Figure 11-12. Basic Electrical Circuit.

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TM 9-8000
a.  General (Fig. 11-12).  A very basic circuit
current flow goes down if the resistance goes up. An
example of this would be the lighting circuit that is going
consists of a power source, a unit to be operated, and a
bad in a truck. Suppose the wiring circuit between the
wire to connect the two together.  if the unit to be
battery  and  the  lights  has  deteriorated  due  to
operated is to be controlled, a switch will be included in
connections becoming poor, strands in the wire breaking,
the circuit also.
switch contacts becoming dirty, or  other, similar
b. Automotive Circuits (Fig. 11-13). The body and
problems. All of these conditions reduce the electron
path or, in other words, increase resistance. And, with
chassis in an automobile are made of steel. This feature
this increased resistance, less current will flow.  The
is utilized to eliminate one of the wires from all of the
voltage of the battery stays the same (for example, 12
automobile's circuits.  By attaching one of the battery
volts). if the resistance of the circuit when new (including
terminals to the body and chassis, any electrical
light bulbs) was 6 ohms, then 2 amperes will flow. To
component can be connected by hooking up one side, by
satisfy the equation, 12 (volts) must equal 12 (amperes
wire, to the car battery and the other side to the body.
times ohms resistance). But if the resistance goes up to
The practice of connecting one side of the battery to the
8 ohms, only 1.5 amperes can flow.  The increased
automobile body is called grounding. Virtually all current
resistance cuts down the current flow and, consequently,
automotive manufacturers ground the negative side of
the amount of light.
the battery. This is referred to as an electrical system
with a negative ground. Vehicles with a positive ground
c.
A great majority of electrical troubles on
are very uncommon at the present time.
automotive vehicles result from increased resistance in
c. Series Circuits (A, Fig. 11-14). A series circuit
circuits due to bad connections, deteriorated wiring, dirty
or burned contacts in switches, or other such problems.
consists of two or more resistance units (electrically
With any of these conditions, the resistance of the circuit
operated components) that are connected together in an
goes up and the ampere flow through that circuit goes
end-to-end manner so that any current flow in the circuit
down.  Bad contact points in the ignition circuit will
is dependent on a complete path through all of the units.
reduce current flow in the circuit and cause weak sparks
The following characteristics of series circuits are
at the spark plugs. This will result in engine missing and
important:
loss of power.
d. If the resistance stays the same but the voltage
increases, the amperage also increases.  This is a
condition that might occur if a generator voltage regulator
became defective.  In such a case, there would be
nothing to hold the generator voltage within limits, and
the voltage might increase excessively. This would force
excessive amounts of current through various circuits
and cause serious damage. If too much current went
through the light bulb filaments, for example, the
filaments would overheat and burn out.  Also, other
electrical devices probably would be damaged.
e. On the other hand, if the voltage is reduced, the
amount of current flowing in a circuit will also be reduced
if the resistance stays the same. For example, with a
run-down battery, battery voltage will drop excessively
Figure 11-12. Basic Electrical Circuit.
with a heavy discharge. When trying to start an engine
with a run-down battery, the voltage will drop very low.
This voltage is so low that it cannot push enough current
through the starter for effective starting of the engine.
11-12. Circuit Configurations.
TA233528
11-11