trailer lights being brighter, or all lights being dim. That is a common problem on trailers equipped with bulbs that have two filaments.

One of those filaments is for the running lights, while the other is for the turn/brake lights. Each filament has a wire going to it, but they share the same return or ground via the light case connected to the trailer's frame. When properly wired and grounded, the electrical current flows from the positive terminal of the battery, through the wire to the filament, through the filament, through the base, through the ground, through the vehicle frame back to the battery's negative terminal. When there is a false or faulty ground, the flow can go back from the base through the other filament and follow the wiring to the bulb on the other side of the trailer. There the current can go through that filament and base, following its ground through the trailer and vehicle frame back to the battery's negative terminal. When this happens, none of the filaments receive the proper voltage and will appear dimmer, or two of the filaments on one side could be brighter, depending on what function is sending the current. Again, the fix is a proper ground.

The Case For LED

Many boat trailers now come equipped with LED lights, and there are several companies that offer kits to allow you to upgrade to LED lighting. Other than the initial cost of LEDs, which is higher than that of traditional incandescent bulbs, there are few downsides to the new technology, especially for boat trailer applications.

An LED is a solid-state, semiconductor device that has been specially designed to emit light. LED stands for light-emitting diode, a tiny diode that generates light when a small electric current is applied. Unlike traditional, incandescent lights that use bulbs, LED lamps burn cooler and don't use delicate wire filaments that stretch, weaken, and eventually fail. This feature means LED lamps last longer because they withstand road vibration and shock much more effectively than bulbs. In fact, studies show that the life expectancy of an LED is 100,000 hours — or six times that of an average incandescent bulb.

LED trailer tail lights are brighter, which means other vehicles will see your brake lights better in the daytime. But make sure you purchase LED trailer lights that are waterproof because there are similar lights designed for cargo trailers, and that's not what you need. And because these lights use a fraction of the energy of regular bulbs, they're a smaller pull against your battery, which means you'll also improve your battery life over the long haul.

LED units also illuminate faster; when a vehicle's brake lights are applied, LED lights on the trailer respond instantaneously and provide a brighter, more focused illumination. In comparison, incandescent lights take about a quarter of a second to get to 90-percent brightness. A study by the University of Michigan's Transportation Research Institute found that LED lamps respond, on an average, 1/5-of-a-second faster than standard incandescent lamps. Using test results at 65 mph, LEDs improved reaction time and decreased braking distance by 16 feet.

LED bulbs that are designed to fit into traditional incandescent bulb fixtures are available. However, it's not recommended to mix LEDs and incandescent lights on a boat trailer. Incandescent lights draw much more amperage than LEDs do, and if they pull that current through the LED fixtures, the LEDs can burn up. Therefore, it is recommended that all lights on a conversion be replaced with LEDs.