Air con makes strange hissing noises
 

1999 SL500, 118k miles.

The air conditioner in my car intermittently makes a hissing noise for about 3 seconds every 30 seconds or so. It sounds like it is exhaling or breathing out. The sound comes from the centre vent nozzles. It does not happen when the "EC" button is pressed (i.e. the air conditioner is disengaged). I am not sure how well the air conditioning function is working (it's not very hot around here at the moment); it seems to be OK but not particularly brilliant. Any ideas?

It's probably a little low on refrigerant...that's one sign when the level is getting low you will hear it hiss through the expansion valve.

If it is getting worse then you will hear the hissing all the time when the compressor is running, then it will stop working all together...then you know you have a real leak, otherwise you might just need to have it topped off.

Wow, thanks for the quick reply!

I really question if the refrigerant is low. My 1990 SL has done this since I bought it and both the A/C and heater work great. I seems to do this when the climate control system is making an adjustment. I believe this is normal operation. Does anyone elses SL do this?

My car has always done that and my mechanic says it is "normal" for the 129s (at least for my 1999 129)

Well, my dealer removed and replaced the refrigerant free of charge. He said the level was correct. The car still makes the hissing noise, albeit the air con works fantastically. My wife says it has always made that noise. I am beginning to wonder whether it has always made the hissing noise, too. I suspect it has, and that I didn't pay any attention to it until a passenger asked what it was.

Could it be the vacuum operating the flap behind the centre air vents ?
This opens and closes as the blower speed varies on Automatic with the air con on.
N T-J