[AMC-List] What I've learned about auto A/C

[Tom Jennings <tomj@xxxxxxx>]

I've built one auto A/C system from scratch, and doing a
second as a retrofit. I'm hardly an expert, but I've done a
LOT of research and asked some actual experts. There's a lot
of bullshit out there, especially small sites selling magical
"drop in" replacement fluids.  Nothing beats knowing how it
actually works though! Then you'll be able to tell a crock of
poo from chocolate ice cream. Here's a summary of what I know:

www.acsource.com is a great place. The guy who runs it has 35
years experience; provides advice to sensible do-it-yourselfers;
sells parts at reasonable prices, and answers tech question
email. I think Mark Price turned me on ho him; thanks!!!


First, you have to have a basic understanding of how A/C works:
when the freon gas is compressed, it gets really hot (same as
any gas). The hot gas cools in the condenser in front of the
car; it turns to a high-pressure liquid. This is hosed to the
evaparator under the dash, where the high-pressure liquid gets
squirt through a small hole (the "expansion valve"), turns to
a gas again, and soaks up heat (eg. gets cold). Exactly the
same as a spray can does -- the expansion valve is basically
the spray top.  The compressor sucks it in and cycles it through.


R12 vs. H134a: there are three practical differences; they inform
the Retrofit Rules below:

 * One, the pressure on the low side is higher for H134a than
 for R12. Hence Retrofit Rule #1.

 * Two, when H134a goes from a liquid to a gas in the expansion
 valve, it doesn't soak up as much heat as R12 does; about 90%
 the amount. Hence Retrofit Rule #2.

 * Three, the oil used in older R12 systems doesn't mix well
 with H134a, so internal parts don't get lubricated well,
 and wear out fast. Hence Retrofit Rule #3.

Retrofit Rule #1: When you fill an R12 system with H134a, you
will put about 10% less H134a in than you would R12. I assume
you have a TSM. See below for correct method of charging. No
need to freak out here.

Retrofit Rule #2: you simply need to pay more attention to the
condenser, so that as much heat as possible is taken out of the
hot freon. Make it bigger if you can afford it, but all of the
following is more than adequate: seal condenser sides so that all
fan air goes through, not around, the condenser; add an electric
fan if you have mechanical (drive with relay from clutch wire);
bigger fan. No need to freak out. With a good cooling system,
a retrofitted R12/H134a system could actually be colder than
a factory, typically sloppy system.

Retrofit Rule #3: dump out the old oil, if any, and replace
with the recommended PAG oil, preferably with a dye to help find
leaks. Small amounts of ester oil left behind will do NO harm;
apparently it will color the new oil but this is harmless.



The expansion valve: on our older cars, it turns out that there
are very few different, and semi-standard types.  You can get
H134a calibrated ones cheap. They are physically a major PITA to
replace unless you have the evaporator unit on the bench; if you
twist the copper lines on the coils you will wreck it instanter.
Use two wrenches and you're OK; but not easy upside down under
the dash!

But it seems to be just fine to simply run the R12 valve and
forgetaboutit.

Fittings: older systems use ordinary flare-type fittings.
They tend to leak unless assembled perfectly (entirely possible).
All new systems use O-ring fittings, which don't seal even
when assembled by monkeys. All new A/C components will come
with O-ring fittings.

This isn't a problem -- www.acsource.com sells very inexpensive
adapters that make O-ring compressors take flare hoses.

Compressors: there is NO REASON AT ALL except originality to
keep those old upright York type compressors. If it's working fine,
no problem, leave it! But if you need to replace/rebuild it,
the Sanden 508 works great, about $250 brand new.

ONE BIG PROBLEM retrofitting a Sanden into older systems is
actually easily solved: the Sandens usually have a #10 hose
on the suction side; my Hornet (and Rambler, and all our older
stuff) usually has #8. The solution is again www.acsource.com:
he sells hose ends that accept #8 hose (from the suction side
of the AMC evaparator) and attach directly to ther #10 O-ring
fitting on the Sanden.

Most places will sell you an adapter that trivially attaches
a Sanden compressor onto the mounting plate for a York.

For the AMC six, the later engines came with a complicated cast
aluminum mount for the compressor and alternator; this is the
way to go even thou it means buying new fanbelts. I don't know
nuttin' about them newfangled serpentines.

I even adapted one of those bracket setups onto
the 1970 232 (old block) in my '63 Rambler.
http://wps.com/AMC/1963-Rambler-Classic-550-Cross-Country/AC/index.html


Making your own hoses: after mounting the compressor,
if you don't already have one, this is the hardest part of
the project.  the entire project! You absolutely need to use
crimp type compression fittings. The crimper runs $450. Untill
I borrowed my father's, what I did was buy lengths of hose,
and the fittings, assembled it all in place (actually running
the hoses and hand-tightening the connections) then marking the
fitting position on the hose with a paint pen and taking them
to the local hose shop to be crimped. A/C barrier hose is stiff,
it cannot be twisted lengthwise, so test first.


Hoses, barrier vs. non-barrier, and leakage: new hoses are
all barrier hose, and contain a (duh) barrier to prevent the
H134a from leaking.  However, you CAN reuse your old, R12,
non-barrier hoses, assuming they are in good, non-leaking and
non-cracked condition. The reason is, it turns out that the
ester oil soaks into the inner layer of the hose and creates
a barrier during normal use. Studies were done about this,
referred to on the EPA site.


And last:

Proper system charging: this is hearsay from experts, I have
not yet done this! Pretty much everyone states that you should
fill the system normally (from the low side) with a gauge set,
initially to about 80% by weight what was specified for R12. Then
on a hot day with motor at 2500 rpm, the A/C blasting with the
windows open, add H134a until the low side pressure is optimum.

I have no idea what optimum is at this point.


My 1970 Hornet: I'm attempting to use the factory evap unit.
I long ago bought an R12-ready Sanden 508, but I'm dumping
the oil out of it and converting to H134a. I have a factory
condenser, about 16x24 (pretty big!), two electric fans, and
a 3-row radiator. I need a few fittings. I hope to
charge it up in a couple of weeks.

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