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1990 420sel
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Discussion Starter #1 (Edited)
I've searched, but couldn't find anyone with similar issues. When starting the car with A/C on set to min. temperature, I often get hot air coming from the upper vents. The lower and center vents blow cooler, though not super cold air. After about 5-7 minutes of driving the hot air no longer comes out and the air blows much colder. After turning the car off and restarting, the process starts over again.

I've just replaced my Climate Control Unit with a reman unit, and I want to make sure it is operating properly. Sounds like a monovalve issue to me, but all the posts I've searched say they get hot air all the time.
 

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1982 280 se aut. ecc. - sold
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1,607 Posts
Just how soon do you expect your AC to cool the hot outside air for you?

gr J
 

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2005 W220 S500 4Matic / 2008 W211 E350 4Matic
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It may very well be monovalve-related. I cleaned mine by removing it (search the forum for that - and be careful removing and handling it, it is a lot like pulling rods of uranium out of a nuclear reactor!) and then soaking it in alcohol, scrubbing gently with a toothbrush, then flushing with distilled water and re-installing. A 30 minute DIY for free, so do it before you start throwing money at it. And be CAREFUL with it, they are VERY costly to replace.
 

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1990 420sel
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324 Posts
Discussion Starter #4
Well I talked with a M-B tech about this issue, and he said that he thought that it was my flaps being slow to close. I didn't catch the exact part that he was mentioning, but he said the flaps for the heater always reopen when you turn the car off, and have to close. Mine is just taking longer to close.

If anyone knows what he's talking about, please chime in.
 

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Typcially, monovalve will cause temp to go up and down repeatedly, like it had a mind of its own.

If you have issue that takes a few minutes and then goes away until car is urned off, then repeats, that sounds too consistent for bad monovalve. I'd concur with flap diagnosis.
 

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1990 420sel
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324 Posts
Discussion Starter #6
brucemendel said:
Typcially, monovalve will cause temp to go up and down repeatedly, like it had a mind of its own.

If you have issue that takes a few minutes and then goes away until car is urned off, then repeats, that sounds too consistent for bad monovalve. I'd concur with flap diagnosis.
Thanks Bruce. Do you know which flap he was speaking of? Is this a DIY without removing the entire dash, or do I just live with this?
 

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1982 280 se aut. ecc. - sold
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My monovalve had a little rip in it. At startup I would get the temperature I wanted. When stopping for something and thus creating other pressure in the monovalvebody after let's say ten minutes hot air would come out and I had to go to the off position.

Why don't you carefully remove the valve and examine?

flaps direct air to the vents IMO so I doubt.

gr J
 

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86 560sec-168K -Ext. Light Ivory/Int. Black/01 E320 4Matic Wagon Brl Silver/Black/ Mad '95 Q45
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a3jeroen said:
My monovalve had a little rip in it. At startup I would get the temperature I wanted. When stopping for something and thus creating other pressure in the monovalvebody after let's say ten minutes hot air would come out and I had to go to the off position.

Why don't you carefully remove the valve and examine?

flaps direct air to the vents IMO so I doubt.

gr J
Where exactly is the monovalve ?? my car consistently generates HEAT ??
 

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1991 420sel
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267 Posts
It could be a monovalve issue. A torn monovalve seal will sometimes provide hot water to the heater core at the beginning, but then the pressure in the cooling system will close the monovalve and keep it closed even if the heat is on. So a torn monovalve could do what you describe. If your monovalve is more than 10 years old, I would change it anyway. Get the Bosch insert kit, about $40 bucks, and change it. Relatively simple job. There are references in the DIY section to articles with pix that make this job easy.
 

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1984 500 SEL euro, 1989 560 SEL, 1980 450 SL, 1981 DeLorean, 2013 F10 BMW
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What your tech was referring to was an issue with the vacuum diaphragms. There are 4-5 depending on the model car you own. Several can be accessed from under the dash or thru the glove box, while others would need the dash removed to replace them. About a 16 hr job to R/R the dash.

By your description, the upper vents are controlled by the vac. diaphragms that would need the dash removed. It sounds as if your s are leaking and are on their way out.

Regards,
 

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1990 420sel
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Discussion Starter #11
500MPL said:
What your tech was referring to was an issue with the vacuum diaphragms. There are 4-5 depending on the model car you own. Several can be accessed from under the dash or thru the glove box, while others would need the dash removed to replace them. About a 16 hr job to R/R the dash.

By your description, the upper vents are controlled by the vac. diaphragms that would need the dash removed. It sounds as if your s are leaking and are on their way out.

Regards,
Unfortunately for me, I think that you and the tech are correct. I guess that I will just live with it, or look into removing the dash myself, or worse yet pony up the $$ to get it done.
 
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