Well, the monovalve rebuild kit is cheap so while you have it out just renew it. My w126 had NO heat as a symptom. That seemd to indicate the monovalve as per archives. Replaced the same and got heat back.
My w126 does not have cold air right after engine start but a bit later, so when it is hot outside I open windows until engine warms up a bit, then I can use climate control w/o getting same hot/warm air blown at me.

Does the a/c have a relay that may have burnt out?

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD