My center vents weren't working. After some troubleshooting I found a BB in the line from the third switchover valve. The previous owner must have thought the servo was bad and plugged the line. There was a leak but it was in the Y fitting just before the center vent actuator. I replaced that fitting. I can now pull a vacuum with my Mighty Vac on the line from #3 switchover valve and open the center vents. HOWEVER, when the ACC is in operation the center vents will not open unless I give the flap a little mechanical assist by lifting inside the vent or reaching up under the dash and pushing in on the vacuum servo arm. Once I initiate the opening process with my finger, the actuator will pull the flap open all the way and hold it open until the control calls for it to close.
Can anyone explain why this would be happening? Is that servo supposed to be picking up additional vacuum through the second line attached to that servo in order to open and it may not be getting additional vacuum from that line?

One has to wonder if Mercedes subcontracted out the engineering of the ACC to Lucas!

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

TTT

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Update on the problem:
I tested the vacuum at the switchover valve. It is fine and pulls about 10 inches of vacuum. When I hook the line up to the center vent and T into it to test the vacuum, I can pull a vacuum and open the center vents. However, when the ACC operates, what happens is that the vacuum increases up the word 'Zero' on the scale on the Mighty VAc. When it reaches the 'o' on zero it stops and does not open the vents. If I give it a few pumps on the might vac up to 1 or 2, then it increases up to 10 on its own and opens the vents. If I release that vacuum thru the mighty vac back down to 1 it will build up again and work fine. If I hold the release until it goes all the way back to 'z' on zero and hold it there for a second, the same problem arises. It will only go back up to 'o' on zero until I give it some help with a couple of pumps.

If I disconnect the line from the actuator and test the vacuum, it is working fine.

Anyone have any clue as to what the problem is?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13