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

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104 Chapter Six
Figure 6-22. Many different types of switches are used in the
complete electrical system of a modern automobile.
Figure 6-23. These symbols for normally open
switches are used on electrical system diagrams.
the seatbelt is buckled; this stops current flow to the
buzzer. Figure 6-23 shows the electrical symbols
for some simple normally open switches.
A switch may lock in the desired position, or
it may be spring-loaded so that a constant pressure
is required to keep the points out of their
normal position. Switches with more than one
set of contact points can control more than one
circuit. For example, a windshield wiper switch
might control a low, medium, and high wiper
speed, as well as a windshield washer device
(Figure 6-24).
Switches are shown in simplified form on electrical
diagrams so that current flow through them
can easily be traced (Figure 6-25). Triangular contact
points generally indicate a spring-loaded
return, with circular contacts indicating a lockingposition
switch. A dashed line between the movable
parts of a switch means that they are mechanically
connected and operate in unison, as shown
in Figure 6-26.
In addition to manual switches, automotive
electrical circuits use a variety of other switch
designs. Switches may be operated by temperature
or pressure. Switches designed to sense engine
coolant temperature contain a bimetal arm that
flexes as it heats and cools, opening or closing the
switch contacts (Figure 6-26). Oil pressure and
vacuum switches respond to changes in pressure.
Mercury and inertia switches are motiondetector
switches, that is, they open and close
circuits automatically when their position is disturbed.
A mercury switch uses a capsule containing
two electrical contacts at one end. The other
end is partially filled with mercury, which is a
good conductor (Figure 6-27).
When the capsule moves a specified amount
in a given direction, the mercury flows to the
opposite end of the capsule and makes a circuit
between the contacts. This type of switch often
is used to turn on engine compartment or trunk
lamps. It can also be used as a rollover switch to
open an electric fuel pump or other circuit in an
accident.
An inertia switch is generally a normally
closed switch with a calibrated amount of spring
pressure or friction holding the contacts together.
Any sharp physical movement (a sudden change
in inertia) sufficient to overcome the spring pressure
or friction will open the contacts and break
the circuit. This type of switch is used to open the
fuel pump circuit in an impact collision. After the
switch has opened, it must be reset manually to its
normally closed position.