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Adjusting POA Valve

69-er on Tue April 26, 2011 7:44 PM User is offlineView users profile

Year: 1968
Make: Pontiac
Model: Firebird
Engine Size: 400
Refrigerant Type: R12
Ambient Temp: 80/100
Pressure Low: 33
Pressure High: 200-250
Country of Origin: United States

I did the pressure adjustment check on my POA valve as outlined by Dougflas. The pressure was about 25 PSI. I read somewhere here that the factory setting was 29.5 PSI. I also read that the specs for altitude compensation was 1/2 PSI for each 1000' in elevation. I'm at 4000' so I adjusted it to 32 PSI.

With ambient temps at the front of the condenser at 100 degrees in the shop or 80 degrees (current outside air temps) going down the road, I can only get 55-60 degrees vent temps. Assuming all other areas of the system are OK, would my adjustment have been responsible for this? I'm not sure the effect of going up or down from the 29.5 PSI has on the vent temps. I remember how one member adjusted his POA down a few PSI with an R134 system and the vent temps improved by a significant amount. I don't know how that would relate to an R12 system.

Also, I noticed the low side was pretty consistent at around 33 PSI whereas the high side would slowly fluctuate from about 200-250 PSI at 2,000 RPM in the shop. The factory fan was pulling a fair amount of air since the vent temps were pretty consistent in the shop or on the road.

Is the low side pressure governed by the POA valve or the TXV? Any ideas on the high side fluctuations? It seems like one of the valves is adjusting for varying conditions.

Thanks again!


mk378 on Tue April 26, 2011 8:48 PM User is offline

The POA regulates the low side. You set it to 32, you got 33, it's working OK.

The POA regulates absolute pressure. It is not affected by altitude. A conventional manifold gauge measures gauge pressure, the difference between the absolute pressure in the line and the atmospheric pressure. For the same absolute pressure, the gauge will read high at high altitudes because the atmospheric pressure is lower. Thus the compensation factor.

The POA system works by not letting the compressor pull down the low side too far. This keeps the boiling point of the refrigerant in the evaporator high enough to prevent ice-up on the air side. In other words the POA acts to throttle down system capacity when necessary to prevent overcooling of the evaporator. Everything else working, lower POA setting, lower low side means lower vent temperature. It's the same with either refrigerant, but the optimum setting for R-134a is a few psi lower because of the different boiling point of R-134a.

The TXV may not be working properly. If the evaporator doesn't get enough liquid refrigerant (undercharged or TXV issue) the vent temperatures will be high. Is the sight glass running clear? Make sure the sensor bulb is properly attached to the evaporator outlet line and fully insulated from ambient heat. Also of course check for reheating from the heater core.

69-er on Tue April 26, 2011 9:14 PM User is offlineView users profile


So, you wouldn't recommend lowering the pressure back to 29 PSI? FYI, here in the desert, the humidity is very low.

On the sight glass, I noticed that it cleared up pretty early in the charging procedure. It would bubble occasionally during this time, but at full charge and at 80 degrees ambient, the last I remember, it was clear for the most part. I've been told in the past that the sight glass isn't really the definitive answer to the state of charge, although the GM service manual states above 80 degrees it is.

The TXV bulb is positioned properly and insulated with the proper "tar" tape. I there a way to test the TXV?

I'll have to check the heater core for reheating. Didn't think of that.

Dougflas on Tue April 26, 2011 10:33 PM User is offline

To test the POA on the vehicle, use 1500 rpm, fan in front of radiator. Unplug the blower motor. Low side shold read 28 to 30. To test the tXV on vehicle, see my notes on test Chrysler epr's. Epr and POA do basically the same thing.

Also, charge this system via sight glass. Charge it until there are no bubbles at idle, then add an oz more. The drier will store any extra.

Edited: Tue April 26, 2011 at 10:36 PM by Dougflas

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