Wacky Air Conditioner

I just purchased another diesel Benz. This time it is a Gold 84 300D turbo with 80,000 miles for a mere $1500.00. The car is beautiful, even came with a car cover. Everything seems fine (drum roll please) except, the air conditioning system, and the clock and tach is not working at all. I have spent the last two days reading old posts here and really have enjoyed the information. I qanted to ask a couple of questions before I break out the tools.

Here's what is happening. The air will only come on when the car is first started and will run for only 4-8 minutes, I timed it several times. It will only run on the minimum temp setting, any other setting and nothing. If you try to change it while you are getting the initial blast of very cold air, it stops dead, nothing at all. If you push any of the control module buttons, try to adjust the temp or the fan speed, it stops and the seat belt light will come on for about 3 seconds then goes off. The air will not work until the car sits for awhile and is restarted.

I read earlier that it could be the mono valve and/or the climate control module.

Any other suggestions?