Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Rain Sensor Module (RSM)
The rain sensor (RSM) detects water on the windshield via optic measurement. An infrared light beam of known intensity illuminates part of the windshield. The intensity of the reflected beam is measured and compared with the transmitted intensity. If there is water on the surface of the windshield, the light beam is deflected a bit and will lose some of its intensity. If the beam is not reflected in its entirety, it is interpreted as water on the windshield and the windshield wipers are activated. On vehicles with rain sensor (RSM), the automatic function replaces the standard interval wipe function.
The rain sensor module (RSM) transmits signals to the central electronic module (CEM) using serial communication. The signals contain information about:
- how much rain is hitting the windshield
- a sudden splash on the windshield
- the outside light conditions.
The central electronic module (CEM) requests a certain windshield wiper speed based on this information.
The rain sensor module (RSM) is in front of the rear view mirror on the inside of the windshield. Cars with rain sensor modules (RSM) have a special windshield.
There are diagnostics for the rain sensor module (RSM).
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.