Ok, let's check some easy stuff first. I think there are TWO fuses involved with the climate control. #8 and #14 come to mind, but won't swear to it. Check all the fuses with voltmeter. I have seen them look great and be bad. Next, just for fun, try jumpering the presure switch on the reciever/dryer. That is the device with the glass window in the top of it where you can see the freon/bubbles when the system is running. I think it will be on the right side near the headlight. Just pull the connectors off the switch and short them together for a second with the car set for max A/C. With any luck, one of these two tricks will get the clutch to engage and them we can work on the finer points of the system actually cooling. BTW, when I bought my '83 300D, the connector had been pulled off the compressor. I plugged it back in, set the A/C to max and when the clutch kicked in, it STALLED the engine. And you know how hard that is to do on those torquey diesels. Compressor was completely seized up.