The combined rain-light sensor
Here you will find useful knowledge and handy tips relating to the rain sensor - light sensor in vehicles.
Important safety information
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.
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.
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.
Given the modular structure, up to five functions can be integrated in the current generation of rain-light sensors.
In addition to recording rain and light, the following sensor functions can also be covered as options:
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.
The following service work is illustrated on a 4th generation rain-light sensor by way of example, but can also be undertaken on sensors constructed in the same way.
This video provides important repair instructions on replacing the rain-light sensor.
In order to remove the rain sensor from the windshield without any damage, the pressure on the coupling medium (gel) must first be relaxed. This takes approximately 30 minutes at 20°C. The coupling medium may be destroyed by rapid, careless movements or jolty pulling actions.
Air inclusions of up to 1 mm in diameter are permitted between the windshield and coupling medium immediately after assembly. Over time these inclusions are displaced by the force exerted by the sensor springs.
The optical sensor is fastened on the windshield with the aid of a coupling medium, which forms a level layer on the optical sensor. The coupling medium (gel) smoothens out unevenness between the optical sensor and windshield, producing a defined optical interface. Incorrect assembly may result in malfunctions or sensor failure.
Depending on vehicle manufacturer, different fastening elements or coupling media may be used. Therefore, please observe the relevant vehicle manufacturer's removal and installation instructions !
In the time between disassembly and reassembly, the sensor must be protected from dirt, mechanical and electrical damage. The sensitive coupling medium should be covered by a clean and dry plastic film. To protect the sensor from electrostatic damage, it should be put into intermediate storage in a dry plastic box.
This video shows you how to carry out a basic function check of the rain sensor and troubleshoot using a diagnostic unit.
The function of the rain sensor can be easily tested by simulating precipitation on the windscreen.
The windshield wipers should now start and the wiper frequency adapt automatically to the simulated "rain intensity". If there is no response but all other manually activated wiper stages are working properly, a diagnostic unit is needed for further testing.
The individual functions of the rain-light sensor are monitored by the respective superordinate control units. Any errors which occur are stored in the control unit's error memory and can be read using a suitable diagnostic unit. Depending on the system, additional parameters can be displayed and used for troubleshooting.
The following diagnostics functions can be used as part of troubleshooting:
In this function, the fault codes stored in the error memory can be read out and deleted. Information about the fault code can also be called up.
In this function, the current measured values of the sensor can be displayed in the control unit and evaluated.
In this function, the sensor can be taught on the control unit. The rain sensor must be initialised after the following work:
The various diagnostic options have been shown on the mega macs 66 diagnostic unit by way of example. The respective extent of testing and variety of functions may differ depending on the vehicle manufacturer and depend on the respective system configuration of the control unit.