Heater always on

The heater in my 91 300CE seems to always be blowing. The climate controls are non-functional and have no effect on the heat coming out of the side and defrost vents. What
could be causing this?
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Loose vacuum hose is primary culprit... check for loose vacuum hose in engine compartment.
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I looked and nothing seems to be loose. Could there be another likely cause?

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It may not seem obvious... but look on your firewall area... there should be two yellow checkvalves... one in and two out... check those connections... make sure the nipples are not broken.
Other thing to check is on the side where there is only one... pull it out and make sure you got good vacuum.
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I messed around with the checkvalves and tubes a bit and voila, the climate controls started working again. They probably still need replacing soon though. Thanks!
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Replace those checkvalves... very very simple job... Welcome!
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Although it's a simple job my local MB stealer quoted me $34 apiece for those little pieces of plastic. Any idea where these can be obtained for a realistic price?
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Try any online part store... I remembered paying $10 each... been a while though.
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I replaced one of the checkvalves (the one with two connections) but it hasn't fixed the problem. Should I replace the other one? From what I've read the the one I replaced is connected to the line that controls the heater.
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What type of vehicle are we talking about here?

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91 300CE

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