DeTomaso Mailing List: July 98, Message #218

I see a basic lesson on AC is in need here.  

The AC is a pump.  It pumps heat from the inside of the car to the outside.
 It does this by taking advantage of the chemical state change of a
chemical compund.  By some coincedence or they designed it that way, freon
is a good chemical to use.  When it changes from a liquid to a gass, it
absorbs alot of heat.  Then when it changes from a gas to a liquid, it
releases alot of heat.  It is also a coincidence that if you pressureize it
to about 20 psi, it turns to liquid and if you release the pressure, it
turns back to gas.  

Now we have a system.  use a pump and pressureize the freon to 200 psi and
it turns to liquid.  THe liquid releases lots of heat, in both the hoses
and the condensor.  If the heat cant be released, then the pressure builds
up too high.  Then the cooled liquid freon passes through the expansion
valve (just a small orfice for the most part) and the liquid expands to a
gas.  Now this gas wants some heat real bad. So as it passes through the
evaporator (wonder where they came up with that name) the gas evaporates
and absorbs heat.  As a matter of fact it absorbs so much heat that it will
freeze the water that condenses on it.  From there the gass passes back to
the compressor and the whole thing starts over.  I left out the part where
the dryer removes moisture from the freon.  

Now, all the other little stuff are extras.  Some systems will cycle based
on a pressure switch in the low pressure gas side.  As the compressor pumps
all the gas to liquid, the pressure on the low side drops and the
compressor is shut off.  When the pressure goes back up from the
evaporating gas, the compressor comes back on. (GM)  Then you have the
system where there is a temp sensor in the evaporator core.  As the core
gets cold, it turns off the compressor.  Then when it starts to warm back
up, the compressor comes back on.  Sometimes they make this temperature
adjustable.  (pantera and old underdash units)  Both of these methods do
two things, they keep the pressure down in the condensor, and prevent the
Evaporator from freezing up.  We need this type of temperature adjustment
because in the pantera we dont have the little trap doors that bypass the
air around the evaporator like the big cars do.  This is also the same
reason why we adjust the heat with a water valve instead of little trap doors.

One of the problems mentioned is when that little orfice in the expansion
valve plugs up with crud, the freon cant flow around the sytem and it dont
cool any longer.  It is also hard on the compressor and hoses since the
pressure builds up real high.  This is where that little GM low side
pressure switch would prevent failures of hoses.

Dave D

At 10:18 AM 7/6/98 +0000, you wrote:
>Mike,
>Your are absouletly correct. The Job of the expansion valve is to
>monitor the temperature of the evaporator. It will constantly adjust
>itself to prevent the freezing of the evaporator coils. If the expansion
>valve temperature probe is not correctly positioned in the coils of the
>evaporator that could also cause the system to freeze up.
>
>Cary
>
>> ----------
>> From: 	MikeLDrew@aol.com
>> Reply To: 	MikeLDrew@aol.com
>> Sent: 	Wednesday, July 01, 1998 6:35 PM
>> To: 	Multiple recipients of list
>> Subject: 	Re:  Compressor Cycling
>> 
>> Tim wrote:
>> 
>> >My AC has worked great for 7 or 8 years now.  My only complaint is
>> that the
>> system seems to cycle too often.  It blows really cold, but not for
>> long
>> enough before if cycles.  I wonder if there is any way to increase the
>> cycle
>> time that the compressor is on?
>> 
>> >>>Hmm.  I'm far from an A/C expert, but I'll give this one a shot,
>> 'cause it
>> saw it happen to P.J. Couillard.  His '72 had some funky hard lines in
>> the
>> system under the dash that you could see from the passenger seat.  We
>> noticed
>> the same thing happening, and I could see that the metal pipe would
>> completely
>> freeze over, then un-freeze, then freeze again...
>> 
>> The A/C would blow ice-cold for a few minutes until this hose froze
>> over (and
>> was blocked off), then it would quit until it warmed up and melted off
>> (and
>> dripped condensation all over my feet.)  Thus his system would cycle
>> back and
>> forth like yours does.  I think he replaced the expansion valve and
>> cleaned
>> (or discarded?) the screen and that solved it.
>> 
>> Might want to try having a passenger monitor the status of your hose
>> (if
>> visible) as you drive down the road.
>> 
>> Am I WAY, WAY out to lunch here guys?  (I'm sure you won't resist the
>> temptation to tell me if I am! :>) )
>> 
>> Mike
>> 
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