Which vacuum control valve controls the center vents(300SD)?
 

My center vents only work on economy, and close when the AC is on. I pulled a valve from the yard and might try that before I bypass anything, but I need to know which of the 6 valves controls the center vents. I tried pulling the lines off of the bottom 3 valves one at a time to see if that would close the vents while in economy mode, but they stayed open. removing the top 3 will take a little more effort so I just wanted to check here before I proceed. BTW, the CCU was recently rebuilt. Thanks

The switchover valve for the center vent is the third one down, in the bank of switchover valves behind the left passenger kick panel. Connecting a mightyvac to the outgoing vacuum line should tell you if the element is holding vacuum.

The vacuum element, for the center vent, is behind the pushbutton controller. It is the most challenging one to replace. Plan on about 4 hours. The difficult part is disconnecting the arm from the flap. (The clip is very similar to the element for the defrost flap.) Note: poke some rags into the area below where you are working as the little bits tend to fall into the console, only to be discovered years later.

It's not a bad job but is a little tedious the first time.

Thanks Sam, I've been told that the center vent and defrost pods are connected to the same line. Is that Y-connector accessible so that I can disconnect the pods and test them separately?

This section of the FSM should help: Mercedes-Benz 126 Service Manual Library

You will see in 83-604 that it isn't really a Y-connector. The center element and the defrost element are connected by a check valve and orifice set-up. (That's how you can identify which one is connected to the center element.) To verify the center element, I just pulled the glove box and used a set of clamping forceps on the line to the defrost element before pumping the mightyvac. Much easier than trying to access it from below.

That link didn't work for me, but I was able to find my CD-ROM manual. I'd post a PDF but it's just over the size limit for this forum.

I'm thinking of blocking the outlet of the 4 point distributor that goes to the defrost pod and see how the defrost flaps operate when just using the red/white line from the switchover valve designated for the defrost. The defrost vents always open up when I press defrost, so I think that the pod is okay and my center vents open up on economy so that should be good too. Maybe there's something going on with the check valve or switchover valve.

My center vents are working again.
 

The problem was the scoop air flap pod. The center vents wood stay open and hold vacuum when I pumped it up with the mityvac, so I figured that they weren't getting enough vacuum due to a leak somewhere else. I tested the scoop air flap line and it was dead, so I plugged the line and now the center vents open up. I think that the scoop air flap pod is located near the driver's foot well and is one of the easier ones to replace, does it open up to blow warm air on the driver's feet?

The defrost pods operate in reverse of the others - no vacuum means vent is open. Pinching the defrost lines shut guarantees air to the defrost vents.

I'm still looking for a flow diagram through the airbox. It explains what the scoop air flap does. From what I recall, the center vent flap allows air into the center vent plenum. The scoop air flap downstream of the plenum closes to force air to the dash vents. I don't remember where the air goes if it stays open. Maybe the side dash vents or to the footwell vents.

Sixto
83 300SD