Automotive Wiring And Circuit Diagrams.pdf

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CHAPTER

Wiring and

Circuit Diagrams

Upon completion and review of this chapter, you should be able to:

Explain when single-stranded or

multistranded wire should be used.

Explain how temperature affects

resistance and wire size selection.

Explain the use of resistive wires in a

circuit.

Explain the purpose and use of printed

circuits.

Describe the construction of spark plug

wires.

Explain why wiring harnesses are used

and how they are constructed.

Explain the purpose of wiring diagrams.

Explain how wire size is determined by

the American Wire Gauge (AWG) and

metric methods.

Identify the common electrical symbols

that are used.

Describe how to determine the correct

wire gauge to be used in a circuit.

Explain the purpose of the component

locator.

Introduction

Today’s vehicles have a vast amount of electrical wiring that, if laid end to end, could stretch for

half a mile or more. Today’s technician must be proficient at reading wiring diagrams in order to

sort though this great maze of wires. Trying to locate the cause of an electrical problem can be

quite difficult if you do not have a good understanding of wiring systems and diagrams.

In this chapter, you will learn how wiring harnesses are made, how to read the wiring

diagram, how to interpret the symbols used, and how terminals are used. This will reduce

the amount of confusion you may experience when repairing an electrical circuit. It is also

important to understand how to determine the correct type and size of wire to carry the

anticipated amount of current. It is possible to cause an electrical problem by simply using

the wrong gauge size of wire. A technician must understand the three factors that cause

resistance in a wire—length, diameter, and temperature—to perform repairs correctly.

Automotive Wiring

Primary wiring is the term used for conductors that carry low voltage. The insulation of pri-

mary wires is usually thin. Secondary wiring refers to wires used to carry high voltage, such

as ignition spark plug wires. Secondary wires have extra thick insulation.

Most of the primary wiring conductors used in the automobile are made of several strands

of copper wire wound together and covered with a polyvinyl chloride (PVC) insulation (Figure

4-1). Copper has low resistance and can be connected to easily by using crimping connectors or

soldered connections. Other types of conductor materials used in automobiles include silver,

gold, aluminum, and tin-plated brass.

Primary wiring

refers to smaller wire

with light insulation.

Secondary wiring

refers to larger wire

or cable with heavier

insulation.

AUTHOR’S NOTE: Copper is used mainly because of its low cost and availability.

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PVC insulation

Stranded conductor

Figure 4-1 Stranded primary wire.

Stranded wire

means the

conductor is made

of several individual

wires that are

wrapped together.

Stranded wire is used because it is very flexible and has less resistance than solid wire.

This is because electrons tend to flow on the outside surface of conductors. Since there is more

surface area exposed in a stranded wire (each strand has its own surface), there is less resis-

tance in the stranded wire than in the solid wire (Figure 4-2). The PVC insulation is used

because it can withstand temperature extremes and corrosion. PVC insulation is also capable of

withstanding battery acid, antifreeze, and gasoline. The insulation protects the wire from short-

ing to ground and from corrosion.

A ballast resistor

reduces the current

flow through the

ignition coil to

increase the life of

the coil. The

resistance value of

the ballast resistor is

usually between 0.8

and 1.2 ohms.

AUTHOR’S NOTE: General Motors has used single-stranded aluminum wire in

limited applications where no flexing of the wire is expected. For example, it is used

in the taillight circuits.

A ballast resistor is used by some manufacturers to protect the ignition primary circuit

from excessive voltage. It reduces the current flow through the coil’s primary windings and

provides a stable voltage to the coil. Some automobiles use a resistance wire in the ignition

system instead of a ballast resistor. This wire is called the ballast resistor wire and is located

between the ignition switch and the ignition coil (Figure 4-3) in the ignition “RUN” circuit.

Spark plug wires are also resistance wires. The resistance lowers the current flow through

the wires. By keeping current flow low, the magnetic field created around the wires is kept to

Resistance wire is

designed with a

certain amount of

resistance per foot.

Electrons

Spark plug wires are

often referred to as

television-radio-

suppression (TVRS)

cables.

Electrons

Conductors

Conductor

(A) Stranded wire

(B) Single strand wire

Figure 4-2 Stranded wire provides flexibility and more surface area for electron flow than a

single-strand solid wire.

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Battery

Switch

Starting bypass

Ballast

(primary

resistor)

Coil

primary

winding

Coil secondary winding

Figure 4-3 Ballast resistor used in some ignition primary wiring circuits.

a minimum. The magnetic field needs to be controlled because it causes radio interference.

The result of this interference is noise on the vehicle’s radio and all nearby radios and televi-

sions. The noise can interfere with emergency broadcasts and the radios of emergency vehi-

cles. Because of this concern, all ignition systems are designed to minimize radio interference;

most do so with resistance-type spark plug wires. Spark plug wires are targeted because they

carry high voltage pulses. The lower current flow has no adverse effect on the firing of the

spark plug.

Most spark plug wire conductors are made of nylon, rayon, fiberglass, or aramid thread

impregnated with carbon. This core is surrounded by rubber (Figure 4-4). The carbon-impreg-

nated core provides sufficient resistance to reduce RFI, yet does not affect engine operation. As

the spark plug wires wear because of age and temperature changes, the resistance in the wire

will change. Most plug wires have a resistance value of 3,000 Ω

to 6,000 Ω

per foot. However,

some have between 6,000 Ω

and 12,000 Ω

. The accepted value when testing is 10,000 Ω

per

foot as a general specification.

Because the high voltage within the plug wires can create electromagnetic induction,

proper wire routing is important to eliminate the possibility of cross-fire . To prevent cross-fire,

the plug wires must be installed in the proper separator. Any two parallel wires next to each

other in the firing order should be positioned as far away from each other as possible (Figure

4-5). When induction cross-fire occurs, no spark is jumped from one wire to the other. The

spark is the result of induction from another field. Cross-fire induction is most common in two

parallel wires that fire one after the other in the firing order.

Cross-fire is the

electromagnetic

induction spark that

can be transmitted

in another wire close

to the wire carrying

the current.

Glass and

cotton braid

Insulation

Jacket

Core

• hypalon – normal

• silicon – high temperature

Figure 4-4 Typical spark plug wire.

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Cable bracket

Plug cables

7 and 8

separated

in cable

bracket

Firing orders and

cylinder numbering

vary among the

different engines

Example:

firing order

1-5-4-2-6-3-7-8

Left bank

engine

cylinders

Figure 4-5 Proper spark plug wire routing to prevent cross-fire. (Reprinted with the permission of

Ford Motor Company)

Wire Sizes

An additional amount of consideration must be given for some margin of safety when

selecting wire size. There are three major factors that determine the proper size of wire to be

used:

The number

assigned to a wire to

indicate its size is

referred to as

gauge.

1. The wire must have a large enough diameter, for the length required, to carry the

necessary current for the load components in the circuit to operate properly.

2. The wire must be able to withstand the anticipated vibration.

3. The wire must be able to withstand the anticipated amount of heat exposure.

Wire size is based on the diameter of the conductor. The larger the diameter, the less the

resistance. There are two common size standards used to designate wire size: American Wire

Gauge (AWG) and metric.

The AWG standard assigns a gauge number to the wire based on its diameter. The higher

the number, the smaller the wire diameter. For example, 20-gauge wire is smaller in diameter

than 10-gauge wire. Most electrical systems in the automobile use 14-, 16-, or 18-gauge wire.

Some high current circuits will also use 10- or 12-gauge wire. Most battery cables are 2-, 4-, or

6-gauge cable.

Both wire diameter and wire length affect resistance. Sixteen-gauge wire is capable of

conducting 20 amperes for 10 feet with minimal voltage drop. However, if the current is to be

carried for 15 feet, 14-gauge wire would be required. If 20 amperes were required to be carried

for 20 feet, then 12-gauge wire would be required. The additional wire size is needed to pre-

vent voltage drops in the wire. The illustration (Figure 4-6) lists the wire size required to carry

a given amount of current for different lengths.

Another factor to wire resistance is temperature. An increase in temperature creates a sim-

ilar increase in resistance. A wire may have a known resistance of 0.03 ohms per 10 feet at

70°F. When exposed to temperatures of 170°F, the resistance may increase to 0.04 ohms per 10

feet. Wires that are to be installed in areas that experience high temperatures, as in the engine

compartment, must be of a size such that the increased resistance will not affect the operation

of the load component. Also, the insulation of the wire must be capable of withstanding the

high temperatures.

In the metric system, wire size is determined by the cross-sectional area of the wire. Met-

ric wire size is expressed in square millimeters (mm 2 ). In this system the smaller the number,

the smaller the wire conductor. The approximate equivalent wire size of metric to AWG is

shown (Figure 4-7).

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To t a l

Approximate

Circuit

Amperes

Wire Gauge (for Length in Feet)

12 V

1.0

1.5

100

150

200

Note: 18 AWG as indicated above this line could be 20 AWG electrically.

18 AWG is recommended for mechanical strength.

Figure 4-6 The distance the current must be carried is a factor in determining the correct wire

gauge to use.

Metric Size (mm 2 )

AWG (Gauge) Size

Ampere Capacity

0.5

0.8

1.0

2.0

3.0

5.0

8.0

13.0

19.0

Figure 4-7 Approximate AWG to metric equivalents.

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