Rain sensor & headlight sensor testing - repair instructions & fault diagnosis

Initially integrated in the mirror base of the rear-view mirror, the rain sensor is fitted on the inside of the front windshield outside the driver's field of vision. Within the sensor area, the sensor detects when it is raining and transmits its information to the windshield wiper's control electronics. The wiper frequency in the intermittent wiper setting can therefore be automatically adapted to the intensity of the rain. Thus, manual intervention by the driver is now more or less unnecessary. 

The combined rain-light sensor combines two functions in one module.

• Detection of rain and automatic control of the windshield wiper
• Recording of ambient brightness and automatically switching the vehicle lighting on and off

The rain sensor is usually activated by actuating the intermittent or automatic wiper function on the steering column switch (Figure 2).

Turning the light switch to the AUTO position (automatic daytime running light control) activates the light sensor and the lighting is switched on or off depending on the ambient brightness (Figure 3). 

The rain sensor detects precipitation on the windshield using an optoelectronic method of measurement. The sensor element comprises one or more light emitting diodes (transmitters), a prism and a photodiode (receiver). A light beam produced by the light emitting diode passes via a prism to the windshield, is reflected several times by the outer windshield surface and forwarded to the photodiode.

The combination of sensor position and dry windshield surface enables maximum reflection of the light beam (Figure 4). 

Raindrops on the windshield change the reflection properties to the extent that all light beams no longer reach their target and are deflected by the drops of water (Figure 5). 

The greater the intensity of the rain, the less light reaches the photodiode. The evaluation electronics use the irradiance to calculate the amount of rain currently on the windshield and transmit the information needed from the windshield electronics in order to control the wiper speed. The sensor continually takes measurements so that wiping can be individually adapted to the amount of precipitation. If heavy rain or spray from a vehicle driving in front is detected, the system automatically switches from the intermittent setting to the highest wiper setting. 

The external lighting conditions are measured by an optoelectronic detector. Using an upstream filter glass, the sensor records special wavelengths in order to distinguish between artificial light and daylight. The ambient light and front-end lighting are measured by two sensors, which function independently of one another. 

The illuminance around the vehicle is recorded using the ambient light sensor over a large spatial angle (Figure 6). 

The light intensity in front of the vehicle is recorded with the front-end light sensor over a small angle (Figure 7). 

In order to do this, the sensor's receiver characteristics must be matched to the installation situation. A special algorithm uses the sensor data and by including further information from the vehicle electronics detects various lighting conditions (day, night, dusk or passing through a tunnel/bridge) and switches the daytime running light on or off accordingly. 

The system overview illustrates how the sensor can communicate with other system components in the vehicle via the LIN interface by way of example. Here the sensor is switched on by the overriding control unit and supplied with voltage. The sensor only provides the system with information, but does not itself have direct access to the system and/or actuators. 

Should the rain sensor malfunction, the wiper module of the vehicle in question takes over intermittent-wiper control. Regardless of the sensor function, the windshield wiper can always be actuated manually using the control lever.
 
Should the light sensor fail, the daytime running light is switched on in the safe "Light on" status. Regardless of the sensor function, the light can however be switched on and off manually via the switch.