my w210 1997 e320 developed a strange problem .The AC WORKS FINE EXCEPT THAT THE AIR COMMING FROM THE DRIVER SIDE VENT IS HOT!
How to explain this ?!
Omar
Abu Dhabi
United Arab Emirates

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Omar
UAE
1997 silver , Avantgarde
E320 W210
101000 km

is it hot or ambient? likely is a defective duo-valve, there are two vaves hence the duo part, the best way to test them is turn the temp to low, and then hit the EC button to turn off the a/c compressor. if the temp is still hot you will need to verify the duo vave for that side is getting the signal to close (they are open by default, power closes them)
if the air you feel is ambient and not heated, you may have a low freon condition or a partially clogged receiver dryer

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As posted "you may have a low freon condition or a partially clogged receiver dryer"

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I had the identical problem, and it was a clogged duo valve. Do a search and you can take it apart and clean it with some wet sandpaper.

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2008 S550
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While this can be caused by the duovalve, low refrigerant flow is a much more common cause. You won't have "hot" air coming from the left side, but ambient air will feel hot compared to the still-cold right-side vents.

Thanks to all of you who took the time to answer the question.

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Omar
UAE
1997 silver , Avantgarde
E320 W210
101000 km

Did you get it fixed?

Especially with a 12-year old car, low freon would be the most likely culprit. Studies have shown that normal leakage rates are about 6% per year, so after 10 years or so, the freon will likely need a recharge. My '99 E430 hit its time earlier this summer.
I recommend recharging first, since it's simple and cheap, and then if there are still problems, diagnose further.

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Prost!

I would recommend reclaiming whatever refrigerant is in there and recharging to specs by weight. If the system still shows a warm left-side vent (and the problem isn't the heater), there is a restriction in the system.

It might be good to note that it is very bad to run your compressor with insufficient refrigerant flow, be it due to refrigerant level or a restriction. Refrigerant flow in the evaporator is necessary to return oil to the compressor. Insufficient flow causes insufficient oil, and eventual compressor damage.

I went to the AC "doctor" who found the freon pressure to be fine. He rescheduled the car for a follow up visit this week to check the system (including the valve)

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Omar
UAE
1997 silver , Avantgarde
E320 W210
101000 km

OK, now you can move on to:

http://www.eclassbenz.com/node/27

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Prost!

it was fixed! it turns out that the AC IS o.k. but the heater was on .a simple solution was made ; the hot coolant hose that feeds the dashboard was disconnected and pluged with a sparking plug!! this means in winter I may not be able to defrost the windsheild. but herein the middle of arabian desert we rarely have cold days in winter

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Omar
UAE
1997 silver , Avantgarde
E320 W210
101000 km

Doesn't that mean the duo-valve was bad?

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Prost!