Re: [DMCForum] Accumulator located

["Walter Coe" <Whalt@xxxxxxx>]

> I thought the system always operated below atmospheric
pressure even after
the refrigerant was introduced following the vacuuming.

A properly functioning a/c system is always under pressure
of at least 28 psi (when running).  When at rest, the static
pressure should be much higher such as around 60 to 80 psi
depending on ambient temperature.  Why the pressure doesn't
go below 28 psi is because there is a low pressure cycling
switch.  The compressor runs until the low side gets to
between 28 - 32psi which causes the low pressure switch to
cut the compressor off.

When the compressor cycles too often, real idiots like to
first adjust the low pressure switch to make the system
cycle longer.  If the system was already low on refrigerant
then dorking with the adjustment can make the low side
pressure lower than atmospheric pressure.  Then it sucks air
& moisture.  If it was short cycling for a different reason
then the high side can get too high and explode.  Only early
VINs don't have over-pressure protection.

> If you open a charged system then you will see the
refrigerant gas escape,
but that is due to displacement caused by the atmosphere
rushing in until
they equalize, right?

Greg, remember the article I posted on 'dark sucker theory'?
When you open a dark closet, you don't actually see the
light rushing in but rather the dark coming out.  Opening a
charged system is very similar to draining air from a car
tire.

> then he would shoot a little Freon in, then vacuum it
again.
Repeating this process a few times before finally filling
the system.

Yes, this is very good to do.  But few professional do it
due to extra cost & time involved.  It's basically the
difference between doing your own work vs paying a flunky to
do it for you. (sarcasm)  Some people use nitrogen gas to
purge the system of air.  With new compressors, it is a good
idea to pressurize the system and turn the compressor pulley
several times to seat the seal & distribute the oil.  Then
pull a vacuum & charge.

> I don't see how a
schrader valve would let any outside atmosphere in, unless
it was defective,
due to its design.

The schrader valves on the DeLorean's a/c system are
identical(*) to the ones used on car tires.  If you push it
down with your finger nail then air comes out.  If there is
a hard enough vacuum behind it then the atmospheric pressure
can be enough to push the stem in letting air suck past it.
(*) = due to R-134a conversions, new a/c shrader valves are
made of special materials to resist decomposition.

Walt

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