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#1
01-07-2008, 01:10 PM
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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. 
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#2
01-07-2008, 05:24 PM
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Quote:
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.
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1967 250 S 1967 280SE 1968 250S 1971 280 SE 1973 220 1974 280 S 1975 450 SEL 1976 220 D 1979 300SD 1983 300 SD 1985 230 TE euro 1986 280SL 1986 500 SEL euro 1986 190E 2.3 1990 300 SE 2003 C230 K 2004 C230 K 1987 560SEL 1987 300SDL now in canada 2005 C230 1998 C43 AMG Sold to DON 1993 190E 2.6 300TD Former daily driver Totaled current: R320 CDI Ram 2500 Cummins
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#3
01-07-2008, 06:02 PM
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If you still have the old duovalve, would you want to sell it. I need one for parts..
gary@siebring.com
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#4
01-08-2008, 10:14 AM
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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.
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