I had to replace the evap on my 90 560SEL.

Get thee a sniffer and see where the freon is leaking from. For my car, it was alarming whenever I held it in front of the vents.

The expansion valve is under the dash on the left hand side outside of the HVAC plenum, so a leak at the EXP valve will not show up at the vents. You can exchange the exp valve without dissassembly of the HVAC unit.

It's a tedious job, but no harder than the W124 evap I had to exchange on my wife's 95 E320 wagon....

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles