On my wife's '85 300D. Little or no air is coming out of the center vents. Air is good and cold.

What can I do to correct?

The vents on this car are vacuum operated. The push button sends signal to electro/mechanical switch-over valve and that allows vacuum to pull center vent open. How much time to you have to test, & equiptment to check the vacuum system.

I had the exact same problem with
 

my '85 300D and found that the connector that connects the "damper" door for the two center vents and the linkage to the vacuum actuator arm was gone. The vacuum actuator was moving but it was not opening/closing the damper door. You should be able to take a bright flashlight and look into the vent. If the damper door is opening you will be able to see it in there in the vertical position. If it is not opening it will not be in the vertical position. I used a tie wrap to connect them back together. The other thing I did (to insure MAXIMUM air flow out of the two center vents) was to tie wrap the defrost vacuum actuator in the closed position. When you mash the a/c button the vacuum actuator for the defrost is supposed to close off and the center vent vacuum actuator is supposed to open. The other thing you will have to check is to see if you have a vacuum leak in there somewhere and insure that the vacuum actuators are working properly. Try to put a vacuum tester on the line going to the center vacuum actuator and put some vacuum on it prior to looking into the vent opening. Pump it up and you should see the damper door open. A vacuum leak or the actuators leaking will keep them from working properly. The vacuum actuator for the defrost is located to the upper right of the gas pedal. You have to remove the kickpanel underneath the steering wheel to get to it.

I would suggest calling the 800 MB number and purchasing the "Electrical Troubleshooting Manual" for the 300D if you do not already have one. It is an excellent manual for use during troubleshooting.
Jim
'85 300D
'95 E320
'97 CRV

Thanks guys. Do I access the center actuator through the drivers side as well? I have a vacuum tester, but I have to say, without a diagram, the vacuum system has been one of lifes great mysteries for me. I have let my tech mess with running down vacuum problems.

Does the electrical troubleshooting manual deal with vacuum also?

the electrical troubleshooting
 

manual does show the vacuum systems. The vacuum actuator that operates the center vents is a PAIN to get to. I think the dash has to come off to access it. I did get to the point where I could see it by removing the glove box and the panel below the glovebox.

Jim,

You are right, the actuator is a pain to get to. It appears to move a little but not enough to open the vent for the center.

While I was poking around, I found a rubber tube that runs just under the dash. it has a 6" section missing between the glove box and the center vent housing. It appeared as though it was connected to the other piece by a foam tube that has disintigrated. Any idea if this is related?

FWIW I had the same problem with my ’91 TE. In this case the battery had leaked acid over the years and melted the vacuum line, and also part of the apron. If yer ’85 is like my ’91, check the hoses by the battery.

Hope this helps….

…Tracy

About what is the diameter
 

of the rubber tube that you are missing? Is it vacuum line diameter or is it larger? If it is larger then the foam piece is the "aspirator" that makes cabin air move accross the inside air temperature sensor. Can you see the "damper" door moving inside the vent? If it is moving just a little bit then the connector sounds like it is intact. You should then be looking for a vacuum leak. Also, the vacuum line that goes to the defrost actuator ties into the center vent actuator. Try to connect your vacuum testor up to each individual actuator and insure that they work then look for tubing leaks.

Jim,

It's the large tube that runs past the temp sensor. The vent flap doesn't seem to move at all, but I think I detected a little movement from the actuator.

How the heck do I get in there to check anything?

The tube with the foam casing is the aspirator tube. It runs to the temp-sensor. A new one is not expensive and is an easy DIY job.

Bill did you
 

get it sorted out?