My 81SD heat has been acting up lately. This morning driving in the heat continued to blow no matter how much I turned the temp control down. Only thing turning it down would do was to cause the fan to blow at a hi setting, but heat continued unregulated.
I know there is a temp sensor under the dash, behind the glove box? Could that be it, or has the monovalve bought it?

sounds like the monovalve to me. I just had a weak connector there when I had the same problems as you.

Weak connector? Hmmm. never thought of that. Will check it out.

had that problem here too. cleaning the connections also inside the wire plug to the monovalve solved it.
you could also check your in dash sensor . mine went out so i had constant heat.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

Well the problem I have is weird. I can get in the car in the morning and turn the Heat on and it does nothing, then after about 20 miles it will turn on for about 3 or 4 min. then cut off and may not come back on again for awhile. First it will click and tick..tick...tick under my dash near the CC, but sounds near the glove box. If it's 30 degrees outside it definitley takes 20 miles for it to come on and if its 45 degrees outside it'll to it's tick..tick...ticking then come on after about 4 miles down the road. I'm not sure if this is my CC, Monovalve, or relays....It was cold driving to work this morning..Here in Austin it's not that cold much but when it is cold it's cold.

Wow guys wait a minute, are you sure the 81 HAS a monovalve? I thought these would have the dreaded "servomotor" system, but I could be wrong.....
Gilly

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My 1981 SD has a monovalve....

Pulled the plug connector to the monovalve and noticed the two small posts were both gray. Scraped and sanded them, used some alcohol, then a little electrical contact grease and put it all back together. Has worked great ever since. Just a little corrosion made it act up.
Nothing like a no cost repair.