I have an 83 300D Turbo and the center vents are not working. I verified that the vacuum element works, and the silanoid that controls the flow of vacuum to the center vent element is working. I have tested the leads to all the silanoids and they all have about 10.5 volts running to them when the silanoid is supposed to open. All except the lead labled "3" that contols the center vent silanoid. That lead only measures about 7.5 - 8 volts. Any thoughts? I have tried a control unit that I know works, and still only get 7.5 to 8 volds on that lead.
Any help would be great ... I want to put my car back together, but not before I get my vents working.

 

If they are anything like the center vents on my car, they only activate under high cooling demand conditions, and never during heating. Mercedes designed the ventilation system to give priority to the windows for defogging (safety first!), over passenger comfort. So what you're experiencing may be normal.

 

According to the manual the center vents should be coming on when I select the bi-level button ... I will however check to see if the lead goes to 10.5 volts when it's crancked all the way cool.

 

Got it ... the problem was in the switch on the temp selector. True they only work when all the way cold ... but only if the switch in the selector is working. I fixed the switch and PRESTO!!!!

 

Thanks for posting back the fix.