Troubleshooting Trailer Lights
by Alan Glos
from Messing Around In Boats)
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).
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
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.