Download Ground zero used to have a strictly negative connotation. But when it

EyeOnElectronics
Ground zero used to have a strictly negative connotation.
But when it comes to automotive applications, it forms
the basis for dependable electrical system performance.
P
Photo: Mike Dale
Mike Dale
robably the most misunderstood
part of a wiring schematic is the
part you often don’t see. There’s
no end to the information on how
power is supplied, how juice
reaches various components and
how various devices interface with each other.
What’s typically missing is the information
about the “grounds” that form the other half
of electrical circuits.
Grounds are not no-brainers. What’s
ground to one device may not necessarily be
ground to another. The loss of ground may
cause a circuit to generate false or noisy information or, worse yet, stop working altogether.
Let’s look at what the term ground really
means, what the effects of a poor ground are,
how to recognize common ground-related
problems and what to do about them.
Ground (or earth) as an electrical term
Ground voltage differentials are a form of noise to which the electrical system has to be immune. On this vehicle, the resistance between
the engine ground and battery negative is .5 ohm. Here, the two
ground points are not quite at the same place, electrically speaking.
really has its origins in the early days of radio.
Back then, a wire stretched between the
trees on a farm, say, made up one half of the
aerial; the other was literally a rod or pipe
driven into the ground. But this didn’t always
work as intended. When the reception was
poor, for instance, the farmer would often
have to pour a bucket of water on the ground
rod to improve the quality of the connection
to “earth.”
In the context of automotive electronics
and electrical systems, ground simply means a
common reference point. When we say an automobile has a 12-volt electrical system, the
part we leave out is that it’s 12 volts positive
with relation to the ground, or common, connection.
There’s a second purpose in automobiles
for the ground system. Not only does it give a
common reference to the electrical system, it
also acts as the return path for the flow of
electricity. By using the metal of the car body
or chassis as ground, the wiring harness needs
only to consist of wires for supply of B+ voltage as well as carriers of information between
the sensors and the computers.
Most automotive grounding systems come
up somewhat short of being ideal. Why? One
reason is that steel is not nearly as good a conductor as copper. Electricity must pass
through the typical spot welds holding the
steel body panels of a car together from one
grounding point to the next. Problem is,
there’s electrical resistance in both the steel
and the spot welds. This means that different
parts of the body can be at slightly different
electrical potentials relative to each other.
Another factor is the ground cable arrangement. The negative side of the battery is typically tied directly to the engine block via a cable to provide the return connection for the
high-amperage starter motor. But this cable
very often has some resistance. This once again
means that the car’s body is not necessarily at
exactly 0 volts relative to battery negative.
continued on page 18
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January 2001
Eye On Electronics
For heavy power loads such as solenoids, fan motors and the like, the
consequence of the ground connection not really being at exactly 0 volts
doesn’t matter, at least most of the
time. Instead of seeing the total system voltage, the device will see the
difference between the true ground
value and system voltage. When the
loss is a volt or so, the load is designed to still operate properly. The
trouble comes when the cables and
connections from the battery become
excessively resistive. The voltage
drops across these poor connections
can rob the starter motor of the current needed to crank properly.
The story is a little different when
it comes to information lines—say, a
signal from a sensor to a PCM. When
the information is digital, it’s in the
form of a pulse train of 1s and 0s represented by 5 volts and 0 volts, respectively. There are two things to
notice about the choice of voltages
used: One is that the difference between the two is much closer than
the difference between system voltage and ground, so an offset of 1 volt
becomes much more significant.
The second thing is that the plus value is typically 5 volts. Theoretically, it
could be 1 volt that equals a 1 and 0
volts that equals a 0. The reason 5 volts
was chosen is to provide noise immunity. A design feature built into logic ICs
called a comparator looks at the information waveform and compares the
received voltage to an internal reference voltage. The comparator is set so
that any signal received that is less than
.8 volt is counted as a 0 and anything
above 3.5 volts is counted as a 1. This
allows a noisy signal to still transmit its
information.
Analog sensors are much more
susceptible to ground offsets and
electrical noise in the system. The
output of these sensors is often low
(oxygen sensors, for example) and
any loss due to poor connections can
result in misinformation being sent
to the computer.
There are several basic problems
inherent in achieving reliable
grounding. First, there’s the issue of
the mechanical connection itself. As
an example, take the typical tin-plat18
January 2001
ed brass ring terminal. This is
crimped on one end of a wire to be
grounded and fastened to the body
by a self-tapping sheet-metal screw.
Think of all the things that could go
wrong with such a connection: The
crimp to the wire could come loose,
corrode or become mechanically
damaged in a collision. The sheetmetal screw could back out as the
body flexes during use and thermal
cycling. The terminal and the place
on the body to which it attaches
could corrode due to salt, water or
other corrosive fluids such as battery
acids.
Many devices achieve their electrical connection to ground by way of a
bolt or screw attached directly to the
frame, body or drivetrain. As in the
‘Straightening out
a bad ground
often can be
accomplished by
simply loosening
a screw or a
mounting bolt, then
retightening it.’
ring terminal example, the integrity of
these grounds is dependent on the
quality of the mechanical connection.
While identifying ground-related
problems is not necessarily easy,
there are some scenarios that should
automatically raise a red flag. Multiple, seemingly unrelated failures are
often ground-related issues. Mysteries such as the dome light flashing in
unison with the turn signals are often
fuse- or ground connection-related.
Sensor outputs that are either out of
range high or a constant value can be
an indication of a ground problem.
Poor performance of power loads
such as the starting system, solenoids
and fan motors may be the result of a
poor ground connection, as well.
Finding bad grounds is not always
easy. Often, those sheet-metal screws
I mentioned earlier can be buried in
places that have never seen the light
of day. Connections that look terrible
may actually be okay, and vice versa.
Just finding the location of a ground
sometimes can be an all-day affair.
Fortunately, much of the newer service literature does include ground
location information.
Once you’ve found the connection
point, there are a couple of things
you can do to check ground integrity.
One is to attach one lead of a DVOM
to battery negative and the other
lead to the ground lug, ring terminal
or metal body of a grounded assembly. When you operate the device,
there should be less than a 1-volt
drop across the connection; 100 millivolts is better, especially if it’s in a
low-voltage computer circuit. You
can do much the same with an ohmmeter if the circuit is not powered
up. What you’re looking for is pretty
close to a zero resistance connection.
Straightening out a bad ground often
can be accomplished by simply loosening a screw or a mounting bolt,
then retightening it.
In those cases where the integrity of
a ground remains in doubt, there is
something else you can do as either a
diagnostic technique or as a permanent
solution—hook up a redundant
ground. To do this, attach one end of a
wire to a known-good grounding point,
such as the vehicle body or the negative side of the battery. Then bring the
opposite end to the body of the
grounded part or to the ground terminal in the connector to that part. Assuming that the wire doesn’t get tangled in some moving part, the very
worst that can happen is that nothing
will happen. If the problem really is a
ground issue, the wire may confirm the
diagnosis or, when properly secured,
be a permanent solution.
The key to identifying and fixing
ground problems is to remember what
the ground system does. It provides
the return path for power and information distribution and forms the reference that voltages are measured in relation to. When your diagnosis says
that neither of those two things is happening, it’s time to go hunting for that
bad ground connection!