The rain sensor works by the principle of refraction (bending light???). Try sticking your finger into a bucket of water. See how your finger appears to bend? That's refraction, at least the way I understand it.

From your description, it sounds like the problem can be fixed by simply cleaning the outside of the glass in the area of the sensor.

If the wiper leaves even a light film, or streak of water in this area, the sensor will request another wipe. You can check this easily, by looking closely at the area of the windshield where the sensor is located, during a wipe. if you see any streaks, or if a slight film of moisture remains, clean this area thoroughly. I've everything from regular car wax to glass compound, with good success. Of course, make sure the wiper blade isn't the problem. Let us know how what happens.

Yes, the sensor has two parts. One LED, light emitting diode, and
a photo sensing diode, located some centimeters apart.

The LED sends light into the windscreen. When there no water on the windscreen, the light is reflected back and forth inside the screen, like in a optical fibre, and reaching the photosensor.

When water comes into contact with the screen, light "escapes", so less light is seen at the photosensor end. That means, that any water (or other contamination that will let the light "escape"), on either side! of the screen, can cause the wiper to wipe.

I think they have some kind of material on the inside, to reflect light, but it will not hurt to clean the inside as well?

Do also remember, that the sensor must be fastened correctly to the inside, or else, the amount of light, going into the screen, and escaping a few centimeters away, will be insufficient to declare the screen as clean:-)

br,
syljua

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1998 CL500 W140
1984 230E W123