Defroster Flap
 

1988 W124 is blowing A/C air out the defrost vents when the control is not set to defrost. So far I think I understand the problem: bad defrost flap element. My indy also spotted that while doing some evap/drier work and suggested fixing it, but estimated $800 because the dash needs to be removed (labor intensive) to complete the replacement. I've always been under the impression that the defrost flap and footwell flap elements are accessed through the glove box, and don't involve removing the dash. Am I mistaken?

defrost flap
 

As with most Benz, your air ducting system defaults to defrost when there is a vacuum control failure. I think you should check for vacuum leaks before embarking on the very expensive adventure of removing the dashboard.

The first step for vacuum-element repairs is to get a Mityvac. As RR has said, it is likely that there is a bad actuator, but it needs proper diagnosis. If the Indy has properly diagnosed it, you're in for some work.

The footwell flap is accessible through the radio opening, I've done it, it's possible but painful. The '88 has one rectangular element for the footwells (late '87 up).

Defrost I believe is the one on top of the unit, but if the defrost vents are open, it is not your problem, it will fail open if broken (or no vacuum).

If you get defrost only on all settings, it is the diverter flap and/or others such as the footwell flap that fails closed.

The vacuum-control unit is just to the left of the glovebox (or pass airbag). You can disconnect the vacuum lines one at a time, and hook up a mityvac to see if each pod/chamber holds vacuum (and you can hear most of them move).

Thanks to both of you. I've resisted getting a Mityvac because I've resisted getting into the guts of it enough to DIY anything on the vacuum system. I guess it's time to venture into it.

Elaboration: airflow is not out the defrost exclusively. As far as I know, all of the ACC settings pretty much do what they're supposed to, except that with the normal A/C setting (recirc off, any fan setting), airflow also comes out the defrost vents, in addition to the center and side vents, so I believe the diverter is not the problem. Since the defrost element fails in the open position, and since the system defaults to defrost in the event of vacuum loss anywhere in the ACC system, doesn't that point to either a bad defrost element or loss of vacuum?

How about the question of getting at the defrost actuator through the glove box opening? Are you saying that that is not how it's done, and that the dash would have to be pulled if the defrost turns out to be the problem?

I believe that you are correct that the glovebox side pod is the defrost vent:

http://www.w124performance.com/images/W124_dash/manifold_defrost.jpg

Do you get air through the center vent when the system is cooling? If not, then I believe that the diverter vent is your issue.

Here are some more photos thanks to GXSR: http://www.w124performance.com/images/W124_dash/

and a thread on pod replacement in the 124: http://www.peachparts.com/shopforum/showthread.php?p=197605

Thanks
 

GXSR's post is great and the photos clarify things a lot. Yes, I get air from the center vents when the system is cooling. However, I think I've never really been clear on when the center vents are supposed to work and when they're not (cold air, warm air, EC setting, A/C setting?), so I'm sometimes surprised when they're on and when they're not. Suppose it's time to actually read the owner's manual about that? LOL

Thanks again Babymog.

Regardless of what button is pushed, the center vent will not open unless the system is trying to cool the car.

Check the vacuum supply hose / tube going from the controller behind the master cylinder. It routes below the air box, below [or just in front of] the battery and into the passenger fender to a reservoir, then to the air flap controller [accessible thru left side of [removed] glove box.

This tube on my 94 e320 developed pin holes under the air box, even though you'd expect this under the battery. This tube can be replaced by only removing battery [and RF tire well cover if necessary].

The Climate Control's correct operation and vent usage is in your Owners Manual.

Update re: Defroster Flap
 

Wow, here's something strange. If I'm not mistaken, the defrost vent airflow problem may have gone away. Driving with A/C on yesterday, the CCU functioned as it should, with cold air only at the center and side vents, not out the footwell OR defrost vents. Hmmm. Seems to rule out a general vacuum problem in the climate system, no? Leaving the possibility of a failing defrost mod that still works sometimes. Is that possible, or does the diaphragm fail and that's that?
Second possibility (or is it?). About a month ago, my indy replaced the transmission oil pan gasket and o-ring at the transmission dipstick. However, the pan continued to leak, actually quite a bit (10-15 drips per day). I took it back yesterday morning and was told that the pan was not fully torqued and they fixed that. Shortly thereafter is when I used the A/C and noticed that the defrost vents were not letting cold air out. Is it possible that a transmission pan leak could affect the CCU vacuum to such an extent that it would leave the defrost element in its normal open state? Obviously, I don't know enough about how transmission vaccum and CCU vacuum are related, if at all. TIA for any insights.

the defroster flap element is found by removing the glovebox liner - it is toward the center of the car. It IS kinda hard to get at, but it can be replaced in far less time than it might take to remove the dash

Yeah, I was heading there for a look-see when the problem appears to have corrected itself, as I wrote in my last post above. Thanks for chiming in. What do you know about the connection between transmission vaccum and CCU vacuum? Could a leak in the transmission pan have affected proper CCU functions?