Buick AC, freezing POA valve, warm air

[Gustav4523]

POA valve.jpg (233.8 KiB) Viewed 9841 times

Hey

I have a big interest in old AC systems, manual or ACC like the Cadillac Comfort control or Chrysler Autotemp. My knowledge about the cooling systems is not the best and I have an old 1968 Buick Electra with AC that barely cools. I did check the pressure on the low side and got around 30 PSI with compressor running. Thats according to specs. But! The POA valve gets really cold and frosty after a few minutes, but the temperature of the evaporator drops only a few degrees.

I suspect that the refrigerant don't flow through the evaporator, but maybe through the thin line from the expansion valve to the POA valve.

Could it be a faulty expansion valve or POA valve? Or could it be too little refrigerant, even though the pressure seems good. 2 years ago I got more cool air in the cabin.

Im not sure, but it could be a R12 system. Theres no signs of R134 conversion.

[JohnHere]

From the appearance of the low-side service port adapter on the POA valve, this '68 Buick probably still has R-12, although that's a guess. I can't see the service port adapter on the high side because it isn't shown. I know you said that there are no signs of a conversion, but double check whether there are any R-134a conversion labels on the radiator support or anyplace else under the hood. If not, we can "assume" it's R-12, although a refrigerant analyzer would tell for sure.

If indeed the system contains R-12, the first recommendation is to recharge it the "old fashioned way." By that I mean check the sight glass with the compressor engaged. If the refrigerant stream looks foamy, it's probably low on charge. If you have access to some virgin R-12, charge the system until the foam just disappears and the sight glass clears, then check the center vent temperatures.

What is the high-side pressure, and how is the fan clutch's condition?

[Gustav4523]

Hey, no signs of conversion on the high side either.

If the pressure on the low side is right, can the system be undercharged and the PAO valve be frosty but the evaporator not especially cold? Im not familiar with this. I thought that if system is low on gas, the POA valve should freeze.

Sorry, I havnt a gauge so I can check the high side. My gauge goes only up to 100 psi.

[tbirdtbird]

You really need to get a proper MGS manifold gauge set which gives hi and low pressures if you want to do AC work. You won’t get far without one.
John told you how to check the sight glass and that is a must. I also think the system is low on Freon

[Gustav4523]

Thank you! I search for a proper gauge set.
The system haven't had a charge for many years. The air was quite cold a few years back so could be low on freon. Could a low charge cause a frosty POA valve?

[JohnHere]

Could a low charge cause a frosty POA valve?

It's possible. However, if the A/C system hasn't run for a few years—and given the age of the car—testing the POA valve's pressure setting is advisable. Should you decide to convert from R-12 to R-134a refrigerant, the POA valve will also need to be re-calibrated to a different pressure setting.

[Gustav4523]

Yeah, Im planning to upgrade it. I, and the previous owner, have been using it för about 5 years and the performance has declined over the last 3 years.

R134 and maybe a POA eliminator. Im not sure.

[tbirdtbird]

Avoid a POA eliminator. You can get the valve recalibrated by any number of places on-line

[TheBandit]

It is normal for frost to form on the second half of the POA valve when it's regulating. That's because the POA will set the minimum evaporator pressure, but down stream the compressor can suck down further which will result in lower temps and frost after the POA. I've attached a photo of my POA valve with frost forming; you can see where it starts just after the regulating area of the POA

The small line from the TXV to the POA valve is a pressure equalizing line. Refrigerant does not normally flow through that line. It just balances pressure from the sensing bulb and a spring to set the superheat. Some systems also have a second oil bleed line that allows a small amount of oil to still circulate even if the POA is mostly closed down; that line is connected to the bottom of the evaporator and it's so small, only a small amount of liquid can pass through it and then evaporate in the second half / downstream of the POA. That can also result in some frost.

I have a GM POA system that I've updated to R134a and a few other things. I have a video that helps explain how the system works, which also has links to other videos showing how to test and adjust the POA valve when converting to R134a. You can find that long explainer here: https://youtu.be/jNUb-GDWz4Y?si=gxk2mC4lnMLtcFBQ

I would get a gauge set on it and if you see normal operating pressures, check other parts of the system like the blend door, heater shut off valve, recirculation ducts etc.

[Gustav4523]

Thanks! thebandit very helpful!

I did charge the system and the frost from the valve itself disappear. Cooler air from ac outlets now, but not that cool. The blend door cable was out of function, that's why I didn't get enough heat in the winter. Water valve seems to work and system sends vacuum as it should.

I received my gauge set yesterday. Gonna measure this weekend. Does a low pressure on the high side indicates a bad compressor? And how can I measure if the expansion valve needs replacing?

I think I have to replace the hoses to the AC vents too. They are old and quite hard.

My 1974 Pontiac has a more modern system, with VIR I think. It cools really well on R134.