Trouble-Shooting Trailer
Lights
Being legally- and safety-minded, next season's plans will be on hold until
the lights on the boat trailer/camper are fixed.
by Steven Don
College of Technical Sciences Chair, Montana State University-Northern
The weather is cooling, summer is over, the boat and the
camper are headed to storage, it’s been a great season of
travel and fun…but the trailer lights don’t work anymore.
In my several years of owning and operating a diesel repair
business, and now teaching in northern Montana, this is an
all-too-common malady. The purpose of this article is to take
you through three common issues – dim lights, only one side
working, and no lights at all – in an attempt to improve your
chances of fixing your own trailer lights in the future.
When it comes to lights, it is first important to understand
basic Ohms Law and circuits. Ohms Law relates to
everything electrical, with three basic elements: 1) voltage,
2) amperage, and 3) resistance. In a nutshell, voltage is
electrical pressure, which pushes the current (amperage, or
amps – the movement of electrons), through a load, thereby
achieving some work. A load is anything that resists the
amperage flow (e.g., lights, heater motor, corrosion).
There are three types of circuits, including a simple series
circuit, a more complex parallel circuit, and a combination
of both, a series-parallel circuit. All circuits contain
common parts and pieces, including power source (battery
or alternator), conductors (wires covered with insulation),
circuit protection (fuses, circuit breakers, fusible links),
a switching device (the light switch), a load (lights, etc.),
connectors, and a path to ground.
The Dim Lights Issue
The trailer is hooked to the tow vehicle, the trailer connector
is plugged into the pickup connector at the bumper, the lights
on the trailer are on, but are dim. A load in a circuit will use
up the voltage as the electrons (current flow) get pushed
across each load. A load can be something useful (e.g., tail
lights on the trailer) or not useful (e.g., corrosion). Corrosion
is a huge issue in any electrical circuit, from the battery
terminals through the circuit to the ground. If the bulbs are
dim, then not all of the voltage is present at the bulbs (i.e.,
some of the voltage is being used to push the electrons
through an unwanted load). In this situation, it is important
to locate the corrosion, which can be in many places. Some
examples include corrosion at the ground, corrosion in a
connector (remember that the backs of some boat trailers
get submerged in water), a nick/cut in the insulation around
a wire, allowing moisture/water into the wire (traveling on
gravel roads is very tough on wiring), or insulation being
eaten by mice over the winter when the boat trailer is
stored. Any and all of these will necessitate a wiring repair.
The corrosion must be removed to get the full voltage to the
useful load (e.g., the lights). Remember to do a thorough,
complete, and correct wiring repair using solder (rosin core,
not acid core) and heat shrink tubing.
The Partial Lights Issue
In some instances there will be a trailer with only one side
of the lighting system working. This is usually due to a
broken wire. As with a dim light, do a thorough, complete,
and correct wiring repair. One thing to keep in mind is that
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this issue may require a splice in a section of wire. Ensure
that the splice is the identical gauge (i.e., size of wire) as the
existing wire because the gauge is specified for the current
carrying-capacity of the circuit. If it is too small, there will be
other issues.
The No-Lights Issue
The trailer light connector is plugged into the pickup
connector at the rear bumper, the pickup lights are normal,
but there are no lights on the trailer. The first place to
check is the connector. Are all of the wires in place in the
connector? Next, check the fuse. The fuse is one method
of circuit protection (i.e., protects the electrical circuit from
too much current) that protects the electrical system of
the vehicle. Fuses come in many different forms (e.g., ATC,
glass, mini, maxi, etc.) with different amperage ratings
depending upon the circuit being protected. If the current
flow exceeds the rating of the fuse, then the fuse will burn
open (i.e., it will “blow”), therefore disrupting the flow of
electrons. Without electron flow, the load in that circuit will
not operate.
PHOTO COURTESY OF BIGSTOCK.COM
What causes a fuse to blow? Typically, a hot (electrified)
wire touching a bare metal surface is the culprit. Before
replacing any fuse, a diagnosis must be made as to why
the fuse needs to be replaced. A common fallacy is that if
a fuse blows, then it can simply be replaced with another
fuse with a higher current rating. This can be done, but is a
VERY BAD IDEA. Fix the problem, replace the fuse with the
correct amperage rating, and avoid using homemade fuses
and unsafe wiring.
Be aware that with all of the above issues, a repair is being
made because something is not as it should be (e.g., a wire
is broken, the insulation around the wire is split allowing for
corrosion, the fuse is blown). Something caused the issue
and it is critical to diagnose the initial cause. For example, if
gravel caused the broken wire, do I need to reroute the wire
or place it inside a protective loom? If mice chewed through
the insulation, do I need to re-evaluate my storage options?
Making the fix without taking care of the initial cause will
likely result in just a temporary fix. Take time to make the
repair, do it right, and, do it only once. 
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