Troubleshooting Trailer Lights

by Alan Glos
(Excerpted from Messing Around In Boats)
(click here for more information about MAIB)

I am guessing that many readers have had a run-in or two with trailer lights and have had to offer up the lame excuse, "Gee officer, they were working fine when I left the house." So consider the following.

First, trailer lights are merely clones of the lights (running lights, directional signals, and brake lights) that are mounted internally on the rear of all lowing vehicles. Unlike the vehicle lights enclosed in steel and plastic, trailer lights are exposed to rain, snow, salt, and the occasional dunking in fresh or salt water when the boat is launched. The wires
are exposed and subject to chafe and damage and the connectors (usually brass) are particularly prone to corrosion. Trailers also have small wheels, do a lot more bumping and jarring around than tow vehicles, and as a result the filaments in trailer light bulbs tend to fail more often than vehicle light bulbs.

Trailer light rigs are usually of one of three varieties. Rig "A" has the light fixtures permanently bolted to the metal frame of the trailer and the ground "wire" is actually the entire trailer frame joined to the tow vehicle via the metal trailer hitch. This is the most common rig, but also the one most prone to failures when the fixtures rust and end up losing contact with the trailer frame.

Rig "B" also has the light fixtures permanently bolted lo the trailer frame but has both a positive and ground wire going to each fixture. This is an improvement over Rig "A" but a little more costly as a second ground wire needs to be initially rigged for each fixture.

Rig "C" is a removable light board consisting of all the fixtures and full wiring mounted on a plank that can be bolted to the frame of the trailer when in use but removed from the trailer and stored out of the elements when not in use.

Rig "C" is "The Answer" to most of the woes that beset trailer light problems, but check with your local State Department of Transportation regulations to make sure such rigs are legal in your state, as in many states such a rig must be quasi-permanently attached to the trailer when in use, bolted on for example, rather than tied on with balling twine or some kind of quick release device. The greatest advantage of the removable light board is the ability to remove the lights before launching a boat and submerging all of the light fixtures. No electrical device likes
to get wet.

All trailer light rigs have to be linked to the tow vehicle via some kind of connecting device (plug and socket) and these devices come in several varieties. The most common type for small trailers is a "flat plug" male and female device that has metal prongs and sockets (usually brass) that are encased in a flat plastic jacket. The flat plug typically has four posts, a ground wire and separate wires for running lights, left and right directional signals, and brake lights. Others may be round, all metal plugs and sockets, but they all have the same function.

On most vehicles the plug is hardwired into a black box called a "converter" and unless you know your way around automotive electrical circuits, I would recommend that you let the guys at the local garage or a U-Haul installer rig up the converter unit for you (although these days most light trucks and SUVs come with a factory installed wiring harness that makes it relatively simple to hook up the rest of the rig). Most tow vehicles have the light rig socket mounted externally (again out in the elements), but if you have a choice in the matter, install a "pig tail" wire harness and socket in the trunk or the hatch of the tow vehicle that you can run through the trunk lid or the hatch door rubber gasket when needed. Again, keeping the wiring out of the elements will reduce failure due to corroded plugs and sockets.

Troubleshooting: So you hook up the rig, walk to the rear of the trailer, and have your tow vehicle partner turn on the headlights and test the running lights, turn signals, brake lights, and four way Hashers. If they all work, bow your head and mutter an appropriate benediction to the God of all things electrical, but often one or more of the lights will not work properly and you will need to make repairs.

Tools needed: Screw drivers, knife, steel wool, small wire brush, light power drill, spare light bulbs, and a circuit tester. After years of frustrating attempts to troubleshoot these problems, I finally went to the local NAPA store and sprung $8.75 for a simple 12 volt automotive circuit tester. This device looks like an ice pick with a little neon light in the handle and a wire with an alligator clip attached to it.

To test a circuit, one clips the wire to a bare metal part of the tow vehicle or trailer ground wire and then uses the sharp pick end as a probe to touch fittings or wires. If the circuit is complete, the light in the handle of the tester will light up; if not, no light. You can also use the pick end to "stab" a wire through the plastic covering without the necessity to peel back the covering.

With these tools in hand, let's look at the most common problems and fixes (Note: disconnect the light rig from the tow vehicle when making repairs to avoid blowing tow vehicle fuses, see "Fuses" below.)

Problem: None of the lights work. Fix: Check the ground wire as this common ground provides half the circuit for all of the lights, and if none of the lights work it's a better than average bet that the ground wire is faulty. Start with the tow vehicle. Has the ground wire (usually the white wire in most rigs) become disconnected or is the connector device corroded? Use your circuit tester to see if current is getting through and make repairs as needed to get the ground wire humming.

Problem: Some of the lights work but not all of them. Fix; In this scenario, the common ground wire is probably not the problem but the "hot" wire that forms the rest of the circuit to the light fixture probably is. Use the circuit tester to see if power is getting to the light bulb. First check the wires going into the fixture that doesn't work and work you way back to the tow vehicle to see where the circuit fails. Use steel wool and/or the small wire brush to clean off the brass terminals.

To clean the inside of the hollow parts of the plug and sockets, chuck a small bit or nail in your power drill, wrap steel wool around the bit or nail, and then gently insert the device into the holes and rotate. When you can see clean metal, you know you have a good contact. Use handheld steel wool or a small wire brush to clean the male prongs on the connectors.

Similarly, if the circuit tester indicates that the problem may be in the light receptacle itself, unscrew and remove the light fixture covers and use steel wool to clean the contact points. Also, remove the bulbs themselves and clean the brass sides and lead tips. At the same time, visually inspect the bulbs and replace bulbs that are blackened or have obviously broken filaments. Corroded contacts account for the majority of trailer light failures, and cleaning is the answer. Bulb failure seems to be the #2 problem.

Problem: One or more lights don't work on the tow vehicle or the trailer. Fix: Check the tow vehicle fuse box for a blown fuse. You can blow fuses with a faulty trailer light rig or clumsy efforts to make repairs to same. Check the fuse box for one that is blown, replace it, and see if it does the job. If it blows again, you have a short circuit in the tow vehicle or the trailer light rig.

Other fixes: In rare cases the tow vehicle converter device falls. Again, use your circuit tester to see if the output directly from the converter works. If not, you may have to replace the converter. This has happened twice to me over 30 years, once on a factory installed unit in a Toyota SUV and another on my VW where it turned out one of the wires had broken just inside the converter and was repaired with some epoxy glue.

Broken wires: The metal part of a wire can be severed even if the plastic cover is still intact, witness stretching a wire until the metal strands break even though the elastic plastic cover does not. Look for obvious breaks and replace or re-solder as needed, or use your circuit tester starting at the light fixture working back to the tow vehicle until you find the break that interrupts the circuit.

Rusted beyond repair fixtures: I have found that even good quality trailer light fixtures rust out somewhere around 5-10 years, depending on how long the rig is left outside in the elements, so be prepared to replace units often if you find you are spending too much time trying lo restore them. A salt water environment results in even shorter working life for these fixtures.

All of the above notwithstanding, if you operate a trailer long enough, you are bound to come across One Of Those Problems That Has No Logical Solution (OOTPTHNLS). To
troubleshoot OOTPTHNLS, first try a different tow vehicle to find out if the problem is in the light rig or in the tow vehicle and proceed accordingly. If you still can't find the problem, enlist the help of the local garage ($ouch$') or better yet a friend who actually knows his or her way around automotive electrical circuits. If you don't have such a friend, cultivate one.

Trailer lights perform a valuable and necessary function, but as noted they are prone to malfunction and the occasional breakdown. I hope some of the steps outlined above can help you make basic repairs that will gel you street legal and roadworthy again. Happy trailering.