Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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68 Chapter Four
Shielding Metal
Shields, such as the ones used in breaker point distributors,
block the waves from components that
create RFI signals. The circuits of onboard computers
are protected to some degree from external
electromagnetic waves by their metal housings.
Ground Straps
Ground or bonding straps between the engine and
chassis of an automobile help suppress EMI conduction
and radiation by providing a low-resistance
circuit ground path. Such suppression ground straps
are often installed between rubber-mounted components
and body parts, Figure 4-36. On some
models ground straps are installed between body
parts, such as the hood and a fender panel where
no electrical circuit exists, Figure 4-36. In such a
case, the strap has no other job than to suppress
EMI. Without it, the sheet-metal body and hood
could function as a large capacitor. The space
between the fender and hood could form an electrostatic
field and couple with the computer circuits
in the wiring harness routed near the fender panel.
For more information about ground straps, see the
section on “Diagnostic Strategies” in Chapter 4 of
the Shop Manual.
EMI Suppression
Interference suppression is now a critical automotive
engineering task because the modem automobile
has an increased need for EMI suppression.
Figure 4-36. Ground straps are installed in many
areas of the engine compartment to suppress EMI.
(Reprinted by permission of Robert Bosch GmbH)
The increasing use of cellular telephones, as well
as onboard computer systems, are only two of the
factors that have made interference suppression
extremely important.
Even small amounts of EMI can disrupt the
operation of an onboard digital computer, which
operates on voltage signals of a few millivolts
(thousandths of a volt) and milliamperes (thousandths
of an ampere) of current. Any of the interference
transmission modes discussed earlier are
capable of creating false voltage signals and
excessive current in the computer systems. False
voltage signals disrupt computer operation, while
excessive current causes permanent damage to
micro-electric circuitry.
As the complexity and number of electronic systems
continues to increase, manufacturers are using
multiplex wiring systems to reduce the size and
number of wiring harnesses, which also reduces
EMI. Multiplexing is a method of sending more
than one electrical signal over the same channel.
SUMMARY
Electricity can be generated in several ways. The
most important way for automotive use is by magnetism.
Magnetism is a form of energy caused by
the alignment of atoms in certain materials. It is
indicated by the ability to attract iron. Some magnetic
materials exist in nature; others can be artificially
magnetized. The magnetic properties of some
metals, such as iron, are due to electron motion
within the atomic structure. Reluctance is resistance
to the movement of magnetic lines of force: iron
cores have permeability and are used to reduce
reluctance in electromagnetic fields.
Lines of force, called flux lines, form a magnetic
field around a magnet. Flux lines exit the north
pole and enter the south pole of a magnet. Magnetic
flux lines also surround electrical conductors. As
current increases, the magnetic field of a conductor
becomes stronger. Voltage can be generated by the
interaction of magnetic fields around conductors.
The relative movement of a conductor and a
magnetic field generates voltage. This process is
called induction. Either the conductor or the magnetic
field may be moving. The strength of the
induced voltage depends on the strength of the
magnetic field, the number of conductors, the speed
of the relative motion, and the angle at which the
conductors cut the flux lines. Electromagnetic
induction is used in generators, alternators, electric
motors, and coils. Magnetomotive force (mmf) is a