What should the high side and low side pressures be when checking the air conditioning of my 99 ML 320?
Following may be needed to ascertain this information:
Ambient Temp measured 2" from the front of the condenser is 95-100 degrees F.
Relative humidity is 60-70%.

I just installed a new compressor, expansion valve and receiver/dryer in my 92 300E. I now get 39 degrees at the vents with outside ambient at 80 degrees while driving. When I charged the system at idle ( ambient 95-100 degrees F) my high pressure side was 245 psi and the low pressure side was 39 psi. vent temp. in the car at idle was 55 degrees F. on the highest setting. I put in very close to 2.1 lbs. of R134a (vehicle had been converted from R12). The system held 2.4 lbs. of R12.

With the ML being manufactured in June of 1999 I think it would be a good idea to check the A/C system even though it still does get cold. I have not checked yet but I don’t think it gets nearly as cold as the 300E that I just finished up with.