# Re: ~ long fogging and AC (was: Re: 1st Snow in My A4 (1/2 long))

```Hello again,

> > experimentally measured values. If you can heat that cm^3 up 30 or
> > 40 degrees, it  will hold thousands of molecules of water.
>
> Fine.  And as that cm^3 passes by a cm^2 of almost-freezing
> windshield, what happens?

Yup, if the air is cooled below the dew point, it becomes a bathroom window.

> > A small change in temperature makes a _huge_ change in
> > spongeability. That's why the relative humidity in your house is so
> > low in the winter. Outside, maybe it's 40 F and  80%. By the time
> > that same air comes in your house and gets heated to 70 F, the
> > relative  humidity has gone down to 0.16%. Yes, less than 1%.
>
> Exactly.  Look at it the other way.  Consider a cold, rainy
> winter day, where the interior of your car is 60F at a balmy
> 50% RH.  Run that air by that chilly windshield.

_That's_ prime-time for the compressor. The windsheild must also be _clean_ (if you
haven't cleaned it in the past, oh, two weeks, it's not _clean_).

> > Your compressor would run at 5 F?
>
> Not sure about exact temps, but they would all definitely,
> without question, run below 32F.

Ok.

> Really, this can be argued academically in all kinds of ways.

I'm not trying to be academic, I'm just trying to give some background information so
someone can arrive at a solution.

> But Jim,

> have you ever actually been in a car where you could
> in fact turn on the compressor during a cold day when the windshield
> was fogged?

Yep. The Subaru would clear a _completely_ fogged windsheild in about 10 seconds from
engine startup/compressor actuation. That thing didn't have AC, it had a refrigeration
system. (only worked above ~38 F)

> Honestly, you would be absolutely astounded at the
> immediate and thorough defogging result.  Maybe I haven't explained
> it in principal, but the result is *VERY* definite.

I'm with you.

> That's why
> those of us who've been doing this are screaming so loud now that
> we have cars that won't allow us to.  It is MUCH more effective
> than heating.

I wonder if your (ie, A4 folks) temperature cutout is higher than others? On the 4k, it's
towards the far right of the intake plenum, next to the inlet screen. If you jumper (or
disconnect, depending on the wiring) that bugger, the compressor will not be denied below
X degrees. It seems to be a simple thermo-switch. I don't know if it's normally opened or
closed.

If I get a chance, I'll check the data tables tonight. I know R-134 isn't as good as
R-12, but I can't remember in what ways. Maybe the system _can't_ function at a lower
temperature. The A4 does have R-134, right? Who has another (brand of) car with R-134? At
what temperature does its AC give up?

> > > Now, will someone please find out what microcontroller is in
> > > the climate control so we can chip it?!?
> >
> > 2.2 bar max without heater-core milling! And infinite O2 sensor
> > life. ;-)
>
> Careful with the wastegate adjustment, though; might end up
> blowing out a window...

Especially Eric and PDQ.

cu
James

```