PeachParts Mercedes-Benz Forum - 1991 300e a/c not cold at low rpm

your vehicle was equipped with r-12 refrigerant when it left the factory. you mentioned a new compressor, so i would next ask what refrgierant was installed? i'd bet a buck it was r134-a.

fewer and fewer shops offer r-12 anymore. they just don't want to deal with it.

if your system was converted to r134-a, there should be tags under the hood indicating this. if your familiar with r-12/r134-a service adapters, you would know right off the bat what kind of refrigerant you have. some places skip the relabeling part even though they are not supposed to.

i'll be perfectly honest. i have never done a conversion. i have diagnosed and repaired numerous r-12 systems. what i do know "academically" is that a conversion is a multi-step process that requires diligence on the part of the person doing it. i won't go into all of the processes, but a thorough "vacuuming" of the system - removal of old oils, etc and mixing in of oil suited for r134-a is required. there is a lot more to it than just this, but if any part of the conversion is "cut short", you'll end up with a poor performer. a lot of fly-by-night operations skip the vacuuming.

the pressures on r-12 & r134-a differ, so if your system was changed to r134-a, a lesser amt. of refrigerant would have been used....maybe.

someone who can detect what refrigerant you have would install the appropriate gauges and check your pressures - low & high side.

with that information, you'd move to the next step.