Greetings,
I'm new to forum and new to classic MB's. I bought a 1981 W123/300d three months ago and am trying to fix all the little things that are apparently common to these models. Currently it's a lack of heat at highway speeds. I bought a rebuild kit for mono valve and found the innards to be in good condition but replaced them anyway.
Sitting in the driveway idling, I have complete control of heat and auto fan works fine.
What would be the next thing to check? Again, heat is controllable at idle UP/DN. One more thing, I disconnected the mono valve and get scorching heat at any speed.
I know I'm going to love this car eventually.

Thanks
Pat