Mercedes climate control systems have built-in schizophrenia: they are capable of "blowing hot and cold" at the same time. The intent is to run the a/c as a dehumidifier and then warm the air (if needed). To that end, the CCU and the flaps have a complex set of operating variables. I would guess that some of the flaps are not working correctly.

Your '86 SDL is in some ways similar to my '87 D; there should be a manifold behind the center-right of the dash with a lot of hoses running to the various vacuum pods that control the flaps. You can remove one line at a time from the manifold and test the pod with a Mityvac. Any pod that does not hold a vacuum is probably bad. Replacing pods is another problem, often requiring dash removal.

If the pods all check out good, the other possibility is a bad CCU. This is easier to get at than the pods but more expensive to replace unless you can find one in your local junkyard.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970