Ambient Light Sensor: Description and Operation

Rain / driving light sensor

The rain / driving light sensor comprises the following sensors and components:

Rain sensor: consisting of 3 individual sensors that pick up the precipitation quantity on the windscreen.

Driving light sensor: consisting of 2 photo diodes that pick up the brightness of the ambient light and of the frontal light.

Brief description of components
The rain / driving light sensor is bonded to the inside of the windscreen in the wiper range of the windscreen wiper.







Index Explanation
1 Photo diode to pick up the surrounding brightness
2 Optical element of the rain sensor
3 Photo diode to pick up the frontal light


This installation location ensures that the sensor surface remains free of dirt and ice. To prevent the sensor surface from fogging over, the sensor is heated.

The rain / driving light sensor is supplied with voltage by the junction box. The signals of the rain / driving light sensor are transferred across the LIN bus to the junction box electronics.







Index Explanation
1 Engine control module
2 Windscreen wiper motor
3 Pump for the headlight-cleaning system
4 Windscreen washing pump
5 RLS rain-driving light sensor
6 JBE junction box electronics
7 Rear window wiper motor
8 SZL steering column switching center


Rain sensor
The rain sensor is an optical sensor. It consists of an optical element. The optical element is firmly bonded to the windscreen. An electronic module is also integrated. Alongside the electronic evaluation unit, 3 optical transmitter and receiver diodes are integrated in the electronic module.

Driving light sensor
The driving light sensor is also an optical sensor. It consists of 2 photo diodes and the corresponding electronic evaluation unit. One photo diode is aligned forwards and it picks up the light that strikes the front (frontal light). The second diode is aligned upwards and it picks up the light striking from above (surrounding brightness). The diode to pick up the frontal light is installed in the housing of the interior mirror in the opposite direction to the direction of travel.



System functions
The following system functions are described for the rain / driving light sensor:
- Detection of precipitation
- Detection for driving lights

Detection of precipitation
If the intermittent mode is switched on at the wiper switch, the rain sensor is activated (lightly push the right-hand steering-column lever upwards).

First of all, one wipe cycle is run as visual feedback of the switching on. Then, the 3 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 3 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 amount of reflected light is thus the measurement of the intensity of the precipitation in the area of the optical element. The electronic evaluation unit uses the missing amount of light to detect the degree of wetness on the windscreen. It sends a signal across the LIN bus. On the basis of this signal, it is assessed in the junction box electronics whether a wipe cycle is necessary.

The sensitivity of the rain sensor can be varied in 4 stages by means of a knurled wheel on the wiper/washer switch. Each operation of the knurled wheel towards "Increase sensitivity" (i.e. turning the knurled wheel upwards) runs a wipe cycle.

With the vehicle stationary and failure of the rain / driving light sensor, the windscreen wiper is switched into a fixed intermittent operation mode. On switching off terminal R, the rain sensor is switched off for safety reasons. To reactivate the rain sensor, the intermittent mode must be set again at the wiper switch.

Detection for driving lights
When the light switch is in the position for Automatic Light Control, the driving light sensor is activated. The change in lighting conditions in front of the vehicle and in the environment of the vehicle is picked up by the light-sensitive sensors. The lighting condition is processed by the electronic evaluation unit and sent across the LIN bus to the junction box-electronics. Depending on the brightness, with the Automatic Light Control active, the low-beam headlight is switched on and off.







Index Explanation
1 Switch position Side lights
2 Switch position Low beam
3 Switch position 'Off'
4 Switch position Automatic Light Control


Notes for Service department
The following general information about the rain / driving light sensor is provided for service staff:

General information
A dry and undamaged windscreen surface is necessary for installation of the rain / driving light sensor. Furthermore, the sensor must be bonded to the windscreen without bubbles.
For fault-free operation of the rain / driving light sensor, there may be no streaks on the windscreen. Moreover, the wiper blades must be OK.

Diagnosis instructions

Initialization of the rain sensor
In order to ensure perfect function of the optical measurement procedure for the detection of precipitation, the rain sensor must be adapted to the windscreen.

Requirement for faultless initialization is that the windscreen glass is dry, clean and free of defects in the area of the rain sensor.

The initialization is performed using the service function "Initialize rain sensor" in the BMW diagnosis system. In this service function, the adaptation values in the rain sensor are deleted. Subsequently, the sensor system is adapted to the windscreen in the following sequence:
- Switch off the rain sensor
- Switch off terminal 15 and terminal R
- Switch on terminal R and terminal 15 and wait for 10 seconds

In the 10-second waiting period, the new adaptation values are determined automatically and saved by the sensor.

A re-initialization of the rain sensor is required:
- After replacing the windshield
- After installation of a rain / driving light sensor

Notes on encoding / programming

Coding
After installation of a rain / driving light sensor, the sensor must be recoded.