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Windscreen Demist

B

berrytastic

Member
Good evening all,

a question on the operation of the windscreen demist.

On thursday I went to you the windscreen demist (as the screen was misty). The temperature was set at 21.5 (auto), I pressed the demist button, and as expected the icon illumiated, the fan speed inceased, but it pumped out hot air.

This may seem an odd question, but I had never noticed it sending out hot air to demist the screen, I always thought it was at the temperature as displayed, the a/c works fine when on auto, and the air temperature is cool when on a lower temperature setting.

Can someone please let me know if I have just being missing this (I have had the car for 2 years, so a big miss)
 
As far as I can recall it just puts the air onto the windscreen and puts the air con on with a faster fan speed to get rid of moisture inside on the screen. I don't think temperature comes into it.
 
That is what I thought.

But as I said the temperature was set at 21.5 and when I pressed the demist button, the fan speed increased and the temperature went to hot?
 
Is it not possible that the ambient temperature inside the car was below 21.5 and the heater was automatically activated to raise the temp accordingly? The auto function will attempt to raise or lower the temp as quickly as it can until the selected temp is reached and only then will the blown air temp be the same as the temp selected.
 
If I am honest I am not sure. That seems a very plausable explaination.

I was just surprised at how hot the air on the demist was.

I guess I will need to keep an eye on it and see if it happens when the air temperature is warm.
 
I think the system is designed to demist in the quickest possible way, thus when pressing the screen demist button it overrides the set values and switches on the aircon, fan and temperature up to max. De-selecting it reverts the system to the existing set values.

However I have always understood that it is not good practice to switch in the aircon system at anything other than idle/low revs to protect the aircon compressor clutch. This seems to be ignored if you press the demist button at higher engine revs.
 
With the 944, when you press the demist button, the fan goes to max and also to max heat so quite possibly the same.
Alasdair
 
On the 3.2 when you set it to demist using a highly complex pattern of lever positions, all that happens is a lot of noise is emitted, and the screen takes 10 minutes to clear, instaed of 12 minutes if you hadn't done anything! [:(]
 
Following on from the original question/problem, I checked the operation again last night on my way home from work.

The control panel was set to AUTO and 21C, with the fan running at 4 bars and cool air (noticable cooler than when the a/c is not selected) coming from the vents.

I then selected demist screen, the fan speed increased to full bar and the air flowed out the vents on the top of dash as expected, but the temperature rocket up to hot (equivilant to Hi on the temperature setting, to the point I needed to open the window to let the hot air out. The temperature was still set at 21C.

So the question still remains, why does the temperature increase?

The only why I can demist teh screen without melting at the same time is to select demist and set the temperature to Lo

Could some check if there car does the same so I can understand if there is a problem.

Thanks
 
On my every day car, I know for a fact that if demist only is selected the air con does not work even when it was running.... the indicator light on switch goes out.... therefore it would then kick out the type of heat (21*) I guess that would feel quite warm.

Not forgetting also that if using climate control & heat inside vehicle generated through occupants & sun (!) if you can see it through all the rain, the air con would cut in to a colder setting to get temp back down to 21....

However, if demist & 'upper' vents are selected, air con would still work.
I'm assuming the same principles would work on my 996!

Does this explain anything to help?

Paul
 
This happens on my BMW 530D Touring with climate control so I always press the MAX button if the system doesn't self regulate quickly-this despite recently replacing the "hedgehog" in the heater unit on top of the transmission tunnel.
What is the hedgehog?-I hear you ask-well its the generic title for the climate control module -this senses the heater duct temperatures & is a diecast "stiff bristle" looking device with the electronics encapsulated into the top where the loom plugs in.
When it failed (a known phenomenon)-everything went haywire.
 
With regards to the air from the windscreen vents feeling "quite warm" when I select the demist, the air coming out is not warm, it is hot (i.e. max and nothing like the cooler 21C coming from the vents when the demist is not selected).

I am sure there must be a fault with the controls somewhere, as I am struggling to believe you should get cool air coming from the vents on one setting, and when you select demist it should rocket up to hot. Currently I have to open the windows at the same time as to not start sweating.

But I am not sure, so I guess I need to keep an eye on it or get it checked.
 
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