Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help




Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Tech Help

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 07-21-2009, 11:45 PM
Registered User
 
Join Date: Jul 2008
Posts: 1,236
Lightbulb W126 Climate control theory - does it hold water?

Hello all,

I have spent many hours trying to figure out why I have intermittent center vent/defroster issues on my '90 300SE.

The problem is (and it seems to be a common one) that the AC doesn't always go through the center flaps, it often comes out the defroster. Switching to EC makes the air come back through the center flaps very quickly. When it is HOT outside, the air always seems to come through the defroster and when it is cooler, the center vents work fine.

I got a private email from another member here asking me if I managed to figure out what was wrong with my car. The symptoms described matched mine exactly, even the intermittency of it all.

Here's my response to that user. I posted it here because I am hoping that somebody more familiar with W126 CCU will read it and tell me that I'm dead on target or way off.


Here goes:

I have a theory as to why this happens as it does.

What I think is causing it is a defective (leaky) vacuum pod that controls the recirculator flap. This is the flap that decides whether you are getting fresh air or recirculated air. Since all of the flaps in the system are controlled by vacuum, one leaky pod introduces a vacuum leak that can rob vacuum from the other pods causing them to misbehave.

The reason I think it's the recirculator pod is because of the way the overall system behaves when working correctly. In the morning when the car isn't as hot, the climate control unit is content to let you have fresh air. When it gets hotter outside (and inside) the CCU decides that it needs to give you recirculated air in order to cool down the car quickly. When the solenoid valve that controls the recirculator flap opens, the vacuum leak in the recirculator pod allows air into the system and therefore robs vacuum from all of the other pods. The pod that controls the center vent and defroster is designed so that with no vacuum, you get defroster. This is a safety feature so that you can defrost your window even if there is a huge vacuum leak. Smart, really.

So, when the recirculator flap's solenoid opens, the huge leak in the vacuum pod lets air into the system and the center vent pod loses vacuum very quickly and switches to defrost. This will tend to stay this way, because now the car isn't getting as much cold air, the temperature rises, and the CCU continues to try to cool the car down with recirculated air.

There are some tests I did that lead me to this theory. The first was the observation that when cooler outside, the system seemed to work fine. The second thing I noticed was that if the system was working great and I hit the recirculate button, the system very quickly switched to defrost. Also, as you noticed the EC setting works fine because it is a vent setting and doesn't recirculate automatically. Another thing that lead me to this theory is that when the system is in that state where it goes back and forth between working and not, adjusting the temperature up thereby decreasing the difference between the actual temperature and the desired temperature, the CCU decides that recirculated air is not necessary anymore and the center vents start working again.

I have not had the money or time to fix mine yet.

If you do decide to plunge in, please let me know if I was correct!!
__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
Reply With Quote
  #2  
Old 07-22-2009, 09:20 AM
Moderator
 
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Of course you can block-off the 2 vacuum lines to the fresh air/recric vacuum elements & see if your theory is correct.
__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator
Reply With Quote
  #3  
Old 07-22-2009, 04:59 PM
Registered User
 
Join Date: Jul 2008
Posts: 1,236
Very true, and I actually suggested that to the person that emailed me, because I have been too busy to dig into it just yet. I guess I'm going to have to bite into it this weekend, it's effin' hot down here in FL.

So, I suppose I'll report back soon.....
__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
Reply With Quote
  #4  
Old 07-27-2009, 07:32 PM
Registered User
 
Join Date: Jul 2008
Posts: 1,236
Success!

My theory was correct! I dug into my dash this weekend and removed the two pods that control the air scoop, and they were both shot!

I took them out and plugged the vacuum lines going to them, and my center vent has worked properly ever since!

Since my car has no passenger airbag, getting to them was very easy. All I had to do was pull out the glovebox, and the whole thing took less than 10 minutes to do.

I decided to pull apart the vacuum pods, and the results of that can be seen here:

http://www.youtube.com/watch?v=sovTANrf3pM


So,

If you have a W126 that has this strange center vent issue, I strongly suggest you start here first, since these are the easiest pods to get to, and are also the most likely culprit.


Cheers!
__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
Reply With Quote
  #5  
Old 07-28-2009, 04:15 AM
Registered User
 
Join Date: Jan 2008
Location: Melbourne Australia
Posts: 833
Great Video there T.P.

How much were those two valves?
Reply With Quote
  #6  
Old 07-28-2009, 10:14 AM
Cal Learner's Avatar
Registered User
 
Join Date: Aug 2006
Location: Marysville, CA
Posts: 781
Well done. Your symptoms sound familiar with the intermittent, alternating center vent, then defrost vent. In my case, I notice that only when the A/C is engaged. Then, cold air comes out the side and center vents initially, but soon, the center vents are not blowing and the side vents are blowing more air, and sometimes, the defrost is blowing cold, too (but not always). I realize that one or more of the vac pods is bad. Questions: does the recirculator flap operate even when the RECIRC switch isn't on? Is there anything noticeable, like an audible opening/closing sound, or increased/reduced airflow when the RECIRC switch is pressed? What I'm getting at, is how you would know if the recirc system is even working? The 10 minutes it took you, did that include removing glovebox and the two recirc vac pods? Is it likely that installing new pods then would take only a few more minutes? Thanks.
__________________
1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't
Reply With Quote
  #7  
Old 07-28-2009, 05:51 PM
Registered User
 
Join Date: Jul 2008
Posts: 1,236
Hi Cal,


In my case, recirc was not working at all, and one of the things that lead me to realize that the pods were leaky was that hitting the recirc button would cause me to lose my center vents. Allow me to explain.... The center vent pods is spring loaded so that if vacuum fails, you get defrost not center vent. That's a safety feature. So, if you lose vacuum in that system anywhere, you will likely lose your center vents in favor of defrost.

So, whenever I would select recirc, or the CCU decided to recirc by itself as it does when it's really hot outside, I would lose center vents. Apparently, the CCU compares the inside temp to the setting on the temp dial and adjusts the vents appropriately.

Ivanerrol, I still haven't ordered the two pods as I am broke at the moment so I don't know what they cost yet. My guess is $50-75 each.

And yes, it took me all of 10 minutes to pull out the glove box and the two pods. All you need is a flat-blade screwdriver to push in the little catch on the bottom of the pod that locks it into place. You just disconnect the arm from the lever, push in the little tab with the screwdriver, then twist the pod out like a bayonet light bulb. You can remove the hoses either before or after you do that.... Surprisingly easy.

The glovebox is easy too. You pull out the little plastic plugs all the way around top and bottom, pull out the light assembly and unplug it, then the whole thing just pulls right out.

The easiest test I can think of to see if your recirc pods are shot is just to hit the recirc button when your AC is on and the center vents are working. If you lose your center vents shortly after pressing the button, there you go....
__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
Reply With Quote
  #8  
Old 07-30-2009, 08:50 AM
Cal Learner's Avatar
Registered User
 
Join Date: Aug 2006
Location: Marysville, CA
Posts: 781
OK, thanks for the explanation. Bottom line, I guess, is that if airflow from center vents is lost (even intermittently) during normal A/C operation, the recirc pods are bad?
__________________
1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't
Reply With Quote
  #9  
Old 07-30-2009, 05:02 PM
Registered User
 
Join Date: Jul 2008
Posts: 1,236
I'm not saying that it's always the case that they are bad, but because of their design (different from the other pods) they seem to be prone to fail.

I've also heard of center vent issues on many W126 cars, and I am speculating that the recirc pods are a likely cause of this.

Again, I would test the recirc button to see if this is the case. If you lose your center vents as soon as you press recirc, then I think you have proved it enough to go digging in.
__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
Reply With Quote
  #10  
Old 03-30-2010, 08:37 PM
Registered User
 
Join Date: Jul 2008
Posts: 1,236
Hello everyone. Since I first started this thread, I have learned a LOT about 2nd generation W126 Climate control systems.

Here we go......

First off, there are three (not two as I previously said) vacuum pods that are prone to leakage. Luckily all three are accessible when you pull out the glove box. It's no coincidence that the three suspect pods are all double element pods, while the single element pods used in the footwell and diverter flap are much less prone to fail. Invariably, it's the middle element that fails on these double element pods.

My theory is pretty much dead on correct it turns out, but there's more to the picture than just the pods.

The vacuum valve unit turns out to have bad solder joint issues due to poor design of the input pins. To make matters worse, this unit is sealed shut. I wound up investigating the unit when I got done replacing my busted vacuum pods in the recirculator flap and somehow my recirc still wasn't working. Testing my pods and the circuit with a vacuum tester showed that everything was air-tight so there must be an electrical issue either in the CCU or the switchover valve. After trying my spare CCU and getting the same results, I grabbed my spare vacuum valve (thanks pick n' pull).

My spare vacuum valve worked on the recirculator function, but the center vents were now misbehaving..... What gives?

On a hunch, I started wiggling the wire harness that goes into the vacuum switchover valve, and tada! Things started working and not working when I wiggle the plug.

After taking the thing out and playing with the pins, I realized that indeed there were bad solder joints on the other side. This thing is melted shut so what do I do? Ahhhh...... See picture one, this will save you a lot of money....

Using a jigsaw, I carefully cut away the plastic behind the plug socket revealing the solder joints! See picture two... This is AFTER I touched up the joints. Indeed two were visibly broken and the rest just looked weak. Sorry the picture sucks, it's a cheesy phone. I nicked the edge of the circuit board with my saw, but didn't do any damage luckily. If I were more careful not to let me saw get so close to the board, that could have been easily avoided.

So, after re-touching the solder joints I noticed that this valve (dated '90) was ever so slightly different than my junkyard spare (dated '89). See picture 3. Notice that output #2 (for the recirculator flap) has a brass "resistor" installed in the nipple on the '89 valve, but that little brass "resistor" is missing or was never installed on output #2 of the '90 valve. The end result of this is that on the '90 valve, the recirc flaps open and close almost instantly, and on the '89 valve they can take up to 15 seconds to fully open or shut. What a difference! Even if this is purely by accident, I recommend anyone who digs this far into their system to go ahead and yank that sucker out of the #2 port, it just makes things work so much better.


So, I put everything back in and BAM! My flaps are working like brand new. The soldering fixed the intermittent valve, and here's the really cool part. See picture #4. That's the valve after it's been installed. Notice how the solder joints are visible and accessible even with the valve installed? If they ever break again, all you have to do now is pull the cover off the side of the center console and go to town with a soldering iron! The thing can now be re-soldered in place!


I hope this information proves useful to you. Please go back and see my YouTube video if you are just tackling this problem for the first time.


Cheers!
Attached Thumbnails
W126 Climate control theory - does it hold water?-sawtovalve.jpg   W126 Climate control theory - does it hold water?-vavlecutopen.jpg   W126 Climate control theory - does it hold water?-valvessidebyside.jpg   W126 Climate control theory - does it hold water?-valveinstalled.jpg  
__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
Reply With Quote
  #11  
Old 03-31-2010, 05:17 PM
Registered User
 
Join Date: Mar 1999
Location: Williamsburg,VA, USA
Posts: 106
126

I wish I had this info a couple years ago when I still had the 126. Frustration about the climate was one of the reasons I let it go. Great work! Thanks for sharing
Reply With Quote
  #12  
Old 03-31-2010, 07:27 PM
Registered User
 
Join Date: Jul 2008
Posts: 1,236
One thing I forgot to mention in my last post is that the missing "resistor" on port two of the valve causes a couple of things. First, it makes the recirculator flaps work really fast, which is good. The second thing it does is make my theory about broken pods that much more correct. By this I mean that with that resistor removed, a leaky recirculator pod will rob the entire climate system of vacuum immediately if that port is open, whereas if the resistor was there, there may still be enough vacuum for the other components to operate albeit much more slowly. So when testing your pods be aware that there are two sets of behavior depending on if your port #2 has that little resistor or not. It may have very well been the case that if my valve module had that resistor there, my symptoms of front center vent air disappearing immediately upon switching on the recirc switch may have either happened much more slowly, or possibly not at all. So there is a trade-off if you pull that resistor out; your recirc pods work much quicker but if they ever leak, you will have much more dramatic symptoms due to the increased vacuum loss possible with a wide open port.
__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -4. The time now is 12:24 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page