I just had this happen to me on another non-MB car. It can be deceiving. My problem was caused by a stuck TXV. After replacing the TXV and R/D, I discovered a massive leak from an O-ring on the high side. I suspect what happened was:

1. TXV got stuck
2. high side pressure got too high
3. O-ring on high side became compromised
4. refrigerant leaked, causing what appeared to be a simple low charge condition

I checked my pressures when it first started acting up, and it was about 300 on the high side and vacuum on the low side (R12). When I checked again later in the afternoon it was about 100 on the high side and vacuum on the low side.

I recommend checking for leaks, particularly on the high side. After charging, you will know pretty quickly whether or not your TXV is stuck.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280