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Davidf · 06-15-2015, 04:31 PM

All the vintage GM air conditions (i.e. what was used in the '67-72 trucks) use a POA valve. So, any a/c shop that knows their stuff should be familiar with a POA valve. A POA valve lets the compressor run constantly by limiting the amount of refrigerant that can pass through the evaporator based on pressure. The colder the refrigerant, the lower the pressure. So, if evaporator is about to freeze up, the pressure drops below a given value of 29.5 psi (R12) and the valve restricts the refrigerant flow until the evaporator temp rises again. But, it never shuts off the flow and thus the occupant does not notice a rise in air temp at the vents. Make sense? I think I got that right. There is some interaction with the expansion valve as well. The EV meters the refrigerant change from liquid to gas.

The stupid POA update tube simply monitors refrigerant pressure and turns off the compressor if pressure gets too high. Not very elegant and during the off state, the evaporator gets too warm.

POA valves are readily available on Ebay and they are all the same no matter what vehicle they came out of.

06-15-2015, 04:31 PM	#66
Davidf Registered User Join Date: Nov 2014 Location: Austin Posts: 749	Re: Converting factory AC to use R134a All the vintage GM air conditions (i.e. what was used in the '67-72 trucks) use a POA valve. So, any a/c shop that knows their stuff should be familiar with a POA valve. A POA valve lets the compressor run constantly by limiting the amount of refrigerant that can pass through the evaporator based on pressure. The colder the refrigerant, the lower the pressure. So, if evaporator is about to freeze up, the pressure drops below a given value of 29.5 psi (R12) and the valve restricts the refrigerant flow until the evaporator temp rises again. But, it never shuts off the flow and thus the occupant does not notice a rise in air temp at the vents. Make sense? I think I got that right. There is some interaction with the expansion valve as well. The EV meters the refrigerant change from liquid to gas. The stupid POA update tube simply monitors refrigerant pressure and turns off the compressor if pressure gets too high. Not very elegant and during the off state, the evaporator gets too warm. POA valves are readily available on Ebay and they are all the same no matter what vehicle they came out of.