I've got an 87 190D 2.5 turbo diesel...an unusual benz as some of you may know. Climate control exhibits following symtom: intermittantly (don't you love those?) the ac compressor refuses to start, bringing on instant discomfort (I live in South Florida - 90 degrees today!) When the compressor does run, the ac is very cold...works great. Things I've done are: checked pressures, they're fine....checked low pressure switch...fine, replaced climate contol unit in dash.($$$)..the compressor will start most times within a few seconds of turning on the ac, sometimes it takes a minute, sometimes several minutes, sometimes it won't run for days, then it just comes to life. Sometimes it seems that revving the engine starts the compressor, this leads me to the RPM sensor, but I'd like to test it before I change it...looks like a PIA to change....and how do you test it ? it puts out alternating current signal !? I've also tried another (although used) compressor control relay located behind the battery, with the same results. I basicly understand the system, and I know I can jump the relay and just leave it on (especially from may to novemeber!) but I'm kind of nutty in that I want it to work properly....any ideas ? thanks !!

 

Some turbos had a full load switch on the throttle linkage to disengage the compressor on full throttle (I have never seen a turbo 2.5, but I have a 2.5 N/A so i'm guessing here). Make sure it's not sticking.

A faulty rpm sensor on the compressor will cause the a/c to turn on for a few second and then disengage and not re-engage until restart. If the a/c restarts while driving then it won't be that.

Also checking the evaporator temp sensor is worth a try, might have an intermitant open circuit.

 

Try to put more oil into the system....(release some of the gasses and fill with only the oil charge.)
I would try running it whne you released the gas..(before you put in the oil)
My previous car's compressor wouldn't start when it was overcharged..