Had a similar problem. My heat stopped at 62mph and came back on as I went slower. Replacing a the mono valve ($30-40)is simple and worked for me as well as several others on this forum.

Do a search on "mono valve" and "climate control" and you will get a lot of info.

MOno valve is on the wall next to the battery to the left of the engine (looking at it). Four screw (don't drop them) is all you need.

I got one for free at a salvage yard as I was strapped for cash, but I would go new if I could have.



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81 300D 250K+ (odo doesn't work!)
Want another one.