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Troubleshooting Article Archive:  September 2005      
Electrical Problems?  Don't Overlook the Ground
My colleague Joe Baker and I were working on a challenging clutch pump job recently when I joked that the three most important aspects of 12-volt electrical work are ground, ground, and ground.  The phrase "electric circuit" indicates a complete circle in which the electrons need to flow back to the battery.  The ground is as important a part of that circle as the hot wire.

Ground problems usually show up in three forms:

*  Loads come on when they're not supposed to.  For instance, you activate the reverse lights and the brake lights come on.  The reason lights come on when they're not supposed to is that the lights you turned on lack a good ground, so they ground through the hot side of neighboring light fixtures, causing those fixtures to light up.

*  Loads don't come on when they're supposed to.  For example, you turn on the beacon lights and they don't come on.  A ground doesn't have to be wholly missing in order for the load to fail to come on.  A 12-volt relay, for example, will likely not work at all if there's, say, a 5-volt potential difference between hot and ground, instead of the 12-volt difference it should have.

*  Components fail because of extended operation with intermittent or inadequate ground.  People often wrongly attribute these failures to mechanical overloading or some other cause.  A 12-volt device that gets a full 12 volts from the hot wire and is poorly grounded will burn up in the same way as it will by getting 7 volts from the hot wire and being properly grounded.  In both cases the effect is the same:  less than the full, steady 12-volt potential difference the device needs to operate.  (See photo above of two electric clutch coils of the sort that run a hydraulic pump or air conditioning compressor.  The coil at left failed from inadequate ground.)

Bear in mind that on truck equipment, it's not enough just to ground your device to the local metal.  On a flatbed tow truck, for example, the pylon needs to be grounded to the deck, the deck to the subframe, the subframe to the chassis, the engine to the chassis, the cab to the chassis and the battery to the chassis.

Overlooked ground problems cause money to be wasted by replacing perfectly good components that turn out to have been simply not grounded properly.  When diagnosing electrical problems, give the ground its due and remember it's as important a component of an electric circuit as any other part.