No heat at 70mph
 

This one's got me stumped, '85 300D turbo. Last week WU and I went to visit my Dad up in Rockford, about a hundred mile ride. Took off up I-55 all nice and toasty ( it was about 15* out) after it was nice and warmed up I took it up to 75 mph and noticed a couple miles later cool air was blowing from the vents. Hit the temp wheel for some more heat and got non, looked at the temp gauge and it was up to 95*. We slowed down to make the tollway ramp (about 35 mph) and the heat came back on, temp also went back to 85. Got off at the next exit to take a look and found nothing odd under the hood, all fluids were fine, so off we went. Holiday traffic was heavy so we stayed under 70 all the way up to I-90 and had perfect functionality of acc. Then the same thing as soon as I hit 70 (about 3000 rpm) lost the heat, tried slowing to 55, no way didn't get heat again till we slowed to 35 for a toll. I have repeated this 4 or 5 times in the last week, always the same speed it goes out. Around town and under 70 the acc works flawlessly. Sorry for the long post but I wanted to give as much info as possible.
Any ideas would be greatly appreciated as driving in Chicagoland under 70 can be skeery!! and driving while shivering is no fun either,
Thanks. James

MonoValve = 99.9%

what would cause the monovalve to give up at 70 and return to normal after slowing down? And doesn't the monovalve fail in the open position? It was my first thought also but it just don't make sense.

Similar to my post today "So the mono valve is broken". Look at this link which explains more HEAT .
The rebuild is easy -

Fishslayer, Don't try using logic to understand the darned monovalve, it just works crummy at speed because of back pressure or something in the Benz "Climate Control System"
Seen this b4.

I spose you're right about the logic end of it, I have replaced alot of these over the years. This one was replaced (not rebuilt) by the PO only 14 months ago. Stranger things have happened tho, Thanks Guys, James

Check the small screen, if its getting plugged up you may have some scale or crud in the cooling system that is responsible for it not operating properly.
Has anyone ever seen the pulse that monovalves make at idle with the heat on?
I never understood why it does thet.

That's the jest of the thread I started today - what breaks on the thing. I can understand the coil - but examining the cylinder didn't really notice anything odd. But the darn thing works again and that's all that matters :P

I had exactly the same symptoms in my '82 300D. Rubber diaphram in the monovalve was torn. Installed repair kit, fixed problem.