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Troubleshooting Article Archive:  March 2004          
Magnetism and Light Bar Wiring
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.