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

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66 Chapter Four
even installed a metal shield inside their distributors
to further reduce RFI radiation from the
breaker points, condensers, and rotors.
As the use of electronic components and systems
increased, the problem of electromagnetic
interference reappeared with broader implications.
The low-power digital integrated circuits now in
use are extremely sensitive to EMI signals that
were of little or no concern before the late 1970s.
For more information about EMI, see the “Logic
Probe” section in Chapter 4 of the Shop Manual.
Interference Generation
and Transmission
Whenever there is current in a conductor, an electromagnetic
field is created. When current stops
and starts, as in a spark plug cable or a switch that
opens and closes, field strength changes. Each
time this happens, it creates an electromagnetic
signal wave. If it happens rapidly enough, the
resulting high-frequency signal waves, or EMI,
interfere with radio and television transmission or
with other electronic systems such as those under
the hood. This is an undesirable side effect of the
phenomenon of electromagnetism. Figure 4-29
shows common sources of EMI on an automobile.
Static electric charges caused by friction of the
tires with the road, or the friction of engine drive
belts contacting their pulleys, also produce EMI.
Drive axles, drive shafts, and clutch or brake lining
surfaces are other sources of static electric charges.
There following four ways of transmitting EMI
can all be found in an automobile:
• Conductive coupling through circuit conductors
(Figure 4-30).
• Capacitive coupling through an electrostatic
field between two conductors (Figure 4-31).
• Inductive coupling as the magnetic fields
between two conductors form and collapse
(Figure 4-32).
• Electromagnetic radiation (Figure 4-33).
Figure 4-30. Wiring between the source of the interference
and the receiver transmits conductive-coupling
interference.
Figure 4-31. Capacitive field between adjacent
wiring transmits conductive-coupling interference.
Figure 4-29. Sources of electromagnetic interference
(EMI) in an automobile.
Figure 4-32. Inductive-coupling interference transmitted
by an electromagnetic field between adjacent
wiring.