First step is to verify charge. Of course you'll never know exactly how much is in there since you didn't charge via weight, but at least you can get close. Manifold gauges are a good start; the W210 also has self diagnostics that you can use. If the charge seems OK, you can begin exploring other options.

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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