Extract from BMW TIS explains how it works
Rain/driving lights sensor (RLS)
The rain-driving light sensor has integrated sensors that sense the wetness on the windscreen glass and the ambient light.
Rain sensor
The rain sensor is an optical system. It consists of an optical element bonded on the windscreen and an electronic element. The electronic evaluation unit and 4 optical transmitter and receiver diodes are integrated in the electronic element.
Function
The rain sensor is activated when the wiper switch is moved to the first stop position.
When the rain sensor is activated, visual feedback is given to the driver in that one wiping cycle is executed first. Then, the transmitter diodes emit an infrared light which is guided across the optical element to the windscreen. If the windscreen is completely dry and clean, the infrared light is fully reflected to the receiver diodes.
If there is water or dirt in the area of the optical element on the windscreen, full reflection of the infrared light is no longer possible. Part of the light penetrates the windscreen so that only part of the light arrives at the receiver diodes. The electronic circuitry detects how much water is on the windscreen on the basis of the amount of light that is being blocked out and send a signal to that effect via the K bus. The ZKE basic module then performs a wipe cycle. In the case of very heavy rain, the intermittent operation goes into stage 1 or stage 2 of the continuous wiping mode.
A heating element integrated in the rain sensor prevents misting of the optical element or the diodes.
The sensitivity of the rain sensor can be varied in 4 sensitivity stages by means of the knurled wheel on the wiper/washer switch. Each time the adjuster wheel is moved in the ”Increase sensitivity” direction (upwards), a wipe cycle is carried out.
When the vehicle is at a standstill, there is an automatic reduction of the sensitivity so that the wiper usually runs in intermittent mode.
If the rain sensor fails, the wiper runs at a fixed time interval.
For safety reasons, the rain sensor is switched off when terminal R is switched off.
Requirements for faultless operation of the rain sensor are:
No wax smear on the windscreen
Wiper blades OK
Windscreen glass free of faults in the area of the rain sensor
Optical element bonded free of bubbles with the windscreen glass
Initialisation
Due to the optical process, the rain sensor has to be adapted to the windscreen. This adaptation takes place at initialisation.
Requirement for faultless initialisation is that the windscreen glass is dry, clean and free of defects in the area of the rain sensor.
The initialisation is performed using the service function ”Initialise rain sensor” in the BMW diagnosis system. In the service function, the adaptation values in the rain sensor are deleted. This is followed by adjustment of the sensor system to the windscreen based on the following sequence:
Switching off the rain sensor
Switch off terminal 15 and terminal R
Switch on terminal R and terminal 15 and wait 10 seconds
In the 10 seconds waiting period, the new adaptation values are determined and stored.
Initialisation of the rain sensor is necessary:
After replacing the windscreen
Fitting a used rain-driving light sensor
Note: a used sensor must be coded beforehand!
Driving light sensor
The driving light sensor consists of two photodiodes and an electronic evaluation unit. The photodiodes are arranged in such a way that both light striking the sensor from the front and light striking the sensor from above (ambient light) are sensed.
If there is a change in the light conditions, a signal is sent across the K bus. If the automatic headlights system is active, the light control centre switches the headlights on or off.
The light sensing is active as of terminal R, independent of whether the automatic driving light system is switched on or off.
Adjustment of the driving light sensor on the windscreen is by means of coding. This is why a used sensor must be recoded on installation!
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