AC Compressor doesn't stop spinning...
 

1992 300SL with single-zone automatic climate control

I searched throughout the Mercedes forums and could not find anyone with similar symptoms with my AC system and compressor. Here are the symptoms...
-Climate control blows cold air, even when I had the "EC" button pushed or if I turn the climate control off by pressing the "0" button. The compressor stays engaged.
-When I turn the temp wheel on hot, the HVAC system blows warm air, but the compressor still spins and I'm sure it is still cooling, maybe freezing, the evaporator because as soon as I turn the temp dial down to the blue zone, the cold air blows out of the vents immediately.
-When I unplug the compressor (electric plug), the compressor does stop spinning.
-The compressor does disengage and stop spinning periodically for short bursts, but then engages again and stays on.

A month or so ago, I added a little bit of R134 to the system (already converted from R12) and it cooled the system down a to acceptable cooling levels and acceptable PSI for the ambient temp. The system functioned fine for a couple of months. I'm not sure if this is related to my issues. Does anyone have any ideas as to why my compressor remains engaged when it should not be?

Thanks in advance for the help!!

Problem Solved
 

Just thought I'd post a follow up in case anyone else has this issue...

I was able to get the compressor to stop engaging constantly when I evacuated and refilled by AC. The compressor now disengages when I press the "EC" button. I'm suspecting that I may have had too much refrigerant pressure in the system from when I filled it.

This is not at all plausible.

On a '96 or earlier car the compressor engages in response to the evaporator temperature. In retrospect, your system was probably under-filled and unable to drive the evaporator temperature low enough to trigger disengagement of the compressor.

Thanks for the input @bobterry99. Shouldn't the EC button on the climate control unit disengage the compressor regardless of the evap temp? My compressor would not disengage even when the EC button was pushed. i even tried a different CCU...

Yes it should, and this negates the theory that your system was under-filled. Still, I think you can satisfy yourself that the system was not over-filled by locating the high-pressure switch on the receiver, removing its pair of electrical connectors, and jumpering the connectors together with a wire or paper clip. This will simulate an over-filled condition, and I think you will find that your compressor does not unconditionally engage.

Your problem was wiring, a pair of faulty in-dash controllers, a stuck clutch, or a faulty ESCM module. If the system persists in working perfectly, then this is a bizarre coincidence.

Brilliant info, thanks guys!