I don't care about your temps at the vent. If you live anywhere except a desert with 0-10% humidity, you'll be experiencing an iced coil with those kinds of vent temps. I'm speaking from experience here, not just heresay.

R134a is the same cost or cheaper than R152a and on a properly charged and functioning system cools just as good if not better than R152a (especially at idle where R152a struggles). Last summer with it being 109˚ in the summer here, my SDL's A/C was blowing a cool 38˚ at the vents running on 134a and the completely stock system. When set up properly and charged by someone who knows what they're doing, it works just fine.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)