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

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Horns, Wiper, and Washer System Operation 307Figure 14-5. The horn relay can be mounted on thefuse panel. (DaimlerChrysler Corporation)HornsExcept for Chrysler’s air horn, which uses airpressure from the compressor, automobile hornsuse electromagnetism to vibrate a diaphragm andproduce sound waves. A typical horn containsnormally closed contact points in series witha coil. One of the contact points is mounted on amovable armature to which the horn diaphragmis connected.The horn coil is in series with the horn switchor horn relay contacts. When the horn switch orhorn relay contacts close, current flows throughthe horn coil to ground. The electromagnetic fieldcreated by the coil attracts the armature, alsomoving the diaphragm. The armature movementopens the contact points, which open the coilcircuit. With no magnetic field to hold them, thearmature and diaphragm move back to theirnormal positions. The points are again closed,allowing current to flow through the coil. Thismaking and breaking of the electromagneticcircuit causes the horn diaphragm to vibrate.Since this cycle occurs very rapidly, the resultingrapid movements of the diaphragm createsound waves. The speed or frequency of thecycling determines the pitch of the sound created.This can be adjusted by changing the spring tensionon the horn armature to increase or decreasethe electromagnetic pull on the diaphragm. The History from the Bell to theElectric HornMany types of signal alarms have been used oncars as follows:• Mechanical bell• Bulb horn• Compression whistle• Exhaust horn• Hand-operated horn (Klaxon)• Electric hornThe mechanical bell was used on very early cars;the driver operated the bell with a foot pedal. Thebulb horn, similar to that on a child’s bicycle, provedto be inconvenient and unreliable.The compressionwhistle was most often used in cars with no batteryor limited battery capacity; a profiled cylinder providedthe whistle’s power. Exhaust horns usedgases from the engine exhaust; they, too, were footoperated.The hand-operated Klaxon horn amplifieda grating sound caused by a metal tooth ridingover a metal gear. This did not work well, becausethe horn had to be near the driver rather than at thefront of the car. Over the years, the electric horn hasbeen the most popular type of signal alarm.WINDSHIELD WIPERSAND WASHERSFederal law requires that all cars built in, orimported into, the United States since 1968 haveboth a two-windshield wiper system and a windshieldwasher system. Wiper systems on oldervehicles may be operated by engine vacuum or bythe power steering hydraulic system.Modern wiper systems are operated by electricmotors. The washer system can be manually operated,or it can have an electric pump. Many vehiclesalso have a single-speed wiper and washerfor the rear window. This is a completely separatesystem, but it operates in the same way as thewindshield wiper and washer system.Circuit DiagramA typical two-speed wiper system circuit diagramis shown in Figure 14-6. The motor fields are permanentmagnets. The wiper switch controls boththe wiper motor speeds and the washer pump. Thepark switch within the wiper motor ensures that

308 Chapter FourteenFigure 14-7.Low-speed current flow.Figure 14-6.A simple two-speed wiper circuit.when the wiper switch is turned off, the motorwill continue to turn until the wiper arms havereached the bottom edge, or park position, of thewindshield. The circuit shown has a circuitbreaker built into the wiper switch. The circuitbreaker also can be a separate unit, or it can bemounted on the wiper motor.Figure 14-7 shows low-speed current flowthrough the simple circuit. Current flows throughthe wiper switch contacts, the low-speed brush L,and the common (shared) brush C to ground.During high-speed operation, the current flowsthrough the high-speed brush H and the commonbrush to ground. When the wiper switch is turnedto park, or off, the park switch comes into thecircuit.The park switch is a two-position, camoperatedswitch within the wiper motor. It movesfrom one position to the other during each motorrevolution. When the wiper arms are at their parkFigure 14-8. The park switch allows the motor tocontinue turning until the wiper arms reach their parkposition.position, the park switch is at the P contact, asshown in Figure 14-8. No current flows throughthe park switch. At all other wiper arm positions,the park switch is held against spring tensionat the other contact. If the wiper switch is turned

Horns, Wiper, and Washer System Operation 307

Figure 14-5. The horn relay can be mounted on the

fuse panel. (DaimlerChrysler Corporation)

Horns

Except for Chrysler’s air horn, which uses air

pressure from the compressor, automobile horns

use electromagnetism to vibrate a diaphragm and

produce sound waves. A typical horn contains

normally closed contact points in series with

a coil. One of the contact points is mounted on a

movable armature to which the horn diaphragm

is connected.

The horn coil is in series with the horn switch

or horn relay contacts. When the horn switch or

horn relay contacts close, current flows through

the horn coil to ground. The electromagnetic field

created by the coil attracts the armature, also

moving the diaphragm. The armature movement

opens the contact points, which open the coil

circuit. With no magnetic field to hold them, the

armature and diaphragm move back to their

normal positions. The points are again closed,

allowing current to flow through the coil. This

making and breaking of the electromagnetic

circuit causes the horn diaphragm to vibrate.

Since this cycle occurs very rapidly, the resulting

rapid movements of the diaphragm create

sound waves. The speed or frequency of the

cycling determines the pitch of the sound created.

This can be adjusted by changing the spring tension

on the horn armature to increase or decrease

the electromagnetic pull on the diaphragm.

The History from the Bell to the

Electric Horn

Many types of signal alarms have been used on

cars as follows:

• Mechanical bell

• Bulb horn

• Compression whistle

• Exhaust horn

• Hand-operated horn (Klaxon)

• Electric horn

The mechanical bell was used on very early cars;

the driver operated the bell with a foot pedal. The

bulb horn, similar to that on a child’s bicycle, proved

to be inconvenient and unreliable.The compression

whistle was most often used in cars with no battery

or limited battery capacity; a profiled cylinder provided

the whistle’s power. Exhaust horns used

gases from the engine exhaust; they, too, were footoperated.

The hand-operated Klaxon horn amplified

a grating sound caused by a metal tooth riding

over a metal gear. This did not work well, because

the horn had to be near the driver rather than at the

front of the car. Over the years, the electric horn has

been the most popular type of signal alarm.

WINDSHIELD WIPERS

AND WASHERS

Federal law requires that all cars built in, or

imported into, the United States since 1968 have

both a two-windshield wiper system and a windshield

washer system. Wiper systems on older

vehicles may be operated by engine vacuum or by

the power steering hydraulic system.

Modern wiper systems are operated by electric

motors. The washer system can be manually operated,

or it can have an electric pump. Many vehicles

also have a single-speed wiper and washer

for the rear window. This is a completely separate

system, but it operates in the same way as the

windshield wiper and washer system.

Circuit Diagram

A typical two-speed wiper system circuit diagram

is shown in Figure 14-6. The motor fields are permanent

magnets. The wiper switch controls both

the wiper motor speeds and the washer pump. The

park switch within the wiper motor ensures that

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