I've posted in vain in the following thread but got no answers -- AC Vents... Center vs Side Vents

My 94 C280 is having the hot air from some vents (usually side vents, sometimes foot vent), cold air from others (mainly middle) problem. It has been indicated that this is typically a vacuum issue but sometimes could be a heater valve.

I do not know where to begin on this. Could someone give me a link with diagrams or give me some pointers?

Side issue -- had my crank sensor replaced a couple of weeks ago. Ever since -- the transmission shifts have been VERY harsh. I'm told that likely some vacuum line was disconnected during the repair. The A/C vent issue started soon after that, but not immediately after that. Is it possible that these are related?

Thanks very much in advance of any help you can give.

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1994 C280, dark green metallic
105k miles

1991 190E 2.6, Black
191,500 miles
(sold to another forum member)

2003 Chevy Tahoe LT, Redfire Metallic
105k miles

1989 Mustang GT Cobra Convertible
43k miles

do a search for "duovalve" on the heater valve and you will find some good info. the duovalve is located in the rear left (passenger side) of the engine compartment.

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1994 C280
2009 VW Tiguan
1993 Toyota X-tra Cab SE-5


1973 220D ... Gone, but not forgotten
1991 Alfa Romeo 164L Gone, wife MADE me forget it

2006 Hyundai Tucson... just straight out FORGOTTEN!

Have a look here for diagrams, just search the list for your car and work your way in to what you need.http://www.detali.ru/cat/oem_mb0.asp?TP=1.
The problem you have with harsh shifts sounds familiar, mine was caused by the vacuum pipe (white) from the inlet manifold to the gearbox being disconnected at the gearbox end ,it passes close to the TDC sensor and connects at the driver side towards the front of the box, bit of a pig to reconnect but worth the effort. It may be that the loss of vacuum if this pipe is off is also responsible for your other problem,I don't know.
Hope this helps, Geo.

I was having the same problem with my '94 C280. Hot air out of the side vents and cool air out of the center vents. My fix was replacing the "duovalve" or dual valve. If you go to any website that sales parts and look it up you will see a photo. It is located under the cover near the firewall on the passenger side of the engine bay. A fairly easy do-it-yourself project.

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• 2001 E320