Windscreen Wipers and Washers (Automobile)

Auxiliary Equipments and Safety Control Systems

Windscreen wiper is essential for keeping the windscreen sufficiently clean for driver’s visibility specifically for modern high speed vehicles. The washer cleans the driver’s side of the windscreen whenever required. Each motor vehicle is equipped with an audible warning device i.e. a horh. Whenever the driver intends to take a turn or to overtake as a pre-warning, directional indicating lights are used. Central door locking and electronically operated windows are becoming quite common in cars. Even some vehicles use headlight wipers and washers. Electric movement of seats, mirrors and the sun roof are also becoming common in vehicles. Cruise control system in introduced as a means of reducing driver’s fatigue on long motorway journeys.
The supplementary restraint system is an air-bag system, which works in conjunc­tion with conventional 3-point seat belts and prevents, during a frontal impact, striking of driver’s chest and face with the steering wheel. As a part of vehicle security system, recently alarm system has been built as an integral part of the vehicle electronics. The obstacle avoidance radar provides an indication to the driver of how much space is there behind the car and collision avoidance radar can be used as a vision enhancement system.
The principle of adaptive noise control used in the vehicle is that the original noise is picked up by a microphone, inverted and amplified, and replayed by a suitably positioned speaker, which effectively cancels out the noise. In-car entertainment and communication systems, nowadays, are commonly used in vehicles. The chapter discusses all these auxiliary equipments and safety control systems in reasonable details.

Windscreen Wipers and Washers



The main purpose of the wiper system is to clean the windscreen sufficiently to provide suitable visibility at all times. The wiper system must perform the following tasks.
(a) Efficient removal of dirt, water and snow.
(b) Operation in the temperature range of 243 K to 353 K.
(c) The ability to pass the stall and snow load test.
(rf) A service life of around 1500 000 wipe cycles, (e) Resistance to corrosion from acid, alkalis and ozone. For meeting the above requirements, proper design and manufacturing with good quality components are required for both the wiper and washer systems. The actual method of cleaning the screen by the blades can vary provided that the legally prescribed area of the screen is cleaned. Almost all of the wipers are operated electrically. Also, today it is a common practice to fit two wiper blades for the front windscreen and both blades driven from a single motor. As per the law the wiper on the driver’s side must operate effectively and efficiently. Hatchback cars often use a wiper for the rear window and some expansive cars also install wipers for the headlamps.
Considerable driving force is required for a rubber wiper blade to move across a glass surface, especially when the blade has to sweep away a large volume of water or snow. The windscreen of modern vehicles have a double curvature, which requires long articulated wiper blades with the ability to flex to the contour of the glass. Wiper systems generally use two wipe speeds to suit the driving conditions and also an intermittent wipe facility is incorporated.
A car wiper motor on a modern vehicle should be a high powered quiet unit operating on a current of 2 – 4 A. In the past, shunt-wound motors were used but now-a-days the permanent-magnet motor is commonly used. The layout of a typical wiper system is shown in Fig. 30.1. A worm on the armature shaft drives a worm wheel connected to a crank to provide the reciprocating action needed to oscillate the wiper blades. The gear mechanism provides the speed reduction and the torque increase required to drive the wiper blades.
Layout of a typical link type wiper system (simplified).
Fig. 30.1. Layout of a typical link type wiper system (simplified).

Wiper Motor Permanent-magnet Type.

The construction of a single speed motor is shown in Fig. 30.2. The armature with 8-slots is mounted on self lubricating sintered bushes. Two carbon brushes, set 180 degrees apart, rub on an 8 segment commutator generally installed at the driving end. Two strong permanent magnets are bonded to the steel yoke using an adhesive, which is sometimes coated externally with non-ferrous metal to protect it against corrosion. A steel worm, formed on the end of the armature, drives a plastic worm wheel at a speed of about l/10th the speed of the armature. The motor (Fig. 30.2) has the output drive through a pinion gears, driven directly by the worm wheel. At the joint faces of the motor, rubber seals are fitted to protect it from moisture. A polythene pipe is used to vent the gases formed by arcing at the brushes.
Single-speed motor.
Fig. 30.2. Single-speed motor.
The wiper motors now in use (Fig. 30.3) are mostly of permanent magnet three brush types, which are driven through a worm gear to increase torque and reduce speed. The three brushes permit two speed operations. The nor­mal speed is achieved through two brushes placed in the usual position opposite to each other. For a fast speed the third brush is installed closer to the earth brush.
This design reduces the number of armature windings between them, which reduces resistance and consequently increases current and hence speed. Typical values for wiper motor speed and hence wipe frequency are 45 rpm and 65 rpm at normal and fast speed respectively. The
motor must overcome the starting friction of each blade at a minimum speed of 5 rpm. The following equation can be used to calculate torque required by the motor.
 Typical wiper motor (Lucas).
Fig. 30.3. Typical wiper motor (Lucas).
Figure 30.4 shows the characteristics of a typical car wiper motor. The two sets of curves correspond to fast and slow speed.

Two-speed Operation.

As indicated above the two-speed operation is achieved normally by using three brushes. This third brush is thinner than the main brushes and is placed as shown in Fig. 30.5A.
Characteristic curves of a wiper motor.
Fig. 30.4. Characteristic curves of a wiper motor.
Two-speed operation. A. Brush assembly. B. Interconnection of coils. C. Lap wound armature..
Fig. 30.5. Two-speed operation. A. Brush assembly. B. Interconnection of coils. C. Lap wound armature..
When the current is supplied to ‘B’, a low wipe rate of about 45 wiping cycles per minute is achieved, which increases to about 65 when the supply is made available to terminal ‘C. This rise in wipe rate is due to an increase in the current flow through the motor. When brushes ‘A’ and ‘C are in use, the shorter armature path between ‘A’ and ‘C (Fig. 30.5B) allows a larger current flow, which provides a higher rotational speed. As the speed is increased a rise in back e.m.f. reduces the current flow. The diagram illustrates the interconnection of the coils of a lap wound type of armature normally used for a wiper motor. High speed of operation should be avoided when there is a heavy load on the wiper blade, for example in heavy snow or on a dry windscreen.

Shunt Wound Motors.

These motors are rarely used nowadays due to the superiority of the permanent magnet type in respect of power, noise, efficiency, cost, reliability and current consumption. Figure 30.6A shows the layout of a single speed motor using a shunt wound field and Fig. 30.6B illustrates the circuit having a limit switch to provide a self switching facility.
Shunt wound wiper motor.
Fig. 30.6. Shunt wound wiper motor.

Two-speed Operation.

Figure 30.7 illustrates one of the arrangements for achieving a two speed operation. During a speed operation, the current flow­ing through the field winding a divided between the armature and the field. When the switch is moved to high speed operation a resistor is inserted in the shunt field, thereby causing a larger current to pass through the armature resulting in an increase in the motor speed.

Control of wipers

Self-switching Action.

During non-operation period of the wiper, the blades should be set so that they are at the end of their wiping stroke. In practice it is difficult to stop the blades in this position and hence to meet this requirement a limit switch is used. The gearbox of the wiper motor controls and limit switch, which is opened only when the wiper blades are at one end of their stroke. The principle of the limit switch is shown in Fig. 30.8. When the driver switches-off the motor the limit switch continues to supply current until the park position is reached.
Even with this switch, the blade always does not stop at the correct position due to the momentum of the moving parts. This problem is removed by using a regenerative braking. When the motor is switched-off, another set of contacts on the limit switch connect the two main
Two-speed wiper motor circuit.
Fig. 30.7. Two-speed wiper motor circuit.
Limit switch to give self switching action.
Fig. 30.8. Limit switch to give self switching action.
brushes together (Fig. 30.9). Consequently, the current generated by the moving armature creates a load on the armature, which provides a braking action to quickly bring the motor to rest.

Intermittent Wipe.

Due to spray from passing vehicles and light drizzle con­ditions the screen is required to be wiped intermittently. To provide this facility most vehicles have a switch position. To overcome the regenerative braking action provided on a permanent magnet type motor, a current pulse of comparatively long duration is required, which rotates the armature sufficiently to move the limit switch from its braked position. A semiconductor controlled relay is installed on most vehicles to provide this function. The time period between wipes is controlled by the action of a capacitor. The resistance-capacitance (R- C) of a circuit governs this time constant and by varying either R or C the interval can be varied to suit the requirement. The electronic circuit layout shown in Fig. 30.10A illustrates an intermittent wipe action.

In the diagram, two switches interconnect the two main brushes and relay contacts ’1′. The regenerative braking works when the contacts are set in this position. When current flows to terminal ‘A’, the relay is energized and the contacts are closed. This connects to earth causing
 Regenerative braking.
Fig. 30.9. Regenerative braking.
Intermittent wipe control. A. Main circuit for intermittent wipe control. B. Relay control circuit.
Fig. 30.10. Intermittent wipe control. A. Main circuit for intermittent wipe control. B. Relay control circuit.
the motor to operate, irrespective of the position of the limit switch. But if the supply is stopped from A at this point of time, the relay opens and the contacts ’1′ close. Then the motor continues to operate until the earth contact is broken at the limit switch.
Figure 30.10B shows the control circuit for the relay. Once the intermittent wipe switch is closed, the sequence of operation commences as follows :
(j) Current flows through the base of T2 to earth via R\, which switches-on T2 and
energizes the relay to start the motor. («) The motor moves the limit switch to the earth position. Current from T% passes to the limit switch via i?6 and deactivates the relay. This closes the relay contacts ’1′ and provides an alternative path from the negative brush to earth due to which the motor continues to operate.
When the limit switch makes its earth contact, current passes through the base of T\. This switches-on T\ and switches-off T2 causing the capacitor to charge.
(iv) Further rotation of the motor moves the limit switch to the stop position causing the motor to stop suddenly. Although current flow from T\ and T4 ceases, T\ is prevented from switching-off by the discharge current from the capacitor. This current flows in the sub-circuit through the base of T\ and resistances R2 and R3.
(v) After about five seconds the capacitor is charged so that T\ switches-off and conse­quently Ti switches on to repeat the cycle.
To suit the conditions these intervals can be varied on some vehicles by fitting variable resistor control in the capacitor-discharge sub-circuit in place of resistor i?3-

Self-parking Wipers.

On some vehicles the wiper blades are parked off the windscreen. To achieve this arrange­ment the circuit is switched-on so that the current through the armature is reversed after the motor has stopped. This charges the polarity of the brush in the permanent magnet motor so that the armature rotates in the opposite direction. By arranging the gearbox linkage the wiping stroke can be extended by the reverse motion, and this movement parks the wiper blades away from the glass screen.
Overload Protection. During snow or ice conditions the load on the wiper motor increases heavily, which decreases motor speed and under extreme conditions stops it. The decrease in armature speed reduces back e.m.f. due to which is large current in the order of 11A flows through the motor leading to overheating and possible damage to the motor.
For the protection of the motor, a thermal switch is connected in series with the supply lead. A bimetallic strip controls the switch. When the strip is heated by a higher-than-normal current, the contacts are opened.

Mechanical Drive Systems

The motor is normally installed remote from the wiper blades, thereby a mechanical drive is required to transfer the motion to the blades. The two main mechanical drives used are link and flexible rack systems.

Link System.

The link system (Fig. 30.1) is efficient and uses a crank on the output shaft of the motor to reciprocate a transverse link. This in turn drives the levers and partially rotates the shafts connecting the wiper arms. The relative lengths of the levers vary the angle of sweep of the wipers. Self-lubricating bushes are normally used at each connection.

Flexible Rack System.

This system (Fig. 30.11) is more compact and quieter than the link system. Also in this arrangement the motor is located in an accessible place, normally under the bonnet. A crank pin on a worm wheel drives a rod, which connects with, and reciprocates, a flexible rack contained in a rigid tube. The rack is similar to a speedometer cable except that it is wrapped with a wire to from a thread. Drive from the rack to the wiper is by means of pinion, which engages with the rack teeth. Each pinion is secured in a wheel box (gearbox), and its casing is screwed to the rigid tube.
Flexible rack drive.
Fig. 30.11. Flexible rack drive.


As per the statutory regulations in some countries a screen washer must be installed to clean the driver’s side of the Windscreen. Most of the vehicles install an electrically-operated pump to supply water or cleaning fluid in the form of two or more jets for spraying on the windscreen. On some vehicles an extra pump is used for a separate headlamp wash system, and some of these vehicles are also fitted with headlamp wipers.
The small centrifugal pump is either fixed directly on to the water reservoir or fitted in the hydraulic line. A permanent magnet motor drives the pump and is controlled by a switch often operated from the wiper switch stalk on the steering column (Fig. 30.12). The pump is of self-priming type and a filter is installed at the inlet of the pump. Polythene tubing is used for the jets. A typical motor consumes about 3 A and supplies about 0.75 litre/min fluid at a pressure of 66 kPa. During winter period a small quantity of methylated spirit is added to the water to lower the freezing temperature.

Wiper Blades

The wiper blades are made of rubber compound and are held on to the screen by a spring in the wiper arm. The aerodynamic properties of the wiper blades are extremely important, because with different vehicle designs, different air currents are set on and around the screen area. To reduce air drug, the strip on top of the rubber element is often perforated. The blades have very small contact area on the screen. A good quality blade has a contact width of about 0.01 mm and the tip wipes the surfaces of the screen at an angle of about 45 degrees. The pressure of the blade on the screen should be proper as the coefficient of friction between the rubber and glass varies from 0.8 to 2.5 when dry and 0.1 to 0.6 when wet. These values are also affected by temperature and velocity.
Bosch wiper system controls the pressure of the blade onto the screen, which varies infinitely depend­ing on vehicle speed. At high speeds, the air stream can cause normal blades to lift and judder, which seriously reduce their cleaning effectiveness. If the original pres­sure is set to compensate for this problem the pressure at rest could deform the arms and blades. In fact in the rest position of the blades, pressure should be very low to avoid damage. The pressure should rise with increase vehicle speed and heavy rain.
Sensor in the Bosch pressure control system (Fig. 30.13) determines the air stream velocity and the intensity of the rain. An ECU evaluates the data available from these sensors and in turn passes an appropriate signal to the servo motor. The system responds very quickly to the situation. For example when the car is overtaking, the deluge of spray is cleared by increased pressure and if the screen dries off, the pressure is reduced to prevent scrapping.
Windscreen washer.
Fig. 30.12. Windscreen washer.
Wiper blade pressure control system.
Fig. 30.13. Wiper blade pressure control system.

Maintenance of Wiper Systems

Clear visibility is a must for safe driving ; therefore a check on the operation of the driver’s wiper and washer is included in the annual M.O.T. test. Routine maintenance should include the following:

Wiper Blades.

Blades should be replaced when the rubber starts cracking, tearing or becomes hard and brittle. The metal portion of the blade should be perfect and the fixing to the arm must be proper. A wiper blade must not work on the dry screen as this overloads the motor and also severely scratches the surface of the glass.


Methylated sprit maybe used to remove traffic film from the screen and wiper blades. The screen and blade surface should not be contaminated with polishes containing silicone and wax.

Wiper Arms.

These arms should be inspected for the perfect condition of the spring to provide required force (about 350 grams) to the blade. The arm should not be bent as this can cause the blade to chatter during operation.

Wiper Faults


Noise occurs due to slackness or tightness of the mechanical drive system. Also it occurs when the moving parts in the linkage come in contact with other parts such as the metal tubing of the screen washer tube.
When noise is not found through a visual inspection, then each part is checked independently Flexible racks are checked for tightness, and also the force required to move the rack through the tube is measured when it is disconnected from the motor and wheel-box. A maximum force of 27 N is a typical value. The tube holding the rack must be free from dent or kink and the radius of any bend should be more than 230 mm. The rack should be lubricated with a grease to provide a smooth movement.

Motor Faults.

Unable to operate and low operating speed are two normal faults that can take place in the motor. In both the cases it is necessary to check the motor by using a voltmeter to ensure that the motor is receiving the full battery voltage. To test the motor is situ, a pair of test leads is used to connect the motor directly to the battery. However a spare wiper motor plug simplifies this job. This test indicates the possible faults in the switch and wiring.
After a reasonably long time of operation the brushes wear down and the commutator becomes dirty. On many models it is recommended to replace the brush when the main brushes are worn to a length of less than 5 mm, or the stepped portion of the third brush has worn away. The complete new brush sets including springs and plastic mounting plate is normally reinstalled. The commutator should be cleaned using a petrol-moistened rag or a strip of glass-paper when the surface is extremely blackened. Some motors use a screw for adjustment of the armature end-float, a typical setting of which is 0.2 mm.

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