The last time I drove my 300 turbo deisel (1985), hot air would only blow from the vents when the car was parked or stopped. As soon as I started driving, the hot air would cool down and eventually the vents would only blow cold air. This problem must be remedied soon as the winter is about to kick in full force. Please, any suggestion are welcome.

 

I had the same problem and it turned out to be the mono valve, just behind the first fire wall,if you unscrew the 4 screws and pull out the valve its self you will see the rubber diaphram has a tear in it and it starts sucking air at higher rmp.
You can find parts at www.adstico.com they have all mercedes parts and easy to find.
thanks Jamie

 

Thanks Jamie,

I will take a look at the mono valve this weekend. Do you know where I can find a diagram showing me how to get at this mono valve?

-peter