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Fog on OUTSIDE of windshield, Is there a fix?

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Old 07-31-2009, 10:07 PM   #1
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Drives: 2006 CLK 350 Coupe
Question Fog on OUTSIDE of windshield, Is there a fix?


I'm new to my 2006 w209 and live in a humid area. I get big patches of fog on the outside of my windshield, much more than I ever experienced with other cars. The wipers clear it but it comes back in seconds.

The defroster duct is passing too much cold air on the windshield and it is cooling it condensing fog on the outside.

I use auto setting on the climate control and it is the worst. If I go manual and kick the fan up to 5 or 6 then it is a little better because the system has to blend warmer air to maintain the cabin temp with such high fan flows. The noise of the fan is bothersome and it is inefficient to use that much conditioned air.

Can it be fixed?

I thought about putting a cover over the ducts on the dash but that is hokey.

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Old 07-31-2009, 11:53 PM   #2
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consult your user explains how to maintain the fog....
something like 3 degree diffrence to keep the fog off...
i know its in the manual though.
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Old 08-01-2009, 10:40 AM   #3
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Haha I had the same problem this morning, it just went away!
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Old 08-01-2009, 11:40 AM   #4
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I believe what is said in the owners manual although I'm calling BS on MBZ. It's humid here often and in the summer the car can get hot and you need the 20 -30 degree drop of AC air to cool it off. If it was maintained at 3 degrees it would never cool the interior.

Car manufacturers favor low air flow when the interior reaches the set point for several reasons. Less noise, less air blast, and less air through the system so the AC compressor can cycle and be more efficient. Less air through the pollen filter too so it lasts longer. I agree with all of that.

The problem is less air flow needs cooler air to maintain the temp. When they bleed it on the windshield it condenses on the outside. I have a patch on the passenger side that is over 1 foot long and 6-9 inches tall. Very annoying. It comes back in 2-5 sec after using the wipers on it.

So now I wonder if the defrost flapper is hung open more than it should be or if it is controlled by the ECM can it be reset by MBZ? I'm just wondering if my car has a problem or are all W209 CLK like this??

I am considering buying one of those carpet dash mats to put up there and tape off the defroster holes on the carpet. It would fix it but I think they degrade the interior and I would have to yank it off when I needed defrost.

What does it take to dissassemble the dash to get to the defrost flapper valve? Or is there some other bypass built in the system? Could be some weather stripping have come off internally or hung the flapper open?

Wonder if Rain-X on the outside of the windshield would help?

Please help, and thanks for comments so far.

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Old 08-01-2009, 04:59 PM   #5
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Location: Norfolk UK
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I do not understand this problem ... unless I have misread something here.

Firstly, it is possible to direct your a/c air to feet/face/screen with the unit controls.

Then you have manual control of the vents by using the thumbwheels. You can close off air to the windscreen completely. if required.
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Old 08-01-2009, 08:19 PM   #6
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maybe euro spec cars are different, I don't know. I notice from your info that your model (240) is not sold here. Here are the facts.

USA spec 2006 CLK350 car.

2006 climate controls may be different than 2004 in your car.

The thumbwheels on the upper dash only control the dash ducts not anything else

The controls on the climate center do not fully shut off the defrost duct to the windshield regardless of position or selection. This is very common on most all of about 40 cars I have owned in this country. My Porsche was the exception. Therefore, I do not doubt, by design, that MBZ wants some amount of air to the defroster at all times. It is just too cold which causes the condensation.

The defrost button will dispense with it but it blows a lot of hot air. Besides, the fog comes back as soon as the rapid defrost mode is off.

Besides I really want to use the auto mode where the car selects the ducts and distribution and fan speed for me. It is a nice feature.

I am running my temp settings straight up in the middle, 72 deg F, which the manual suggests. So I am not trying to freeze the interior or anything like that.

What I need to know is this common in humid areas with this MBZ or do I have a climate control issue that needs to be addressed???

This problem was pretty much non existant on most other cars i owned in this climate. Keep the help coming, Thanks!


Last edited by ETN550; 08-01-2009 at 08:21 PM.
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Old 08-02-2009, 10:28 AM   #7
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Location: Norfolk UK
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ETN 550 - thanks for clarifying your question.

It's true that the CLK 240 was never marketed in the US - just Europe, Far East & Australia. The body and the car interior will be identical to yours. However, the engine is a V6 2.6l M112 as used in the C class range.

I have the a/c unit called Thermotronic, which sounds similar to your set-up. This does a good job of managing the climate 'inside' the car, using a number of temperature sensors distributed around the interior. Your problem of course, is that you want to control the climate 'outside' the car to prevent condensation on your chilled windscreen.

I have witnessed this on rare occasions here in the UK on days of very high humidity, where a small area of my windscreen mists up on the outside.

My solution (if the a/c is set to Auto) is to manually reduce the fan speed by a division or two, and it generally disappears quite quickly.

I believe that there is a d-i-y diagnostic test that can be run to checkout the a/c system operation - but I haven't ever tried it.
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Old 08-02-2009, 10:28 AM

2006, car, cars, fog, fogging, fogs, lot, mercedes, moisture, morning, prevent, rain, remove, roadster, windshield, windshields

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