I checked the 124 climate manual for the expected performance values. It may not be exactly correct, but the 124 has a very similar a/c system as the 126.

Under your conditions the discharge air should be 5C - about 41F. High side pressure should be about 250, your low side is fine.

The one thing which strikes me as odd is that your compressor is cycling. There's no reason it should be doing so. Compressor cycling on these cars is controlled by an evaporator temperature sensor which feeds the pushbutton control unit. The PBU then turns the compressor off/on to prevent the evaporator from falling below the minimum temperature selected by those wizards in Stuttgart. Mercedes is very conservative and switches off the compressor when the evap reaches ~38 degrees. Perhaps you have an ETR or PBU problem?

How certain are you about the temperature and relative humidity? Humidity has a tremendous affect on discharge air temperature. According to the charts, the expected output air temp for our conditions in Dallas this A.M. (75F and 90% humidity) is 12C - 51F.

- JimY