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 TM 9-8000
air system. Air horns also may have their own
is actuated when the transmission is shifted to reverse.
compressed air system. The air horn is controlled by an
The purpose of this is to warn pedestrians when the
air valve.
vehicle is backing up.
17-13. Backup Alarms. Large vehicles some- times
utilize an air or electric horn at the rear that
Section III. WINDSHIELD WIPES
utilizes a pulley and cable system to link the motor to the
17-14. Vacuum Wipers. Vacuum-operated windshield
windshield wiper arms and blades. Because of the erratic
wipers utilize the negative pressure that Is present in the
operation of the vacuum wiper system, it has been
engine's intake manifold as a vacuum source to operate
replaced almost completely by the electric wiper system
the windshield wiper motor. Because the pressure in the
in modern automotive applications.
intake manifold varies with engine loading, the speed of
the wipers varies greatly with engine loading to the extent
that the windshield wipers actually will stop when the
engine is loaded heavily, such as when the vehicle Is
wipers are the most popular type for use on modern
climbing a hill. Some vehicles utilize an auxiliary vacuum
automotive equipment. Electric windshield wipers
pump to help alleviate this problem. This pump usually is
operate at a constant, easily controlled speed, making
built into the engine's fuel pump (para 4-3). The
them much more desirable than their vacuum
windshield wiper motor transmits its power to the trans-
counterparts. One of the drawbacks to using electric
mission linkage in the form of reciprocating motion. The
windshield wipers in early automotive equipment was the
transmission
for
a
vacuum
system
heavy electric current requirements that their motor
Figure 17-26. Electric Wipers
17-20
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