The center vents only blow ambiant temp air when the dial is set at the right point....nothing comes out at other times.....

the '79 300SD does not have a monovalve...it has the evil vacuum servo designed by some guy in Chrysler that spent the remainder of his life in a straightjacket in a padded cell in the 60's. I suspect LSD had a part in its design but I can't prove that.

These will die and would say few from the bone yards will actually work......unless you saw it work on the car it came off of.

There is a related servo amp behind the glove box. One can fry the other....recomend replaceing both at the same time.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche