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

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Electrical Diagrams and Wiring 103
various touch-type switches into the overall
vehicle design.
• The master computer or transmitter can be
programmed with timing functions for convenience
features, such as locking doors
above a given speed or unlocking them when
the ignition is shut off.
ELECTRICAL
SYSTEM POLARITY
We discussed positive (+) and negative () electrical
charges in Chapter 3. We learned that like
charges repel each other and unlike charges
attract each other. We also noted that the terminals
of a voltage source are identified as positive and
negative. In Chapter 2, we defined magnetic
polarity in terms of the north and south poles of a
magnet and observed that unlike poles of a magnet
attract each other, just as unlike charges do.
Similarly, like poles repel each other.
The polarity of an electrical system refers to
the connections of the positive and negative terminals
of the voltage source, the battery, to the
insulated and ground sides of the system. All
domestic cars and trucks manufactured since
1956 have the negative battery terminal connected
to ground and the positive terminal connected
to the insulated side of the system. These
are called negative-ground systems and are said
to have positive polarity.
Before 1956, 6-volt Ford and Chrysler vehicles
had the positive battery terminal connected
to ground and the negative terminal connected
to the insulated side of the system. These are
called positive-ground systems and are said to
have negative polarity. Foreign manufacturers
used positive-ground systems as late as 1969. In
both kinds of systems, we say that current
leaves the hot side of the battery and returns
through the ground path to the grounded battery
terminal.
In your service work, it is very important to
recognize system polarity negative or positive
ground before working on the electrical system.
Some electrical components and test equipment
are sensitive to the system polarity and must be
installed with their connections matching those of
the battery. Reversing polarity can damage alternators,
cause motors to run backwards, ruin electronic
modules, and cause relays or solenoids to
malfunction.
COMMON
ELECTRICAL PARTS
Many common electrical parts are used in various
circuits in an electrical system. All circuits have
switches of some kind to control current flow.
Most circuits have some form of protective device,
such as a fuse or circuit breaker, to protect against
too much current flow. Various kinds of solenoids,
relays, and motors are used in many circuits, and
whatever their purpose, they operate in similar
ways wherever they are used.
Before we look at complete circuits and system
diagrams later in this next chapter, we should learn
about some of the common devices used in many
circuits.
Switches
Switches are used in automobile electrical systems
to start, stop, or redirect current flow. They
can be operated manually by the driver or
remotely through mechanical linkage. Manual
switches, such as the ignition switch and the headlamp
switch, allow the driver to control the operation
of the engine and accessories. Examples are
shown in Figure 6-22; the driver or the passengers
control a remotely operated switch indirectly. For
example, a mechanical switch called a neutral
safety switch on automatic transmission gear
selectors will not let the engine start if the automobile
is in gear. Switches operated by opening
and closing the doors control the interior lights.
For more information about switches, see the
“Copper Wiring Repair” section in Chapter 6 of
the Shop Manual.
Toggle Push-Pull Push Button
Switches exist in many forms but have common characteristics.
They all depend upon physical movement
for operation. A simple switch contains one or more
sets of contact points, with half of the points stationary
and the other half movable. When the switch is
operated, the movable points change position.
Switches can be designed so that the points are
normally open and switch operation closes them to
allow current flow. Normally closed switches allow
the operator to open the points and stop current
flow. For example, in an automobile with a seatbelt
warning buzzer, the switch points are opened when