Page 329 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Electrical systems 311

cruising at a set speed, the driver can press and hold a button to accelerate the
vehicle until the desired speed is reached. If the driver accelerates from the set
speed to overtake, for example, then when the throttle is released, the vehicle
will slow down again.
Several methods are used to control the throttle position. Vehicles fi tted with
Key fact
drive-by-wire systems allow the cruise control to operate the same throttle
actuator. A motor can be used to control the throttle cable or, in some cases Vehicles fi tted with drive-by-wire
( Fig. 3.110 ), a vacuum-operated diaphragm is used. systems allow the cruise control to
operate the throttle actuator.
The brake switch is very important, as braking would be dangerous if the cruise
control system was still trying to maintain the vehicle speed. This switch is
normally of superior quality and is fi tted in place or as a supplement to the brake
light switch activated by the brake pedal. Adjustment of this switch is important.
The clutch switch is fi tted in a similar manner to the brake switch. It deactivates
the cruise system to prevent the engine speed increasing, if the clutch is
pressed. The automatic gearbox switch will only allow the cruise to be engaged
when it is in the ‘drive’ position. This is to prevent the engine overspeeding if the
cruise tried to accelerate to a high road speed with the gear selector in position
‘1’ or ‘2’.
The speed sensor will often be the same sensor that is used for the speedometer
( Fig. 3.111 ). If not, several types are available, the most common producing a
pulsed signal, the frequency of which is proportional to the vehicle speed.
Conventional cruise control has now developed to a high degree of quality. It is,
however, not always very practical on many roads as the speed of the general
traffi c is constantly varying and often very heavy. The driver has to take over
from the cruise control system on many occasions to speed up or slow down.
Adaptive cruise control can automatically adjust the vehicle speed to the current
traffi c situation. The system has three main aims:
to maintain a speed as set by the driver
●
to adapt this speed and maintain a safe distance from the vehicles in front
●

to provide a warning if there is a risk of collision.
●
The operation of an adaptive cruise system is similar to a conventional system.
However, when a signal from the headway sensor ( Fig. 3.112 ) detects an
obstruction, the vehicle speed is decreased. If the optimum stopping distance

Figure 3.110 Throttle motor connected to cable