Hello!  First I'd just like to thank everyone for all of their help so far and say (bc people have been asking), I will be doing a post to document all of the work being done on my 1990 190e.


With that being said, I it seems to be 3 steps forwards, 2 steps back lol.  Every time I think I'm nearing the end, I notice something else that needs attention.  Now, I've noticed that when my AC is on, I get cold air from the center ONLY; the sides and front windshield blower pump out hot air.  Any suggestions would be greatly appreciated now that the summer is finally kicking in.  So far I've recharged the system (it was very low) and checked the U shaped hose/valve that's under the hood near the fuse box (I did not notice any cracks).


If you get cold air from the center vents your AC system is working fine. What seems to be your problem is the vent flaps in the dashboard. They direct the air flow and it sounds like they have gotten stuck. If you remove the bottom part of the dash (panel above your feet) you should see the vent flap mechanism. You can move them by hand and see if that changes anything.

Well needless to say, I'm happy to hear it may be as simple as that! Question though; is it difficult to remove the bottom part of the dash? My recent experience has been, fix one thing and then knock something else out of whack in the process.

Under the glove box, I removed the plastic panel (3 screws) and did find a flap near the bottom.  It didn't appear stuck though; I moved it back and forth with my hand but didn't notice any difference.  It required some pressure to move back and forth as if there were a piston behind it.

There is one other thing that you need to investigate. The duo valve. It is in the engine bay. It can get stuck. They can be easily removed and cleaned. I didn't think that the duo valve was the issue becasue it typically impacts one side of the vents. For example if the drive side blows cold while the passenger gets stuck on hot.

But as far as vents there is usually several under the dash.

I've done the usual googling but can't seem to find much in the way of what the duo valve looks like, where to find it, etc.

I am NOT 100% sure this is your issue, but it doesn't hurt to clean it.


I haven't had a chance yet but will by Saturday at the latest.  My daily driver is actually in the shop at the moment so this AC problem couldn't come at a worse time!

Some questions, if you please:

1) I looked again in better lighting and, with regard to U shaped hose/valve that's under the hood, near the fuse box, I DID notice a crack. There's a thin red tube over there (next to a green one); the red tube has a black gasket(?) at the bottom that is in pretty bad shape.  Could that have anything to do with this?

2) Where is duo valve located?

3) Where are the vent controls referenced by MB Fanatic?

4) I read an account where someone said that they removed & cleaned the temperature sensor from the dome light and that solved a similar problem; thoughts?

Not sure about the cracked hose. Can you post a picture?

I would think the 190 has a duo valve because it was used on many models up to about 2000. Just don't recall where it would be on yours.

The recomandation of cleaning the temperature sensor is a good one and easy to do. If it is not picking the correct temperature for the center of the car, it will send the wrong air temperature through the center vents.  Try that and let us know what you find out.

Ok; I'll google and look again in better lighting to locate the duo valve and also see about removing the dome light to clean the sensor.  My day job is unfortunately always getting in the way.

There is the red tube with that U shaped connector.

For the life of me, I cannot locate the duo valve.

Hard to say without following the line but my guess would be they are either door vacuum lines or windshield fluid lines.

PROBLEM SOLVED!  Please excuse the caps-lock but I think it's warranted given the circumstances.  The problem was indeed that vacuum hose connector; as soon as I replaced it, the heat immediately stopped coming out of the defroster and side vents.  Now I have cold air all around.  

Based on the research that I did previously, I can say that the red is the heat and green is the cold.  When there is a leak in the heating line, it will leak into other parts of the climate-control system.  $3.50 later, I'm all set for the warm weather :)

Congratulations! Glad you were able to figure it out.

Thank you for sharing the solution as well. Those cars are becoming more rare and there are not many people who work on them anymore. 

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