After few hours driving in town with a lot of traffic my AC was working fine, keeping low temperature inside. Then I had to stop the engine for few minutes and when I turned it on, although the AC button was pushed (glows green), the air coming out was hot. It seems to be working (after I press the AC button there is a "click" coming out of the compressor, along with clearly noticeable change in engine sound (slight RPM increse)), but it behaves as if there is no coolant in the system. After I opened the hood to check electrical connections, belts, etc I noticed a lot of small (oily) droplets all over and around the compressor.
The only thing I thought of is if there is some kind of pressure valve releasing the system freon (if the auxiliary fan is not rotating and the temperature is too high or in any other extreme case)?
If anyone has experienced anything similar or has any idea what could be the cause, please share your thoughts.
P.S. The car is BMW E39.