PeachParts Mercedes-Benz Forum - View Single Post - W126 Climate control theory

tinypanzer · #1 07-21-2009, 10:45 PM

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!!