Detection of precipitation

If interval stage is switched on at the wash/wipe switch, the rain sensor is activated (lightly push the drop arm upwards). First of all, one wipe cycle is run as visual acknowledgment of the switching on. Then, the 4 transmit diodes emit infrared light that is routed via the optical element to the windscreen. If the windscreen is completely dry and clean, the infrared light is fully reflected to the 4 receiver diodes.

If there is water 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. The evaluation electronics use the missing light quantity to detect the degree of wetness on the windscreen. They transmit a signal via the LIN bus. On the basis of this signal, it is assessed in the junction box electronics whether a wipe cycle is necessary.

With the vehicle stationary, the sensitivity is adapted automatically. The windscreen wiper is switched to intermittent operation. On switching off terminal R, the rain sensor is switched off for safety reasons. To reactivate the rain sensor, the interval stage must be set again at the wash/wipe switch.