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

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114 Chapter Six
may not supply an entire system diagram for a
vehicle, but may instead illustrate all circuits
separately, as shown in Figure 6-57.
Schematic Diagrams
A schematic diagram, also called a “circuit
diagram” describes the operation of an individual
circuit. Schematics tell you how a circuit works
and how the individual components connect to
each other (Figure 6-55). Engineers commonly
use this type of diagram.
Figure 6-48. These electrical symbols are discussed
in the Classroom Manual.
be called a “wiring diagram.” System diagrams
show the wires, connections to loads, switches,
and the type of connectors used, but not how the
loads or switches work. Installation diagrams
express where and how the loads and wires are
installed. This is covered later in this chapter.
Figure 6-54 shows the same warning lamp circuit
as Figure 6-55, but in a different format.
System diagrams may cover many pages of a
system and grounds are identified for all circuits.
The diagram is also organized by individual subsystems
at the top. This variation on the grid
theme is another tool to quickly locate the
desired part of the diagram. A Chrysler
Corporation shop manual may not supply an
entire system diagram for a vehicle, but may
instead illustrate all circuits work. Figure 6-54
shows the same warning lamp circuit as
Figure 6-55 but in a different format. System
diagrams may cover many pages of a manual as
ground points are identified for all circuits
(Figure 6-56). The diagram is also organized
by individual subsystems at the top. This variation
on the grid theme is another tool to
quickly locate the desired part of the diagram.
A DaimlerChrysler Corporation shop manual
Some schematics are Valley Forge diagrams,
which present current moving vertically. The
power source is at the top and the ground at the
bottom of the page (Figure 6-58). Figure 6-57
illustrates the circuit for a DaimlerChrysler radio
system. Some of the wires are fully identified with
two circuit numbers, wire gauge, and wire color.
Other wires, such as the two wires connected to
the front speaker, are identified only by wire gauge
and color. The “20LGN” indicates a 20-gauge,
light green wire. Figure 6-59 is the fuel economy
lamp circuit in a GM vehicle. Here, neither wire
gauge nor wire color is indicated. The “green” and
“amber” refer to the color of the lamp bulbs.
Figure 6-60 shows a Ford side marker lamp
circuit. Again, wire size and color are not identified.
The numbers on the wires are circuit numbers.
Note that the ground wires on the front and
rear lamps may not be present depending upon
the type of lamp socket used on the automobile.
Switches
Some manufacturers, such as Nissan, extend the
system diagram to include major switches, as in
the headlight circuit shown in Figure 6-53. This
illustration shows the current traveling from the
fuse block, through the switch, and to the headlights.
If a switch does not work properly, it causes
a malfunction in the electrical system. Switch diagrams
may take extra time to understand, but they
are indispensable in testing and diagnosis.
Each connection is shown as two circles joined
by a line. The grid diagram shows which individual
circuits have power at each switch position.
A drawing of the headlight switch is included to
explain the meaning of OFF, 1ST, 2ND, A, B, and
C. Normally, a drawing of the switch action does
not accompany the system diagram. If the switch