The Trouble With Trailer Lights

By Dan Armitage

One suspect connection can ruin your whole weekend

Photo of a trailer on a foggy road with lights working

"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.

Photo of a voltmeter

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.

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Know the (Color) Code PIC Wire Color Setup

photo of a voltmeter testing a green wire

A typical tow vehicle wiring jack is an in-line four-pin setup. It has three female pins and one male pin. The female pins carry the voltages for the different lights; the male pin is the ground or return for all the circuits. Obviously, the opposite kind of jack will be used on the boat trailer with three male pins and one female pin acting as the ground (see photo). Starting with the male pin and working toward the other side of the jack, the intended use and wire colors are:

White - Ground
Brown - Running (tail) lights
Yellow - Left turn/brake light
Green - Right turn/brake light

"Testing, Testing"

Assuming that the wiring from the tow vehicle is in proper working order, but one or more lights on the trailer is not, you can use a common voltmeter* to test the trailer wiring to locate the problem by following these steps:

  • 1) Set the voltmeter to the 12v range setting. Connect the trailer light system's connector (usually the male end of a four-prong flat connector) into the tow vehicle’s female connector.
  • 2) Turn on the circuit to be tested (running lights, turn signal, brake lights) in the tow vehicle to identify a light that may not be working.
  • 3) Touch the sharp-ended probe of the black wire from the voltmeter to a clean metal surface on the trailer near the light you want to test. Test wires as close as you can to the fixture by pushing the sharp end of the red probe through the insulation covering the wire. Brown is typically used for running lights (there may be two, one for each side; typically the port/left side wire is brown with a yellow stripe and the starboard/right side wire is brown with a green stripe), yellow is used for left/port brake lights and turn signals, and green for starboard/right brake lights and turn signals. If there is about 12.5 volts there, yet no light, the problem is in the bulb or fixture. Remove the lens and bulb and replace with a new bulb if necessary, but first clean the fixture's contacts with a bit of sandpaper and put some dialectic grease on the base of the bulb. Secure back into the fixture and see if the light is on. (Note that piecing a wire's insulation may compromise its corrosion resistance. A dab of silicone sealant or liquid electrical tape at the puncture point is recommended.)
  • 4) If there's no voltage at the fixture, the problem is somewhere in the wiring. Start at a point along the wire, midway from fixture to the trailer’s connector and test again, using the black probe on the trailer frame and the red probe on the suspect wire. If you have voltage at the halfway mark, you know the fault is between there and the fixture. If you have no voltage at the halfway mark, you know the fault is between there and the trailer’s connector. Work your way along the wire, testing until you get voltage, which will tell you where the break is. Fix it and you'll be back in business.

*12-volt test lights are popular tools for troubleshooting trailer wiring featuring sharper probes for piercing insulation, and are easier to read (the light comes on or it doesn't) than voltmeters, though they can't show low voltage situations like a meter can.


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