Re: LONG bypass exterior ambient temp sensor?

[James Marriott <marriott@micron.net>]

Elliott Potter wrote:
> 
> James Marriott wrote:
> >
> > Elliott Potter wrote:
> > >
> > > Now that winter is arriving again, I have another odd irritant/endearing
> > > feature of the Audi climate control to b*tch and moan about.
> >
> > > The car is a 93 90CS, with an automatic climate control.
> >
> > Is this R12 or R134A?
> 
> R12.

This is better for thermodynamic performance, but not so keen for
recharge ($$$) performance.

> > > The irritation is that when it gets below a certain temperature outside,
> > > the A/C won't turn on.
> >
> > The reason for this "malfunction" or "deficiency" is that below about
> > 35-40 dF (2-4 dC) there's pretty much no moisture left in the air. If
> > it's not there, why try to remove it? Also, especially if it's a 134
> > system, it is pretty helpless at these low temps by design. Not Audi's
> > fault, just a fact of the thermodynamics.
> 
> Here in Atlanta, we get this a lot--it's not quite raining, just a kind
> of mist in the air, just 2-5 degrees above freezing.  The windsheild is
> fine when I get in the car, but as soon as it turns on, the CC sends a
> big blast of warm air up to the windshield, and voila--instant fog.

Coupla things to try, though some might seem stupid.
1) If you've been running the AC/defrost for a while and you're ready to
park, turn OFF the AC a few minutes before said parking, but keep the
fan running. This will allow the evaporator (the AC part in the cabin)
to dry out, and help cut down this initial fog blast on startup. The air
may still be warm, but at least it won't be as wet.
2) Clean the windows _frequently_. Water can't condense without a
crystalization seed, just like there's a tiny bit of dust at the center
of every raindrop. I prefer ~20/80 ammonia/water, and use newspaper for
the cleaning. Note:  DO NOT USE THIS ON TINTING FILM. Plain glass only.
3) When getting in the car, leave the door opened until you can get the
car started and the CC going. Try to blow your breath out the door.
Sounds stupid, but having lived where your breath will _freeze_ to the
window--which takes a LONG time to clear when the interior and engine
are at -10 dF--it can help.
4) If it's been raining and you got in the car all drippy, leave the
windows or sunroof cracked the next sunny day (keeping security in
mind). Otherwise the sun will warm up the interior and evaporate all
that water from the carpet, etc and it will condense on the cold
windows.

> I can never seem to hit the buttons fast enough to prevent this.

Bummer on the CC. One advantage of the ancient manual controls as in my
cars . . .

> The other reason I want to disable the sensor is that Atlanta is also
> the Land of Deisel and Burning Oil, and recirculate won't work unless
> the A/C is on.  Just a little annoyance, you know...

Igor, any ideas how to rig this? Perhaps a fog light switch (any blank
spots on the dash?) wired to fake either the AC or the recirc into
working? Recirc, of course, in the winter will fog the windows in about
10 seconds, BTW. You DO NOT want the AC running when it's below ~38 dF,
or you'll just freeze up the evaporator. The only way for the system to
transfer any significant heat is for the evaporator to be much colder
(like maybe 20 or 50 dF) than the air. At these low temps, the
condensate will freeze on the evaporator rather than drip off like in
the summer. Typically this just cuts down the efficiency (heat
transfer), but if you condense enough, you can ruin the evaporator, like
blowing a coolant freeze plug.

> > > Anyway, does anyone know where the exterior ambient temperature sensor
> > > resides, or how to disable it?  Also, what would be the side effects of
> > > disabling it?
> >
> > On the 4kq it's under the cover behind the firewall (under the hood,
> > like with the wiper motor, and on the right side). Yours is likely the
> > same place. Mine is a little, black, marble-sized component with two
> > wires. Dunno if it's normally opened or normally closed. The "fix" for
> > mine would be to either disconnect or short the wires, depending on
> > which way it's configured. YMMV.
> 
> Well, could it possibly be in front of the little aux. radiator between
> the fog lights?

Doubt it. You want it somewhere where it sees pretty close to ambient
temp. The HVAC plenum is isolated from the engine's heat. The radiator
isn't.

> There's *something* there, about the size of a marble,
> with two fat wires coming off it.

Nah. The wires should be about the size of the headlight wires. Well,
maybe not _that_ small. ;-) They do have to carry a _little_ current.
I'd expect only a few amps to run the AC clutch. YMMV, as my car is
manual.

> Seems like an odd place, right next to a radiator, but who knows...

agreed and hth,
James