In your 2nd gen W126, you'll never see a vent temperature lower than ~40-41˚, even with a perfectly functioning A/C. There's a sensor on the evaporator that cuts off the compressor around that point to keep the coil from frosting. On a HOT day, once the car has cooled down inside, you should be seeing somewhere around 45˚ at the center vent when moving, around 48-50˚ when idle.

A P/T table is only good if you're running R12 with the original expansion valve and an unclogged filter/drier. If your car has been converted to R134a or any of the "alternative" refrigerants, forget about going by the book.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

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

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]