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
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.