Hi - I have been doing some simple maintainance myself on my 89 300E since I bought it in 1993. I have a full set of Original Mercedes Service Manuals (paper copy bought about 15 years ago) and they have served me well. Now the hard part, the A/C in this car was converted over to R134A about 3 years ago when my son took the car to college. Last fall the A/C stopped cooling but we didn't mind but now it is extremely hot in DC and I've been trying to fix it all day today but no luck.. Here is what I have done...
Checked the KLIMA relay and the PBU. When the PBU is pressed, the KLIMA relay sends the signal to the compressor and it comes on and stays on. I put a set of gauges on it and I checked the static pressure before starting the testing with the engine off and cold - it read 85-90 psi on both low and high side...
then at idle with A/C on, blower on high and windows open, the low side read about 20psi and the high side read about 150psi- ambient temperature at test time was about 88F. it was not cooling, so I hooked up a 12 oz can of R134a to it and it took up about most of the can... then the low side pressure went up to between 37 - 40psi and the high side went up to about 250-270psi. However still no cool air - it blows air fine its just not cold....
So what could be the problem? since the compressor is holding pressure i'm thinking there shouldn't be a leak but I'm not sure - could the pressure be too high or could it have air in the system, could it be a bad expansion valve - I don't want to take it to the mechanic yet until I get an idea of what the possible problems are - I don't want them to do unnecessary parts replacements...
Any help would be really appreciated...