Air Conditioning Compressor Clutch
 

The air-conditioning system on my 81 300D has always worked well, even in the GA summers. However I have recently noticed that the compressor clutch never seems to disengage, even when the cabin is plenty cool and I move the temperature dial to the warmest setting.

Once at temperature the fan slows or even stops but it seems the clutch never releases.

Normally I would think that the clutch would fail where it would not engage.

Have you placed a white mark on the inside of the front .. on the shaft part... to make SURE this is accurate description of the situation ?
Very strange if true...so lets be sure it is accurate before we go down this road...

After you verify the problem (follow leathermag's suggestion) try unplugging th leads to the compressor. Do this w/engine shut off, not running. After disconnecting, start engine. See if the compressor clutch is still engaged. If yes, the problem is mechanical. If not, it would seem to indicate a control issue (pressure switch, relay, etc). Good luck & keep us posted.

With the engine OFF..... and your hand on the compressor clutch... when you pull the leads off do you hear or feel a click ?
With the engine OFF..... but with the ignition switch on and the AC switch on ... do the same test... do you hear or feel a click ?

Good points, glad you reminded me about disconnecting without power. I'll check that today.

But you do agree that the clutch should drop out once the temp setting has been reached.

You do not have the type system I have studied....
Some cars have systems which run the compressor all the time.. they have bigger accumulators....and typically orifice type design... and I do not know the specs on your car... but in general as far as I have encountered on these early 80's MB's.. yes... it should be an on demand compressor engagement.
Dropping out once the temperature has been reached...NO.... as your system may be set so that it makes sure the high side is completely filled ... so you have reserve ready... that would be controlled by a pressure switch most probably.... a high side... as compared to the low pressure cut out designed to protect the compressor in case of lack of refrigerant.

If you have an acc3 system, your compressor will always be on when you are in defrost, total vent and normal settings no matter where your temp dial is as long as there is over 10psi pressure in the drier and the etr reads over 2degC.

What is etr in that system ?