Seems like Steve has taken care of you... but just to further assist you, I would check for fault codes in the system.
First of all, see if your poly-V-belt is slipping. Number 52 is for stored codes (on my 95 C280...), hopefully yours is the same... or similar to that. And for me 51 is the number of current slips.
After you do that, have the key in the on position, but not with the car on. Turn the system to LO & Auto. Then press the REST button and the DEFROST (front) for about 5 seconds or until diA appears in the LCD. By pressing the AUTO button, you will come across several codes... record what numbers you get, and post them back. Good luck!
(Hope this was clear.)
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)