chap32 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 32, the reader should
be able to:

•
•
•
•
•

Prepare for ASE Electrical/Electronic Systems (A6)
certification test content area “A” (General
Electrical/Electronic Systems Diagnosis).
State Ohm’s law.
Identify the parts of a complete circuit.
State Watt’s law.
Describe the characteristics of an open circuit, a short-toground, and a short-to-voltage.

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KEY TERMS:
circuit • continuity
ground (return) path • grounded
insulated path
kilowatt • load • Ohm’s law • open circuit
power source • protection
shorted • short-to-ground • short-to-voltage
Watt’s law
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CIRCUITS

A circuit is a path that electrons travel from a power source (such as
a battery) through a load (such as a light bulb) and back to the power
source. It is called a circuit because the current must start and finish
at the same place (power source).

For any circuit to work, it must
be continuous from the battery
through all wires and
components and back to the
battery (ground).
A circuit that is continuous
throughout is said to have
continuity.
Figure 32–1 All complete circuits must have a power source, a power path, protection (fuse),
an electrical load (light bulb in this case), and a return path back to the power source.
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Parts of a Complete Circuit Every complete circuit contains:
1. A power source, such as a vehicle’s battery.
2. Fuses, circuit breakers, and fusible links, which are
Protection from harmful overloads (excessive current flow).
3. An insulated path for current flow from the power source to
the resistance. This path from a power source to the load (a
light bulb in this example) is usually an insulated copper wire.
4. The electrical load or resistance converts electrical energy
into heat, light, or motion.
5. A ground (return) path for the electrical current from the
load back to the power source so that there is a complete
circuit. This ground path is usually the metal body, frame, and
engine block of the vehicle. See Figure 32-2.
6. Switches and controls turn the circuit on and off. Figure 32-3.
Continued
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Figure 32–2 The return path back to the battery can be any electrical conductor, such as
the metal frame or body of the vehicle.

Continued
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Figure 32–3 An electrical switch opens the circuit and no current flows. The switch could also
be on the return (ground) path wire.

Continued
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Open Circuits An open circuit is any circuit that is not complete,
or that lacks continuity. See Figure 32–4.
No current at all will flow through an incomplete circuit.
An open circuit may be created by a break in the circuit or by a
switch that opens (turns off) the circuit and prevents the flow of
current.
In any circuit containing a power load and ground, an opening
anywhere in the circuit will cause the circuit to stop working.
A light switch in a home and the headlight switch in a vehicle are
examples of devices that open a circuit to control its operation.
Continued
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Figure 32–4 This figure shows examples of common causes of open circuits. Some of these
causes are often difficult to find.

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Use a Schematic as a Road Map
The wiring schematic is the “road map” of a circuit and shows all electrical

paths. If an open occurs in a circuit, the current stops flowing and the
electrical load device does not work.
Trace the circuit by following the path from the battery through the power
side component, load, and on the ground. Check for voltage at various
points in the circuit to locate where the open is in the circuit.

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Short-to-Voltage If a wire (conductor) or component is shorted to
voltage, it is commonly called shorted.

Figure 32–5 A short circuit permits electrical
current to bypass some or all of the resistance
in the circuit.

Continued
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A short circuit:
1. Is a complete circuit in which the current bypasses some or all of
the resistance in the circuit.
2. Involves the power side of the circuit.

3. Involves a copper-to-copper connection.
4. Also called a short-to-voltage.
5. Usually affects more than one circuit.
6. May or may not blow a fuse.

Figure 32–6 A fuse or circuit breaker
opens the circuit to prevent possible
overheating damage in the event of a
short circuit.

Continued
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The Short-To-Voltage Story - Part1
A technician was working on a Chevrolet pickup truck with unusual
electrical problems including the following:
1. Whenever the brake pedal was depressed, the dash light and the
side marker lights would light.
2. The turn signals caused all lights to blink and the fuel gauge needle
to bounce up and down.
3. When the brake lights were on, the front parking lights also came on.
The technician tested all fuses using a conventional test light (not a low current
test light) and found them to be okay. All body-to-engine block ground wires
were clean and tight. All bulbs were of the correct trade number as specified in
the owner’s manual.
NOTE: Using a single-filament bulb (ex: #1156) in place of a dualfilament bulb (ex: #1157) could also cause many of the same problems.

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The Short-To-Voltage Story - Part 2
Because the trouble occurred when the brake pedal was depressed, the
tech decided to trace all the wires in the brake light circuit. The problem
was found near the exhaust system.
A small hole in the tailpipe (after the muffler) directed hot exhaust gases
to the wiring harness containing all of the wires for circuits at the rear of
the truck. The heat melted the insulation causing most of the wires to
touch.
Whenever one circuit was activated (such as when the brake pedal was
applied), the current had a complete path to several other circuits.
A fuse did not blow because there was enough resistance in the circuits
being energized that the current (in amps) was too low to blow any fuses.

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Short-to-Ground A short-to-ground is a type of circuit failure
wherein the current bypasses part of the normal circuit and flows
directly to ground.
Because the ground return circuit is metal (vehicle frame, engine, or
body), this type of circuit is identified as having current flowing
from copper to steel. See Figure 32–7.
A defective component or circuit shorted to ground is commonly
called grounded.
For example, if a penny was inserted into a cigarette lighter socket,
current would flow through the penny to ground. Because the penny
has little resistance, an excessive amount of current flow causes the
fuse to blow.
Continued
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Figure 32–7 A short-to-ground affects the power side of the circuit. Current flows directly to
the ground return, bypassing some or all of the electrical loads in the circuit. There is no
current in the circuit past the short.

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High Resistance Another common fault is excessive resistance in
the circuit. This can be caused by several circuit faults including:
Corrosion of wires on the terminals
Poor electrical connections at connectors
Loose ground connection
Any of the above will cause current in amperes to decrease in the
circuit. As a result, the electrical load device may operate, but with
reduced speed or brightness.
High-resistance faults can also be intermittent and cause problems
just when conditions or temperatures cause a problem in the circuit.

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Think of a Waterwheel - Part 1
A beginner technician cleaned the positive terminal of the battery to correct
the problem of slow cranking. When questioned by the shop foreman as to
why only the positive post had been cleaned, the technician responded
that the negative terminal was “only a ground.” The foreman reminded the
technician that the current, in amperes, is constant throughout a series
circuit (such as the cranking motor circuit). If 200 amperes leaves the
positive post of the battery, then 200 amperes must return to the battery
through the negative post.
The technician just could not understand how electricity can do work
(crank an engine), yet return the same amount of current, in amperes, as
left the battery.
The shop foreman explained that even though the current is constant
throughout the circuit, the voltage (electrical pressure or potential) is
dropped to zero in the circuit. To explain further, the shop foreman drew a
waterwheel.
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Think of a Waterwheel - Part 2
As water drops from a higher level to a lower level, high potential energy
(or voltage) is used to turn the waterwheel and results in low potential
energy (or lower voltage). The same amount of water (or amperes)
reaches the pond under the waterwheel as started the fall above the
waterwheel. As current (amperes) flows through a conductor, it performs
work in the circuit (turns the waterwheel) while its voltage (potential) is
dropped.

Figure 32–8
Electrical flow through a circuit is similar to water

flowing over a waterwheel. The more the water
(amperes in electricity), the greater the amount of
work (waterwheel). The amount of water remains
constant, yet the pressure (voltage in electricity)
drops as the current flows through the circuit.

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OHM’S LAW
German physicist, Georg Ohm, established that electric pressure
(EMF) in volts, electrical resistance in ohms, and the amount of

current in amperes flowing through any circuit are all related.
Ohm’s law states:
It requires 1 volt to push 1 ampere through 1 ohm of resistance.
This means that if the voltage is doubled, then the number of
amperes of current flowing through a circuit will also double if the
resistance of the circuit remains the same.
Ohm’s law can also be stated as a simple formula used to calculate
one value of an electrical circuit if the other two are known. See
Figure 32–9.
Continued
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Figure 32–9 To calculate one unit of electricity when the other two are known, simply use your
finger and cover the unit you do not know. For example, if both voltage (E) and resistance (R)
are known, cover the letter I (amperes). Notice that the letter E is above the letter R so divide
the resistor’s value into the voltage to determine the current in the circuit.

Continued
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OHM’S LAW STATED
I = E/R where:

I = Current in amperes (A)
E = Electromotive force (EMF) in volts (V)
R = Resistance in ohms (Ω)

1. Ohm’s law can determine the resistance if the volts and
amperes are known: R = E/I.
2. Ohm’s law can determine the voltage if the resistance
(ohms) and amperes are known: E = I × R.
3. Ohm’s law can determine the amperes if the resistance
and voltage are known: I = E/R.
Continued
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OHM’S LAW RELATIONSHIPS

See the chart on Page 328 of your textbook.
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Ohm’s Law Applied to Simple Circuits If a battery with 12 volts
is connected to a resistor of 4 ohms, as shown here, how many
amperes will flow through the circuit?
Using Ohm’s law, we can calculate the number of amperes that will

flow through the wires and the resistor.
Figure 32–10 This closed circuit includes
a power source, power-side wire, circuit
protection (fuse), resistance (bulb), and
return path wire.

If two factors are known (volts
and ohms in this example), the
remaining factor (amperes) can
be calculated using Ohm’s law.
Continued
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I = E/R = 12 V/4Ω
The values for voltage (12) and resistance (4) were substituted for
the variables E and R, making I = 3 amperes (12/4 = 3).
If we want to connect a resistor to a 12-volt battery, we now know
that this simple circuit requires 3 amperes to operate.
This may help us for two reasons.

1. We can now determine the wire diameter that we will need
based on the number of amperes flowing through the circuit.
2. The correct fuse rating can be selected to protect the circuit.

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