Middle vents not working on 560SL
 

I have a 1987 560SL and the middle vents do not work. After looking a little further, it appears that I am not getting any, or very little, vacuum at all to operate the valves. When I change from defrost to floor air I hear something going on under the dash and the amount of air decreases going to the windshield. Does anyone know what I can do?

I believe this is all temperature dependent, check the operation instructions in the owners manual. I believe the center vents only provide cooled air, so if it is fairly warm where you are, try dialing down into max cool and wait. It may take several minutes. Once the climate control has figured out "Hey, I need to cool off the interior" it will send cooled air through the center vents. If it is already cool where you are, this may not happen, although Max Cool should send at least a little cold air through.
Gilly

How quick should the fresh air vents close after the button is pushed?

OldBenz:
Nothing happens "fast" with this system, except possibly defrost.
I am unsure though what exactly you mean. There are many buttons, which button do you mean?
I also don't see how the question relates to your observation of air from the center vents. The system will introduce cool air as needed, as called upon by the control unit. Whether or not the fresh air flap, which introduces outside air, or recirc flap, which recirculates the interior air, is opened or closed is pretty academic.
Gilly

The direction I was heading was if when the recirc button is pushed, how quick can I expect the fresh air flaps to close. I am trying to determine if the entire control unit is even supplying a vacuum. Nonetheless, I will put your suggestions to the test tomorrow as I am out of town today. Thanks for the response.

MIDDLE VENTS and NOISE
 

I have the same problem on my 86 SL. Also, the blower motor makes this annoying noise when it's on. I've been told that I may have a vacuum problem regarding the vents. I believe I might need a new blower motor. I took the current one out and cleaned and oiled it. However, it is still making noises. Any suggestions for remedies?

I had to replace the blower motor on mine about 8 months ago. The bearings were completely shot which allowed it to hit the shroud. I bought one from a local scrap yard for 75 bucks and it works great. Just remember to check for play if and when you buy used.