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

It should not be possible for the center button to be pressed and the a/c not be on.

I have run into cases where a bad temp wheel will cause the center vents to close on any setting OTHER than MIN setting(coldest possible).

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg

My setting at the present time is in the MIN (coldest)
For some reason the center vents will work if I disconnect the pressure switch at receiver dryier. Doing this the compressor clutch will not engage. Other way to get center vents working is turning blower on with engine and compressor not running.
I will appreciated any other ideas.
Thanks

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Nyck - TX - USA

AC is on but the center vents will not open.
If AC is off beacuse engine is not running center vents will work.

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Nyck - TX - USA

hmm, that's very strange, not sure I know the answer to your problem off the top of my head. Anyone else know?

__________________
1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg

According to my info, the center vent flap and diverter flap are spring-loaded to the "off" position and vacuum-actuated to the "on" position.