Rain/driving light sensor: restrictions due to technical features

Functional description, see BMW Service Technology:

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SBT number:

- Automatic headlamp control

Dusk characteristics
Due to the technological features used, the photo-transistors suitable for automobiles that are used in the rain/driving light sensor are sensitive to light in the infrared range. In weather conditions with dense clouds, infrared light has a very intense effect on the rain/driving light sensor.
During fair weather with low cloud levels or blue skies - above all in the winter season - there is very much less infrared light.
The harmonization for poor weather conditions, which is relevant to safety, can thus mean in individual cases that the vehicle is driven with the lights on even when the weather is fair.
Tunnel characteristics
The very bright rows of yellow lamps that are fitted at an increasing number of tunnel entrances mean that the RLS only switches on the low-beam headlight after a delay.
Malfunctions
If a fault occurs in one of the sensors and control modules involved, the light is switched on for safety reasons until the end of the journey.

The rain/driving light sensor evaluates the degree of reflection of the windshield in order to control windshield wiper operation. Even small areas of dirt, such as those caused by insects, can in some cases trigger wiper operations.

A special variant of the RLS is fitted on vehicles with HUD. On this variant, one of the sensors is not pointed upwards to detect tunnels, but rather downwards towards the road. This sensor regulates the brightness of the HUD. Under no circumstances may the variant for HUD be replaced with a standard variant!