My SDL is running R12, which is arguably more effective than the R152A and I'm not getting those kinds of results either. Center vent blows 40-45 once the cabin cools down and stays there. If it gets to 38, the compressor trips off and comes back on when the temp gets to 45 or so. Reading up on how the ACC system works, there's a thermostat to shut the system off at 40˚ to keep the evaporator from freezing.

Take any and all readings with a probe style thermometer, preferably one with some mass to it to even out the readings you're taking. IR guns are notoriously inaccurate with cold temperatures and with the way the A/C systems in these cars work, I'd be impressed to see a vent temp much below 35˚ in recirc mode with the fan on low.

To get a negative temperature at the vent on a 100˚ day would mean that the system has to have a >100˚ temp split which doesn't exist in any refrigeration system.

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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]