96 C280 W202 duovalve diagnosis
 

My daughter complained that the heat on her 1996 C280 (250,000 mi.) was intermittent. Specifically, that it would blow warm air from the driver's side dash vent, but cold air from the center vent. It was back from college at Thanksgiving time, and I prepared by having a new duovalve in hand, which was the failure I suspected from having reviewed posts on this forum. I had her retrieve codes from the CCU in advance, which seemed to indicate a left side duovalve problem.

By the time I got my hands on the car, it was giving me a code for the auxiliary water pump as well. Of course, there was never a problem with the heat when I drove it. And the forums seem to say that there should be heat from the ends of the dashboard, but no air at all from the center vents. So I wasn't sure that there was a problem. I measured the resistance across both sides of the valve, expecting that they might be different, but they were the same. I did not want to install a $200, non-returnable part just yet.

I found a post in which Gilly explained that the watervaves are designed to fail totally open, resulting in full heat. So I decided to unplug the electical connector on the duovalve to see what happens.

Sure enough, so much heat comes out of the end vents, that the CCU calls for the A/C to get the temperature down to the setpoint. Lots of cold air from the center vents. I replaced the duovalve, and six weeks later, all seems to be well.

Maybe it wasn't the only failure that could have caused the same symptoms, but it worked out ok for me.

so heat now comes from the center vent? as far as i know i have never seen a mercedes if operating correctly blow heat through the center vent, when heat is on high it will usually shut the center vent off completely, and direct heat elsewhere,as i recall it has something to do with heat in the drivers face causing drowsiness?? not sure, sounds like you may have a leaking vacuum servo somewhere.

If you still have the old duovalve, would you want to sell it. I need one for parts..
gary@siebring.com

No, nothing comes from the center vent, which is at it should be. I think when it was working properly, nobody noticed that nothing came from the center. It was only when it was misbehaving and people were uncomfortable that anyone noticed cold air from the center. I wrote this post just to give people an idea of how to replicate an intermittent duovalve symptom to help them in their diagnosis. My daughter was able to verify that the disconnected duovalve caused the same airflow from the center vents as she had experienced while the car was away.

But oddly enough, this problem was first reported to me (remotely, of course) during the previous winter. When I got my hands on the car in the spring, I found that the fresh air damper's vacuum actuator was leaking, causing various noises. I replaced it successfully and was confident that all of the problems were solved. I did test the heat during the summer to be sure, but the duovalve problem never occurred while I drove it, so I mistakenly thought that everything was normal.

It occurs to me as I write this that if I didn't have a duovalve, I might have simply unplugged the A/C compressor for the winter as a temporary, seasonal solution.

I do have the old duovalve - I was planning on taking it apart, but haven't gotten around to it. I'll give some thought as to whether or not I'm willing to sell it instead.