Rear screen wiper description (Wipers & washers)

REAR SCREEN WIPER DESCRIPTION

The rear screen wiper functions are controlled from a push button switch located to the right of the instrument pack or by the selection of reverse gear when the front wipers are operating. The Central Control Unit (CCU) is responsible for monitoring and controlling the operations of the rear screen wiper motor.

The rear screen wiper has three modes of operation; programmed mode, intermittent and slow.

• Programmed mode operates when the rear screen wiper is selected by operation of the wash/wipe switch. The rear screen washer and the wiper operate when the switch is depressed. When the switch is released, the washers stop and the wiper continues for five cycles before stopping.
• Intermittent operates when the rear screen wiper is selected using the latching push button switch. The wiper operates for three complete cycles, then the CCU operates the wiper intermittently every five seconds until the wiper is selected off.
• Slow mode operates when reverse gear is selected and the front wipers are on. The rear wiper operates until reverse gear is disengaged or the front wipers are switched off.

The rear screen wiper is operated by a single electric motor which indirectly drives, via an eccentric drive mechanism, a splined shaft to which the wiper arm is attached.

The motor is a single speed dc motor with permanent magnets. The motor shaft is fitted with a worm drive which rotates a gear wheel. The gear wheel has an offset spigot to which an eccentric cam is fitted which can rotate through 180° in each direction. A coil spring is located around the cam and operates as a simple friction clutch to enable the cam to be rotated when the motor is driven in reverse. A connecting link is attached to the cam and drives two gears located in a link. The connecting link converts the rotary motion of the gear wheel into linear movement of the link. The linear movement is converted back into rotary movement of the splined shaft which moves the wiper arm in an arc in two directions across the rear screen.

The wiper motor has the ability to park the wiper arm off the rear screen when the wiper is selected off. When the motor is operating to wipe the rear screen, the motor shaft is rotating in a clockwise direction. When the rear screen wiper is selected off, the CCU allows the wiper arm to reach the vertical position on the screen. A micro switch actuated by a slip ring on the underside of the gear wheel, signals that the wiper arm has reached this position. The CCU then reverses the polarity of the supply to the motor which causes the motor shaft to rotate in an anti-clockwise direction.

When the motor changes direction, the eccentric cam is rotated through 180° which effectively lengthens the linear movement of the connecting link. This causes the wiper arm to sweep across the screen to its park position. The longer linear movement of the connecting link, causes the wiper arm to rotate further than during normal wiping, parking the wiper arm and blade off screen. A second slip ring on the underside of the gear wheel, signals the CCU that the wiper arm has reached its off screen position and the CCU removes the supply to the rear screen wiper motor.

The wiper arm is located on the splined shaft from the motor and secured with a nut. The wiper arm attachment to the splined shaft has a pivot to which the remainder of the arm is attached. The two parts of the arm are connected by a spring which controls the pressure of the blade on the screen to a predetermined amount.

The wiper blade is pushed onto the wiper arm which has a clip-on feature that allows the blade to pivot. The wiper blade comprises a lever, two yokes and a plastic strip. The rubber wiper is fitted to the plastic strip and held in the yokes. The lever, yokes and plastic strip ensure that pressure applied by the arm spring is distributed evenly along the length of the blade.