R12 to R134a

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Maverick
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Joined: Sun Apr 26, 2009 6:29 pm
Location: Raleigh, NC

R12 to R134a

Postby Maverick » Wed Jul 29, 2015 5:35 pm

If your ride started out with an R12 AC system, this will be interesting.

http://www.e38.org/pparish/oil.htm

As will this on contaminated refrigerants and incompatible desiccants.

http://www.aa1car.com/library/us397.htm
Maverick

User avatar
Maverick
Posts: 1746
Joined: Sun Apr 26, 2009 6:29 pm
Location: Raleigh, NC

Re: POA or STV

Postby Maverick » Thu Aug 06, 2015 1:25 pm

Some Ford and GM R12 AC systems in the 70s and 80s controlled evaporator temps, not by cycling the compressor, but with a valve that meters the flow of refrigerant in the low side line right by the evaporator. The valve is sometimes called a POA (Pilot Operated Absolute) valve which, to me, is NOT descriptive. The Ford shop manual calls it an STV (Suction Throttling Valve) which I can relate to. This is the '78 Continental STV.

Image

and this is where it's located on the passenger side fire wall.

Image

If I understand it, the valve opens to allow flow at a pressure equal to the vapor pressure of the refrigerant (originally R12) near the freezing point of water. The goal is to keep ice from forming on the outside of the evaporator.

The STV in the '78 Continental is speced to open at 28.5 PSI at sea level and 29 PSI at 1000 ft. When a POA/STV R12 system is converted to R134a, it's recommended that that valve be recalibrated to open at the vapor pressure of R134a at or near the freezing point of water. The vapor pressure of R134a is 28 PSI at 32.27*F. So, I guess the STV should be recalibrated, like everyone says, but I kinda like 27* air coming from the vents. :mrgreen:

If the evaporator does freeze up, which I think it might in cooler temps, I'll see about recalibrating the STV. Classic Auto Air charges $99 plus shipping both ways to test and calibrate the valves for R134a. One post I saw said the adjusting screw should be turned 1/4 turn so I'll try that before spending $120.
Maverick


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