Check high and low pressures first to be sure they are out of spec. Undercharging will produce subpar A/C performance, but overcharging will kill the system.

The low pressure port is on the suction hose which is typically larger in diameter than the liquid line and is often wrapped in some sort of foam insulation. It's the one that gets cold and can sometimes be observed "sweating" when the system is running.

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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