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

www.TechnicalBooksPDF.com
100 Chapter Six
insulation, made by wrapping cotton or silk
around wire and then coating it with rubber, was
easily hardened by heat. The insulation often
broke off, leaving bare wire exposed.
A common problem in cars that used dry-cell
batteries was moisture penetration through the
battery’s paper insulation. Current design would
flow to ground and the batteries would become
discharged.
Even washing a car sometimes caused trouble.
Water got into the distributor terminals and
made the engine hard to start. Some technicians
poured melted wax into the space between the
plug wires and the distributor cap terminals.
For protection from heat, moisture, oil, and
grease, wiring was often run through a metal
conduit. Armored cable-insulated wire enclosed
in a permanent, flexible metal wrapping was also
used, especially in a circuit where any voltage
drop was critical.
This is an important point to remember. It may be
helpful at this time to review the explanations in
Chapters 3 and 5 of voltage drops and current flow
in various circuits from the source, through all the
loads, and back to the source. Every electrical load
is attached to the chassis so that current can pass
through the ground and back to the grounded battery
terminal. Grounding connections must be
secure for the circuit to be complete. In older cars
where plastics were rarely used, most loads had a
direct connection to a metal ground. With the
increased use of various plastics, designers have
had to add a ground wire from some loads to the
nearer metal ground. The ground wires in most
circuits are black for easy recognition.
MULTIPLEX
CIRCUITS
The use of multiplexing, or multiplex circuits, is
becoming a necessity in late-model automobiles
because of the increasing number of conventional
electrical circuits required by electronic control
systems. Wiring harnesses used on such vehicles
have ballooned in size to 60 or more wires in a
single harness, with the use of several harnesses
in a vehicle not uncommon. Simply put, there
are too many wires and too limited space in
which to run them for convenient service. With so
many wires in close proximity, they are subject to
electromagnetic interference (EMI), which you
learned about in Chapter 4. To meet the almost
endless need for electrical circuitry in the growing
and complex design of automotive control
systems, engineers are gradually reducing the size
and number of wire and wiring harnesses by using
a multiplex wiring system.
The term multiplexing means different things to
different people, but generally it is defined as a
means of sending two or more messages simultaneously
over the same channel. Different forms of
multiplexing are used in automotive circuits. For
example, windshield wiper circuits often use multiplex
circuits. The wiper and washer functions in
such circuit work though a single input circuit by
means of different voltage levels. In this type of
application, data is sent in parallel form. However,
the most common form of multiplexing in automotive
applications is serial data transmission,
also known as time-division multiplex. In the
time-division type of circuit, information is transmitted
between computers through a series of digital
pulses in a program sequence that can be read
and understood by each computer in the system.
The three major approaches to a multiplex wiring
system presently in use are as follows:
• Parallel data transmission
• Serial data transmission
• Optical data links
We will look at each of these types of system, and
then we will discuss the advantages of multiplexing
over older systems of wiring.
Parallel Data Transmission
The most common parallel data multiplexing circuits
use differentiated voltage levels as a means of
controlling components. The multiplex wiring circuit
used with a Type C General Motors pulse
wiper-washer unit is shown in Figure 6-20. The circuit
diagram shows several major advantages over
other types of pulse wiper circuits, as follows:
• Eliminating one terminal at the washer pump
reduces the wiring required between the wiper
and control switch.
• Using a simple grounding-type control switch
eliminates a separate 12-volt circuit to the
fuse block.
• Eliminating a repeat park cycle when the
wash cycle starts with the control switch in
the OFF position—in standard circuits, the