If you've ever
done repair work involving small parts and close quarters, and you dropped
a small part or tool, you know there seems to be some sort of magnetic
force that sucks that part or tool into the most inaccessible corner of
what you're working on. It's almost an unfailing rule.
Of course there is no actual
magnetic force, but there is one part of a tow truck that often does have
a magnetic field: the outer case of the electric motor on your
beacon or rotator light fixtures inside your light bar. When
troubleshooting wiring, it's important to keep this magnetic field in
mind.
The reason magnetism plays an
important part in short circuits is because on more than one brand of
light bar, the magnetic motor case not only attracts small parts, but is
also 12 volts positive when activated. Additionally, many models of
light bars have clearance between the motor case (12v positive) and the
light bar frame (ground) of less than 3/32 of an inch. Inside a
high-mileage, older light bar, the vibrations of a diesel-powered chassis
can loosen small nuts, screws, washers and bits of disintegrated metal
reflector, tossing them around inside the light bar like dice in a coffee
can.
Many of these small parts,
particularly flat washers, end up attracted to the bottom of the magnetic
motor case where they fill in the 3/32-inch gap between the case and the
light bar frame. This causes an intermittent short circuit between
the motor case, which is 12 volts positive, and the frame, which is
ground. It's important to carefully inspect the small gaps between
the case and the light bar frame because a washer can be overlooked.
Here are two solutions for this
often-overlooked problem. First, make sure there are no loose parts
inside the light bar. If your fixtures and reflectors have a bolt
and/or screw loose or missing, put all new bolts and/or screws in the
fixture and check the tightness of the fixtures. Eliminate blank
reflectors and other surplus parts if they serve no purpose. Second,
when spec'ing your next new truck or buying used, keep in mind that
because of the smoother nature of the engine, gasoline-powered chassis
tend to cause much less vibration in light bars than diesel.
