Air conditioner in 2003 s500

Could be anything from low Freon to a faulty sensor. You'll have to get some gauges and see the high and low side pressures. If there are no leaks and the system is holding refrigerant then you might be looking at a sensor issue such as a low or high pressure switch, evap temp sensor, etc. Other thoughts are compressor clutch is not engaging due to either faulty clutch or coil, perhaps bad fuse or relay. On some Mercedes climate control heads you can do a basic diagnostic of the climate control system not sure if this would apply to yours. So that's all the ideas I have. Good Luck!