Cold air onto windshield but not out of center dash vents. Help please 560sl

Group, My 1987 560sl dose not blow any air out of the center dash vents. All works ok as far as cooling and heating goes but air is discharged from the right & left dash vents, the doors, under the dash, but not the
center vents. There is a full flow of air coming out of the defrost vents onto the windshield & that fogs up the glass outside of the car. I suspect that some door or flap is stuck in the defrost mode.. Has any one had this happen or dose any one know what to do for this type of illness? I am going under dash diving next weekend and could use some tips. Any help would be greatly appreciated.
Bruce Buchanan snipped-for-privacy@the-i.net najurb
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9 times out of 10, on the 107 body, the center vent won't open because the short stroke part of the defroster element has failed. Don't do any 'under dash diving' next weekend.... there is nothing to see! Remove the steering wheel, remove the cluster. Look in the top right corner of the cluster opening. You will see a vacuum element with 2 vacuum lines going to it. That is the defrost element. Disconnect the line going to the SIDE nipple. Cap the LINE going to the element. Don't worry about the nipple on the element.. Put the car back together. Start car. You should have center vents now.

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So, after removing the steering wheel and pulling apart the dashboard, he's supposed to disable the defroster and put the car back together that way? Why would anyone in their right mind do that instead of replacing the failed part?
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Bruce, what setting are you using? On MB's of your vintage, the center vents were only used for "non-heated" air, which meant fresh or a/c, but not heater air, which was only directed out the corner vents, lower and windshield.
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I'd bet thats your case Bruce... my 1983 240D is the same, there is a slider that 'opens' the fan input to these two vents, but it's just outside (or AC if it worked I imagine) air.
HTHs...
-Jason

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Will it be a broken vaccum tube.

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