2. With the ignition off, disconnect the connector on top of the pressure switch connector.
3. Start the engine, let it idle and turn on the air conditioner on the control panel.
4. Gently wipe the viewing window on top of the dehydrator unit (see fig. 8.4).
Fig. 8.4. Air conditioning system dehydrator unit
A Viewing window
In Pressure Sensor Connector
5. Connect the pressure switch connector while watching through the viewing window. This connection should cause the electromagnetic engagement and when this happens, liquid should appear in the field of view. Continue to watch the liquid level, which should rise above the top of the viewing window, until a flow of liquid without bubbles is visible.
(Material borrowed from an online resource: MercedesMan.ru)
6. If the fluid level is lower than required or there is a pressure drop in the cooling system, the pressure switch will prevent electromagnetic engagement and the air conditioning system will not work. In this case, the car should be taken to a Mercedes service station or an air conditioning specialist for inspection and filling. Do not attempt to block the pressure switch to restore the air conditioning system, as this may cause serious damage to the system.