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Discussion Starter #1 (Edited)
So I've been having this wierd problem for about a month and a half now, if the exterior temperature is below about 15* Fahrenheit, the HVAC system will default to defrost when I apply any throttle when the engine is at operating temperature. But if I'm not giving it any throttle it all, or if the engine is not at operating temperature yet, it'll act just fine. But, after about 15-20 minutes of driving at operating temperature, it'll stop acting so strange and resume it's normal operation.

I believe that there is ice in the vacuum check valve for the engine, which is the cause of this, hence why it's only doing it under acceleration. I'm thinking that after the engine gets hot, and starts to radiate enough heat to the engine bay, the ice in the vacuum check valve will thaw out and cause it to work just fine.

Could it be that this is the most likely cause, or is this something involving the actual HVAC module or control head? Could it be an electrical failure, instead of a vacuum related one?

My father has a '97 Chevrolet Lumina, and he had a similar problem with that a few years back, in the summer. His problem was the car would default to the floor position under any throttle, which meant he lost his air conditioning. He replaced the check valve and all was fixed.

I'm hoping it's that simple, the check valve is cheap and easy to access.
 

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Discussion Starter #2
Here, I made a video of it.

YouTube - Hvac system problem in my S Class

The wind noise you hear is the blower when it's in the default defrost mode. The windows are closed and the fan speed is as high as it goes...it's in "auto" mode, and I haven't changed the direction of the airflow by using the buttons. At 23 seconds in, I back off of the throttle, and you can hear the air diverting back into the floor position, and then at 27 seconds in, I lay back into the throttle and you can hear the system defaulting to the defrost position again.

Any and all help is welcome!!!! :) :)
 

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I am sure I am having the same problem. What I noticed that you didn't mention was the the auto lite on drivers side was off at the time. My feet were freezing, but the top half of the car was warm. Once I turned it off and turned it back on to auto then I my feet got warm after about 20 minutes. I have no idean what was going on, but I just thought that I did something to cause this happen like using the system without knowing what the operator's manual says about it. If you find out what is going on please let me know.
 

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Discussion Starter #4
When I had my car in for the oil change today, I asked my technician about it, and he said it would be caused by one of the vacuum check valves underneath the hood...not in the dashboard, like I was told by the dealer, because if it was in there, I'd be able to hear the hissing noise of vacuum loss under acceleration. He also said it was fairly common to see this, where water gets into the lines and causes the loss of vacuum...even moreso on Audis.
 

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Oh you're the guy who made the W140 videos of the test drives and when you bought it?! You kept/keep comparing it to Cadillacs lol.
 
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