Front Demist Function - any technical experts?

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Volvo V90 D5 AWD
On my 211 if I push the front windscreen demist button, the dual zone temperature LCD's go off and the air is directed towards to front windscreen vents.

All perfectly normal.

However, if I do it manually, by turning the LH and RH dials to point to the screen , increasing both temperture rockers to max and calling for max power from the fan, it blows more strongly than on front screen demist.

Why is this - does the bespoke front demist button flow less air in an attempt to make what hits the screen that bit warmer, for example.

The manual doesn't say.
 
Mine does something similar - the front demist is NOT the same as manually directing the airflow to the screen and setting max power on the blower.

What is actually does seems to vary according to both the car/ambient temperature and the engine temperature, though what logic exactly it uses is not clear.

Also, on the W203 if you remove the cabin filter in the engine bay you can actually see the air mixture flap - and in auto mode it seems to be either open or close as the climate control system sees fit, regardless of the position of internal air circulation button.
 
The fan speed will be varying in relation to the differential between the engine water temperature and the internal ambient temperature. the greater the differential the greater the fan speed.
It also uses a minimum water temperature to start the speed variation off.
 
I'm expecting there is a technical reason with it being an MB, I just wondered what it is.
 
Could it be that the windscreen demist is used with the A/con on so the blower isn't as powerfull, just a quess
 
Could it be that the windscreen demist is used with the A/con on so the blower isn't as powerfull, just a quess

On the sLK the a/c goes off when the demist button is used. On the CLK it stays on. Not sure why.
 
IME pushing the demist button starts a logic sequence, including the fan and heater not really working up until the water has some temperature to it.

Most cars that I have had demist better with AC on as it kicks the dehumidifier into the circuit.
 
In this cold weather I am having to opt for the manual fan speed and direct to the windscreen as the auto demist simply is not quick enough - I am told by the dealer that its part of the blueffeciency ... seems a bit bizarre if you ask me. Apparently all the sales guys in the dealership use manual override as the auto demist in weather like this is not quick enough and is waiting for the engine to warm up.... and this is on both E class and C class as well as CLS

My manual fan also gets stronger as engine revs rise above 1500 rpm, which is equally frustrating ... and apparently this is also normal and part fo the car's programming and the fact that it knows better ;)

Marc
 
i had to use the demist on ours for the first time today in this bizarre weather we're having.

in the CLK it was great, windscreen & side windows all clear in no time, but i was very underwhelmed by the performance today, it took forever! this thread has explained why...

correct me if i'm wrong, but the demist button also controls the heated windscreen? if you opt for manual demist, i presume there's no way to turn the heated windscreen on at the same time?
 
i had to use the demist on ours for the first time today in this bizarre weather we're having.

in the CLK it was great, windscreen & side windows all clear in no time, but i was very underwhelmed by the performance today, it took forever! this thread has explained why...

correct me if i'm wrong, but the demist button also controls the heated windscreen? if you opt for manual demist, i presume there's no way to turn the heated windscreen on at the same time?

You have a heated windscreen :dk:.
 
You have a heated windscreen :dk:.

have a just made a fool of myself?! :eek:

the CLK had one, i think, along with the heated wing mirrors. i just assumed the same on this one as it's got the wire lines around the outside section of the windscreen.

will have a look in the handbook later...
 

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