As of the model year 2003.75 a rain sensor is fitted in conjunction with the heated windshield and the interior mirror with automatic glare adjustment.
This sensor is integrated in a shared housing with the sun load sensor unit for the automatic headlamps. The unit is located behind a plastic cover behind the interior mirror.
The rain sensor consists of an opto-electronic measuring and evaluation circuit. The sensor can determine how much rain is landing on the windshield, and can use this information to determine the appropriate setting for the windshield wipers.
This information is passed on to the automatic headlamp module, which controls both the automatic headlamps and the windshield wipers. This module is located to the right of the steering column behind the dash panel (LHD vehicles).
On the basis of the signals provided by the rain sensor, the windshield wipers are then automatically set to the required wipe speed. The automatic control options range from intermittent mode to maximum wipe speed (level II).
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Light emitting diode (LED)
NOTE:The rain sensor is an optical measuring instrument. Contamination (oil, grease, dust etc.) will prevent it from operating correctly. Therefore the windshield must be kept clean, particularly in the measuring region of the sensor.
The value measured without water on the windshield is used as the reference value for the automatic calibration process. Subsequent deviations from this value cause the windshield wipers to be switched on.
If rain lands on the windshield then the light reflected by the windshield has a lower intensity. This loss of intensity is registered by the photodiode and, proportionally to the loss of intensity, the module switches on the windshield wipers with the required wipe speed (in intermittent or continuous mode).
When the automatic windshield wipers are switched on (wiper switch set to intermittent mode) the rain sensor is switched on and performs an automatic calibration according to the current conditions at the windshield.
To perform the automatic calibration, the windshield wipers perform a single wipe regardless of whether the windshield is wet or dry.
If the windshield remains dry then the windshield wipers stop until moisture is registered on the windshield above the sensor.
On vehicles built from 12/2004, automatic calibration only takes place if the wiper switch was not set to the automatic function before the ignition was switched on.
On vehicles built up to 10/2004, the sensitivity of the rain sensor is not adjustable. Permanent changes to the windshield, e.g. due to stone impact, are detected by the system but do not influence the function of the sensor.
On vehicles built from 10/2004, the sensitivity of the rain sensor is adjustable via the rotary switch on the wiper control, which is identical to the rotary switch for intermittent mode on vehicles without automatic windshield wipers.
- Switch setting 1: High sensitivity
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The wipers wipe even if only a small amount of water has been measured on the windshield.
- Switch setting 6: Low sensitivity
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The wipers only wipe if a large amount of water has been measured on the windshield.
Diagnosis
The modular layout allows the sensor to be replaced separately in the event of a defect. If the windshield wipers only come on in intermittent mode when the automatic windshield wipe function is switched on then there is a defect in the sensor.
If a fault is detected while the wipers are in operation then the wipers are left running at the last selected speed.
In the event of failure of the sensor the windshield wipers can still be operated manually. If the wiper lever is moved to the intermittent position, wiping is always performed at an interval of approx. 8 seconds.
If the sensor is sound but no output voltage is measured by the module then a new module should be installed. Diagnosis of the module is not possible.
Diagnosis cannot be performed on the sensor or module.
