Folks, I have been physically disabled for the last 4 months, and when I started driving again last week, I discovered that the AC vents are acting up. I have heat coming out of the two left-most vents, and the right vents have cold air coming out of them. My wife tells me that this summer the ac came out of the two left-most vents, and heat came out of the right-most vents.
Has anyone experienced this, and do you have any suggestions as to how I might fix this? Thanks so much in advance!

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1983 300D (Hans)

Which button of the 5 in the CCU do you have pressed? What happens when you press any of the others?

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

First allow me to explain that I have no vacuum going to the interior of the car, I have a leak in one of the door diaphragms and don't want to tear into the doors, so I have plugged the vacuum in the engine compartment. I had my mechanic open the two middle vents years ago so that a/c blows out of them, as before, I only had a/c blowing out of the right and left-most vents, but they have worked well this way for years.
I was using the second from the left button yesterday, but my wife tells me that this summer when she would drive the car, the same thing happened when using the middle button. I know that the left-most button is the defrost button,which causes heat to blow out of the defrost vents.
Any help and advice is greatly appreciated, thanks so much!

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1983 300D (Hans)

Sorry, I have no advice on the left/right problems, but I do have a comment on the plugged vacuum system. The door system and climate control system have two separate lines exiting the engine compartment (yellow door and green climate control I believe). You can plug the door system without plugging the climate control system.
The door system can be plugged with little detriment, but there are a lot of vacuum pods in the climate control system that should be operating. I suspect that only your door system was actually disconnected and that your climate control system has been operating on vacuum all along and recently one or more of the pods has failed causing your current condition.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I just repaired a guys wagon for that problem...got the suggestion here to check the MONO VALVE for the heater system. I put in a new element because the diaphragm was torn and it works fine now. I can't explain why it heated on only two vents. The car I worked on did the same thing yours is doing.
If you have a vacuum leak on the doors, ONLY unhook the yellow vac line to the cabin. That will disable the doors and not the HVAC vents.

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.

Thanks so much - I replaced this a few years ago, but suspected that this might be an issue. Does this web site sell the monovalve insert, and how much might it be?

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1983 300D (Hans)