I am just now getting around to fixing the air on my 1985 300d. I installed a new compressor. And as soon as I started the engine the clutch engaged. turned the engine off quickly, as their is no refridgerant in yet. The old copressor was disconnected. With the engine running and wires to compressor unplugged I have power all the time,this is with the air/heat controls in the off position. It also seems to me that there is or should be a low pressure switch that would prevent the compressor from running unless there is enough freon in the system. Any ideas why I should be getting power to the clutch? when everything is off? Looks like fixing this may be more involved then I thought.

Is there resistance to turning the compressor with no load in the system? Like freon and oil. If the compressor basically freewheels with nothing in the system the small residual friction alone may have got it spinning. I know nothing about air conditioning by the way so just a thought.