My 1982 - 300SD have the following issue in the CCU.
With engine running and the AC off, the center vents will blow air when the center button in the CCU unit is pressed but the same button will not work when the AC is on. Better saying, center vents will not blow air whit the AC running. This morning I took the covers in the passenger side to check if the relays for vacuum and vacuum hoses was showing any problem but nothing wrong was found there. All relays are working and all vacuum hoses are ok.
I was thinking: will this vent open only at specific interior temperature?
I know that some Mercedes cars, the vents only open after the air reach some specific temperature. Can this issue be one of those temperature settings?
Any suggestion is welcomed.

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Nyck - TX - USA