Also, like I said, do a search on the HVAC self-diagnostic mode. I think you can actually get system pressures in that mode (someone here correct me if I'm wrong). That might be useful in the absence of manifold gauges and might be able to tell you if the charge is low to begin with.

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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