Note that there is also a valve on the drier that monitors the gas level and if there is no gas in the system it stops the compressor from engaging, in order to protect the system.
On the Mercs, you will find that they can be sensitive to the amount of gas in the system.....meaning that your system may not be totally empty but its not empty enough to stop the compressor, but it is empty enough not to produce any cold air.
Id get the gas levels rechecked if the compressor is OK. You either have a leak or the system that filled you car, (and they are mostly automatic) hasn't done the job properly for some reason.
I recently had this experience from Kwickfit. I recently replaced my condenser and drier due to a minor leak and took it down to Kwickfit for a recharge. The plonker who did the recharge wasn't too friendly, didnt know where to connect the hoses until I showed him and then he told me the car would be ready in 20 min !!!!!! It usually takes 20 min to vacuum the system let alone fill it properly.
I came back in 20 min and sure enough the car was out and he was fighting with the controls, didn't even know how to switch the aircon on, and then said to me " its no good, your compressor is knackered" !!!!
A quick look under the bonnet showed that the compressor was running and he was talking a load of rubbish, so I took it into another place, they found very little gas in the system, properly regassed it, which took over an hour and fine since.
