FFFFreeeezing!!

[Marc]

Hi,
I have an 83 500sl euro with no heat.
I had the car for 14 years but didn't need it, mostly it sat. Drove it maybe 10 times a year a short distance to take my kids to Dairy Queen with the top down, when they were young. Unfortunately, as circumstances would have it. It has now become my daily driver. It runs great and I repair things as they come along.
However, the climate control has me baffled. Seems regardless of what I do I cannot get consistant heat, and its freezing. I have read tons of threads about this problem but have not found a definitive fix. I will have heat for a few days. I will then have no heat for a few days. I will have heat if I keep the rpms below 1500. I will have heat when driving along normally and then the air turns cold cannot get it warm again. I will not have heat, blowing cold air, turn of climate control, turn it back on and immediately get warm air. But who knows for how long.
I replaced the push button console probably 12-13 years ago. At the end of last winter I installed a new monovavle rebuild kit. Seemed to help, or maybe the weather just got warmer. Anyway since the problem was back. I did check and the diaphragm did have a small hole again. So I have just installed another monovalve repair kit within the last 2 weeks. I get tons of heat at idle and low rpms when I have the electrical connection disconnected from the valve. However normal driving or highway speeds just blows cold air. When the valve is connected, who knows? Sometimes it seems to work but, mostly I have no heat. I have also noticed at the times that I do have heat I hear a clicking sound from behind the console, as if a relay is kicking on and off. I should also note I have replaced the thermostat. The car does heat up and run at normal operating temperature. You can watch the temp gauge and see the temperature go up and tell when the thermostat opens.
I am an IT guy so I am analytical and understand how to test. Don't have a ton of tools though. The fact that I can get heat when the rpms are low and the valve is disconnected leads me to believe that the valve is funtioning. Also since there is no heat when connected. As I understand it the failure is an open valve so something is making it close. At this point I think that it is the push button console. However, I have removed it, opened the bottom and resoldered the connections. Although this did repair my cruise control, no such luck here. But, what is forcing the valve closed at higher rpms when the electronics are disconnected from the valve (is has to be vacuum, right?). This would point away from the push button console. What other items affect this climate control system? Temperature sensor? What is the Vacuum changeover valve? Does it just control the ducts?
At this point I just need to be able to get some heat. Is there a workaround? A way to force the valve to stay open?
Would greatly appreciate any insight.

Happy ThanksGiving to all!