From: thurman [HillTH@navair.navy.mil]
Sent: Wednesday, July 26, 2000 9:59 AM
To: 928
Subject: [928] Re: AC More Questions! LONG

> Ok, I live here in hot, humid Orlando, Florida and my S4 has no ac. I NEED
> AC. Today I took my shark over to a friend who works on ac systems for all
> cars and he checked my system out and he said that my ac compressor was
> going and that I had a minor laek from the evaporator. Now he is no 928
> expert, but rather a person who knows ac systems. My question to all is,
> how does this system work? I understand there is an outside temp sensor
> imbedded in the alternator cooling hose, an interior sensor on dash which
> by the way is presently covered by a dash mat. What does this expansion
> valve do?? I understand that they fail a lot. What is the system pressure
> suppose to be? He charged my car free of charge and now I have ac. Its cold
> but I think not as cold as it could be. When I slide the front slide
> anywhere but 65 degeees, instant heat. A month ago at an Orlando shark
> owners meeting at Steak and Shake I met a fellow who said he would look at
> my sytem and fix it the right way. Well I lost his card. He lives in West
> Florida or works there. if you know of him please email with directions.
> Thanks for your listening ears and let the responses come on in
>

OK, here is the Reader's Digest Condensed version.

The AC works by boiling refrigerant in the evaporator coil, which is
located under the dash. As it boils, the refrigerant absorbs heat,
cooling the coil. Air is forced through the evap coil, and the now cool
air is blown into the car. The  refrigerant then goes to a compressor,
that compresses it up to around 300psi. The now compressed (and hot) gas
is sent to the condenser coil, which is mounted in front of your
radiator. Air flowing through that coil cools the gas, which under high
pressure, liquefies in the coil. This evap/condensing cycle is what
makes the system so efficient. The liquid refrigerant now goes to the
dryer, where dirt and moisture (water) are removed. After the dryer, the
refrigerant goes to a small orifice (hole). This is the expansion valve.
The effect is similar to what happens in a fuel injector. It sprays a
fine mist.  Expansion valves don't break, BTW, they clog.  And since
this is a closed system, the pressure is really low on the far side of
the orifice (about 100psi). This causes the refrigerant (which by this
time is back in the evaporator coil where we started) to evaporate very
quickly.  So the main components are the compressor, the condenser, the
dryer, the expansion valve, and the evaporator. The compressor can fail,
which will cause pressure loss and no cooling. The condenser and
evaporator can leak. Think of the dryer as a kind of dry sponge. Once it
is wet, it swells, and since there is no place for the water to go, it

stays wet. If its clogged, its trashed, and must be replaced. Any
problem that allows outside air to get into the system for more that a
day or so will clog the dryer with water. The system must be evacuted
with a vacuum pump to remove the air before it can be charged with
refrigerant. The refrigerant in your system is contaminated with air
because of the leak, so your cooling is reduced. All we have left now is
the control. The compressor is cycled on and off by a pressure switch to
maintain a high side pressure of about 300psi. The pressure switch is
usually mounted on the dryer. If the switch fails, the compressor's
clutch will not engage, and the compressor will not run. A small leak in
the system will cause the compressor to run continuously, icing over the
evaporator. If you notice less and less airflow, which can be fixed by
turning the system off for a few minutes, you need refrigerant. As the
leak causes the pressure to go down, eventually condensation will no
longer take place and the system will quit. The in dash controls
regulate the amount of cold air that is sent to the cabin. I don't think
they control compressor cycling. They are in series; either sensor
failing will send the system to full hot. You can find info about the
sensors (and a lot of other stuff), at Greg Nichols excellent website,
http://ourworld.compuserve.com/homepages/nichols/Cat05.htm
Hope this helps.
thurman