The rain sensor is behind the driving mirror in contact with the windscreen and has the purpose of automatically adjust, during the intermittent operation, the frequency of the windscreen wiper strokes as to the rain intensity.
The sensor has a range of adjustment that gradually varies between wiper stationary (no wiping) when the windscreen is dry, to wiper at first continuous speed (continuous slow) with heavy rain.
See also:
How to start diesel versions
1) Ensure that the handbrake is up.
2) Put the gear lever into neutral.
3) Turn the ignition key to MAR. Instrument panel warning lightswill
come on.
4) Wait for warning lights
to turn off.
5 ...
Low battery charge
Turning the key to MAR the warning light on the dial turns on, but it should
go out as soon as the engine is started (with the engine running at idle speed a
brief delay in going out is allowed) ...