My first M-B, an '87 SDL. According to the seller, the A/C compressor was recently replaced and it's still running R-12.
Compressor seems to be working. The air is cool, but not cold by any means. The biggest problem is that there is no airflow from the main vents in the dash. Air comes out the defrost vents and the rear vent coming down the center console. Button position doesn't have any effect on where the air flows. And yes, I did check the sliding flow modulating switches in the correct positions. It seems to be primarily an airflow problem somewhere in the dash. One possible hint may be the fact that the "O" position switch on the panel that's supposed to shut things off completely (I think) only shuts down the ventilation system for about 30 seconds or so. So it could be a control switch problem, but it seems more likely there's a diverter valve somewhere in the inaccessible nether-world under the dash that is not properly positioned.
I'd like to work out the air flow problem before having the system checked for leaks and recharged for the lovely Phoenix summer which officially arrived this week.
Thanks in advance.
Bill Been

Chances are you have a combination of bad lines and the monovalve. The climate control system is run through a series of vacuum lines that toggle vacuum elements open and closed to direct airflow. The lines become dry and brittle and eventually the system loses pressure and isn't able to push and pull the elements open and closed. Its not too expensive but tracing and replacing vacuum lines is time consuming and requires the partial disassembly of the center console, dash and glove box. The vacuum element that controls the defroster/foot vent is directly behind the glove box.

You may also want service your monovalve with a monovalve repair kit (about $25). Its very easy, only takes about 15 minutes and a good place to start.

Finally if you replace the vacuum lines and the monovalve then you are unlucky and it is the 7-port climate control valve that serves as a hub for all of the vacuum lines, its pretty pricey new but you see them on ebay once in awhile.

Do a search on 'w126 vacuum repair', you'll find more info than you ever wanted.

Good luck!

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1988 420SEL 170K

On our car I had the same problem. There are two vacuum lines at the vacuum pump. One small one large. The small one has an inline screen about 2 inches away from the vacuum pump. Our screen was completely plugged with carbon. I cleaned it out......WOW!! Vents worked good and the shifting was much improved!

Turns out about every thing on yhe car is run by vacuum.

This inline screen is small, about 3/8" in dia. and short. I'm not sure if this is an add on or not but its worth a look. Good luck!

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87' 300sdl

Thanks, fellas. I finally figured out how what terminology to use when searching for information on this topic after I posted the thread. I got a bunch of good info, but I am wondering if all the techniques posted by "gsxr" (I have a '91 955cc, by the way) are also valid for the SDL. Or is it a completely different system for actuating the vents and diverters?

Best,
Bill Been

The chassis is w126 for an SEL or SDL. Its the same thing, no surprises.

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1988 420SEL 170K

Thanks, Tim. I should have been more specific as the most in-depth postings I've found regarding vent actuators was on a few early 80s SDs. That's what I was alluding to. That's a different body isn't it? Or am I showing my rookieness?
Best,
Bill

P.S. About two hours after I wrote the above, I investigated what BJ was saying in his post, and shazam! the whole little screen thing is broken in half! I traced these lines to a surprising number of components. It seems amazing that anything works at all, as this one little line off the pump "T"s into four lines immediately after that screen. Thanks again, guys.

Hey Bill,

Don't worry the nominclature takes awhile to nail down. A 300SD is the big W126 while a 300D is the W123 chassis (I think it went to 85 and then switched to the W124).

Anyway, sounds like you are well on your way. On my system I just replaced all the rubber connections instead of trying to figure out which ones were good and which ones aren't. I just bought 3 yards of vacuum hose and went to town. I also did the engine compartment (unlike the sparse diesel, the gas engines have tons of vacuum lines). Some of the pink vacuum elements may have gone bad too but you can get them by the handfull at the junkyard and the bottoms are interchangable.

For more info you can do a search on 420sel and 560 sel climate control systems. The ACC system will be the same as yours and there are LOTS of good posts.

good luck!

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1988 420SEL 170K

I have had the same problem with my 350SD recently. I took it into the shop because I am on the road and unable to diagnose it myself. The system would sometimes cycle to center and floor and eventually cycled to those vents less and less.

It turned out to be a simple 1 hour repair. Two vacuum lines under the hood and one check valve were replaced. The total cost to me was $88 with tax.

Now when I drive - the center vents work perfectly with no leakage in the defroster. Also, the transmission has begun to shift smooth as silk again.

My $0.02...

what's the electrical switch on fire wall w/two hoses [ one larger]mines disconnected.