Detection of precipitation
When the automatic wiper operation is switched ON, the function LED on the wash/wipe switch lights up green. One wipe cycle is run as an additional visual acknowledgment of switching on.
The three transmit diodes emit infrared light that is routed via the optical element to the windscreen. If the windscreen is completely dry and clean in the area of the optical element, the infrared light is fully reflected to the three receiver diodes.
If there is precipitation or dirt in the area of the optical element on the windscreen, the infrared light is refracted. This means that only part of the infrared light is reflected to the receiver diodes. The reflected light quantity is thus the measurement of the intensity of the precipitation in the area of the optical element. On the basis of the missing light quantity, the evaluation electronics detects the windscreen's degree of wetness and requests wiper activity that is appropriate to this (e.g. one wipe cycle, continuous wipe or intermittent operation at different wiper frequencies).
The sensitivity of the rain sensor can be varied in 4 sensitivity stages by means of a knurled wheel on the wash/wipe switch. Each operation of the knurled wheel towards "Increase sensitivity" (i.e. turning the knurled wheel upwards) runs a wipe cycle.
When the vehicle is stationary, the system automatically switches back to stationary mode. That means: The wiper system wipes at a wiper frequency that is one interval stage lower.
Example:
Whenever the driving speed is reduced during continuous wipe at interval stage 1 such that the vehicle comes to a standstill, wiper operation switches from continuous wipe to intermittent operation. Once the driving speed rises back above a specific value, the continuous wipe action is resumed in wiper stage 1. Whenever the driving speed is reduced to such a point that the vehicle comes to a standstill during continuous wiping in wiper stage 2, wiper operation switches as follows: From continuous wipe in wiper state 2 to continuous wipe in wiper stage 1. When the driving speed exceeds a certain value again, the continuous wipe in wiper stage 2 becomes active again.
If the sensor fails, the system switches the windscreen wiper to fixed intermittent operation.
As of 09/2011, the protection function "Ice detection" will be introduced. At outside temperatures of below 0 °C (observe outside temperature display in the instrument panel) and with the rain sensor activated, automatic wiper operation is not possible. The following functions are suppressed:
- Single wipe if automatic wiper operation is activated
- The other wiper cycles if the windscreen is wet
As a result of the suppressed functions, possible damage to the windscreen wiper, for example due to frozen wiper blades, is avoided.
When a speed of 7 km/h is exceeded after the engine start for the first time, the protection function "Ice detection" switches off automatically. As of this time, automatic wiper operation is possible once again without restriction until the next time the engine is switched off and the vehicle enters the sleep mode. Manual wiper operation is not affected by the protection function "Ice detection". That means: wiper stage 1 and wiper stage 2 can always be set manually.