[V8] The Old Gray Mare's hot flashes....

urq urq at pacbell.net
Tue Aug 14 11:33:12 EDT 2007

I'm not at all familiar with the HVAC system in your car, but I suspect it
is not too different from the one in the V8 ... given that assumption ...

The way the system is set up there is coolant flowing through the heater
core almost all the time.  Temperature control is effected via flaps that
route air through and around the heater core.  Given this, what is more
likely to be happening is that the blend doors get misdirected from time to
time ... could be flaky control of the flaps or a bad motor.  On the V8 the
motor that drives the blend flaps is mounted to the airbox.  

There are probably diagnostics built into the HVAC controller ... before
digging in under the hood see if you can't get any clues there.  I think
there may be another list on audifans where you might find people who know
all about this car's HVAC system ... of course there may be someone here who
knows too ...

Happy hunting!
Steve B
San Jose, CA (USA)

-----Original Message-----

I think I have found the problem that has been causing The Old Gray Mare to
have hot flashes.  If you didn't see the original thread, my "new" '90 Audi
100, aka, The Old Gray Mare, has been having hot flashes.  While driving
with the a/c on, periodically the cool air would turn warm, then after a few
minutes, would turn cool again.  

Normally this would take place on "temperate" days.  Days when the outside
temperature was in the seventies to low eighties.  On really intense bright
sunny days, with temperatures well into the eighties and above, the a/c
would blow cold without interruption.  

Last week while driving, I decided to turn off the a/c entirely, since it
really wasn't terribly warm.  I opened the sunroof, and cracked the driver's
window.  Turned the compressor off....selected "econ".  

AND.  Nothing.  Still warm air blowing out inside the car.

"Hmmmmmmmmmm," sez I.  "Very suspicious."  So here is my theory.

The "fault" is not in the a/c at all, and when the a/c is on, it is doing
what it is supposed to do, drive the air temp to what the temperature
selector says it is supposed to be.  

BUT, the heater control valve is vacuum operated.  (Either with or without
an electronic sensor...dunno which type yet).  And IF the heater control
valve is losing vacuum somewhere, then it could be allowing warm coolant to
run into the heater core, and provide heat AND a/c at the same time.
Depending on the temperature required by the air conditioner, it could be
overriding the cool air, until vacuum was restored.

First off, I will check the heater control valve this weekend.

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