air condition BMW 540I 2000 E39 Central Body Electronics ZKE Manual

13
Central Body Electronics ZKE III
Principle of Operation
The optical infra red portion of the sensor operates by the principle of refraction (bending
of a light ray). The rain sensor control module activates the emitter diode which sends a
beam of infra red light through the windshield on an angle. The set angle is important
because it provides the beam with a calculated reflective path back to the detector diode.
The beam is reflected back into the windshield due to the density difference of the glass
compared with the ambient air on the outside surface of the glass. When the windshield is
clean (no rain drops, moisture or dirt) the detector diode receives 100% of the infra red light
that the was sent by the emitter. With this condition, the rain sensor evaluation electronics
determines the windshield is free of rain drops.
The density of water is closer to that of glass than air. When rain starts to accumulate in
the sensor monitoring area, it causes part of the infra red beam to extend past the outside
surface of the glass and into the rain drop. When this occurs, the beam is refracted and
only part of the beam returns to the detector diode.
The rain sensor evaluation elec-
tronics determines the windshield
has a few rain drops (or dirt) on it.
The intensity of the returned infra red beam diminishes proportionally with an increase of
water droplets. The rain sensor control module generates a signal proportionate to the
amount of rain on the windshield and broadcasts it to the GM via the K bus.
The GM activates the intermittent wipe cycle if the windshield wiper stalk switch is in the
intermittent position. It also adjusts the frequency of wiping the windshield depending on
the four position thumb wheel.
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