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Roller Coaster pressure cycling

lust4speed on Fri August 19, 2011 4:14 AM User is offline

Year: 1967
Make: Pontiac
Model: GTO
Engine Size: 400
Refrigerant Type: R12
Ambient Temp: 88
Pressure Low: 19
Pressure High: 170
Country of Origin: United States

I rebuilt the factory system with all new parts on my 1967 GTO. I replaced the POA valve with a replacement R12 valve that cycles the compressor on and off depending on the switch pressure. System has the specified amount of Freon, and has performed decent over the last two years since the rebuild, but has a problem that is annoying. The (adjustable) pressure switch attached to the replacement POA valve turns on the compressor at 38 pounds and off at 19 pounds, and the system cycles through this range over and over. The vent temps will reach a very comfortable 38°, and the compressor shuts off and the vent temp goes up to 56° before it turns back on. If I adjust the switch, I can alter the on off pressures, but there is always the 18 pounds of difference between on and off. The temps can be moved from the 38/56 to say 45/63, and they also follow a definite spread. The compressor is off longer than it is on as the pressure slowly rises up to 38 PSI when it switches back on.

The system consists of a new A6 compressor, a new stock evaporator, new expansion valve, new dryer, new barrier hoses, and a very large double-pass condenser out of a 91 Lexus 400. I'm thinking that I might have received a bum expansion valve. I don't mind purchasing another valve, but would like some opinions on whether it might really be the valve before the hassle of having the system evacuated and removing the fender to get to the well hidden bulb portion of the valve.

The cycling is dead consistent and has never varied. I've taken long trips across country in the summer with outside temps in the 108° range, and it performs the same. The compressor runs for longer on really hot days since it takes longer to get down to the low cutoff pressure, but just when the interior is getting down to a nice level, the output air warms up.

So what's my best course of action?

(My A/C car isn't the one in the avatar)

Edited: Fri August 19, 2011 at 4:16 AM by lust4speed

mk378 on Fri August 19, 2011 6:38 AM User is offline

This is why it came from the factory with a POA valve instead of a simple pipe and a switch.

If you can't revert it to POA it should be set up for control like most TXV systems with an evaporator temperature thermostat, instead of cycling on pressure.

Edited: Fri August 19, 2011 at 6:40 AM by mk378

newton5 on Fri August 19, 2011 11:34 AM User is offlineView users profile

This is the big drawback to the POA eliminator kits. What you describe is pretty much "normal" operation for that setup.
You may be able to add a bit more refrigerant to slow the cycling down a bit but you need to be mindful of the high side pressure while doing so. Getting the "correct" charge can be tricky

The easiest way to fix this is to reinstall a POA valve calibrated for R134a. You'll be much happier with the performance.

Hmm, I see your car is a 67. I believe it had an STV, not a POA. The same advice still applies, though. It should work really well with the original valve setup and the modern condenser.

bohica2xo on Fri August 19, 2011 2:52 PM User is offline

Pretty much standard performance when you remove the POA / STV and go cycling clutch.

Poor performance, accelerated clutch wear - huge improvement.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

lust4speed on Fri August 19, 2011 4:10 PM User is offline

System is operating with R12. So it definitely sounds like I will be going back to the stock POA valve. The stock one was operating when I did the mass replacement, but someone before me had converted to 134A and the system just wasn't performing that great. That's when I decided to replace everything with new parts and go back to the R12.

Do I need to have the old POA valve rebuilt since it was exposed to 134A for several years? Also, is there gradual degradation of a POA valve, or do they either work or not work?

Dougflas on Fri August 19, 2011 4:38 PM User is offline

the Delco/Harrison A6 compressors were never meant to cycle. The clutches are not large enough. Test your POA before using it. Test procedures are on this web site.

JJM on Fri August 19, 2011 10:00 PM User is offline

I never understood the logic of a POA eliminator... is it because it's too "complicated" or something? Using that logic, should power steering, power brakes, and automatic transmission be eliminated too? Do they sell kits for that? It can't be because of cost, because those stupid things are pretty pricey.

Ironically, the POA valve is probably one of the most reliable mechanical devices in MVAC; many 40 year old valves perform perfectly, even as the rest of the vehicle wastes away in the wrecking yard. And if there is a problem with a POA, it is extremely easy to diagnose, and the valve is typically the most accessible component in the entire system.

The reason why R-134a conversions don't work great is because the POA valve is not adjusted for R-134a, but mostly because there is not enough condenser and/or fan capacity.

So big thumbs down from me on POA eliminator kits.


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