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Discussion Starter #1
Here is an interesting behavior that I saw last night on the MO. I completely understand how the HVAC operates.... or I thought I did. I started driving everything normal, pretty cold outside. OAT was 15°F I had the system on AUTO, set around 68°F. System was blowing nice warm air on the feet only.

Here is where is gets interesting.... after about 35 minutes I notice that now the MO is blowing on the feet AND defrost. Why? I don't understand. I thought that maybe my wife pressed a button so I pressed AUTO to ensure it was still in AUTO mode and it was and still blowing out the feet and defrost.

Strange... why would it do that.

On the return trip same starting conditions but a little colder 0°F.... started off on feet and after about 35 minutes it went to floor and defrost for the rest of the 55 minute trip.

Any ideas why it would start to blow out the defrost? Perhaps the MO is not only sensing temp but also humidity in the cabin? Now it couldn't be that advanced... but then again.
 

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SHIFT_FASTER
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On Auto, the system alters where the air comes out depending on some factors. If the cabin is colder than the set temperature, the air will start coming out on the feet. If the cabin is warmer than the set temp, the air will start coming out the mid level vents (if I remember correctly, summer was a long time ago!).

After the cabin warms up close to the set temperature, it will go into bi-level mode, sending air to the feet and (perhaps depending on outside temp) either defrost or mid-level vents.

It's quite a nice system, although in the winter I always have it set to feet and defrost.
 

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Discussion Starter #3
Yes you are paritally correct.... when it needs to "cool" down the cabin it send the flow to the vents. I've never experienced defrost before. I'm really not complaining... just curious.
 

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SHIFT_FASTER
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Gonzo said:
Yes you are paritally correct.... when it needs to "cool" down the cabin it send the flow to the vents.
I think that's what I said.


If the cabin is warmer than the set temp, the air will start coming out the mid level vents.
 

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I've had the floor mode switch automatically to bilevel before. However, I have never seen it switch to floor/defrost. I also notice that if I turn the AC off manually, the mode is still in "auto" but the setting doesn't switch to bilevel after the cabin has warmed up like it does in auto with AC" mode. Anyone else notice this?
 

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Discussion Starter #6
Yes I have noticed too. Even with AC off, which is how I typically operate it, until a mode is selected the system is in auto. Where to warm the cabin it sends air flow to the floor. To cool the cabin it will send air flow to the vents. In either case when the temps are getting above/below desired temp, depending on how you look at it the system will use a combination of vent/floor or if necessary send air flow to either 100%.

So ex. its cold outside and your set temp to 68°F, heated air is sent to the floor. Now lets say that the car is in the sun and the OAT is getting warm too. The inside of the car is getting too warm, then air flow is directed to vents and floor.... if the interior temp still continutes to rise then air flow will be directed 100% to vents.

The reverse is true of cooling the cabin. The MO uses the light/sun sensor on the passenger side to help minimize large fluxuations in auto control due to sun hitting the car. Cute huh? :)
 

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'03 SE 2WD Cold Pkg, VDC
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I tried this on the way to work, 25-30 minutes. AC off, mode stayed to the floor. Fan speed altered though...
 

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Sometimes it takes a long time for the cabin temp to reach what it's set at. After 30 minutes here, the fan speed will only be down one level from max.
 
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