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72 Monte conv to r134 worked for a while

1972mc on Thu June 24, 2010 1:43 PM User is offline

Year: 1972
Make: Chevy
Model: Monte Carlo
Engine Size: 383 stro
Refrigerant Type: r-134a
Ambient Temp: 90
Pressure Low: 20
Pressure High: 175
Country of Origin: United States

This site is awesome. I stumbled upon you guys and have found the true experts!!

I have a 72 Monte that I've owned for 30 years. Back in 1984, I opened the system and that is how it's been for 28 years. Fast forward to 4 years ago and I undertook a full frame off resto. Did all the work myself except for body/paint and engine (put a crate motor in her). She has been on the road for 2 years and finally getting around to the AC.

Got in touch with Classic Air in Tampa and ordered the following:

A-6 compressor (came pre loaded with oil)
expansion valve for 134
poa (mine cleaned-plated-calib for 134)
drier (from Classic Muscle, Winston Salem NC)
liquid line (from Classic Muscle)

S&D hose and fitting with muffler
condenser to drier line

Changed all o-rings to green
Clamped the temp probe back to the low side of evaporator and covered with the tar sealant
Did not flush system-but did blow air through condenser until flakes stopped coming out. Blew air through the evaporator but no junk came out
Cleaned out the case of all nests but did not clean off fins and can't remember their condition as I could not get the evaporator out of the box.(4 years ago!)

Did not remember about the A/C vent control vacuum line connection from firewall to intake until a week after getting system charged. This has been remedied!!

Got system charged up with 52-58 oz r134a, (too much for a 3.75 # r12 system) and after a few connections being tightened, no leaks!! Boy was I surprised. Drove it to Ohio from VA and had marginal cool just out of the lap vents. After running for a few hours it seemed to stop cooling. I was getting freezing up just after the expansion valve. Water was gushing out of the bottom of the case. Would still work after letting thaw for an hour. The front seal on the compressor was throwing some oil (green dye). To complicate this description- upon initial charge up I did not have the re-calibrated POA so I installed a backup r12 POA. Eventually during the trip, I got no cooling but still would get condensation on low pressure side.

Fast forward- I had system taken down (it still had 50 oz freon), installed the re-calibrated POA and got it re-charged. He said it frosted up the lines but no cooling inside cockpit. Then after 10 minutes or so, no condensation or frosting of any lines. An interesting sidenote, The compressor clutch would turn freely when I received it, I noticed after the problems started , the clutch was difficult to turn when ac was off. After he pulled system down, clutch was free again, then very tight after re-charge. Also am getting some minor AC belt squealing upon initial engine startup when AC is in OFF position.

My mechanic does not know how to troubleshoot an r12 POA type system, but is interested in finding out since he has a restored 1972 C-10 still on r12.

I have no gauges or recovery tank, but have another mechanic who does. His initial over the phone diagnosis was the new compressor, since both lines on the compressor are hot. I would not think there is a restriction since high side is 175.

Sorry for the long life story, but more info is better info. Any assistance would be greatly appreciated.

Carl Hanson

1st Gen Monte Carlo Nut

mk378 on Thu June 24, 2010 1:58 PM User is offline

The theory of a POA system is based on the POA regulating evaporator pressure to about 25 psi to prevent freeze-up. The boiling point of refrigerant varies with pressure, so keeping the pressure from going any lower than that will not allow overcooling.

The low pressure test port is on the evaporator side of the POA, so the pressure reading will show the POA performance.

Also, air in the system (failure to evacuate fully before charging) will mess up the pressure / temperature relationship and allow freezing even with the POA properly working as designed.

Never use more than the original specified R-12 charge weight on a conversion job. Ideally you want to be charged very close to the same weight.

Edited: Thu June 24, 2010 at 1:59 PM by mk378

bohica2xo on Thu June 24, 2010 3:59 PM User is offline

System open for 28 years.

"Did not flush system-but did blow air through condenser until flakes stopped coming out. Blew air through the evaporator but no junk came out "

And it stopped working properly after a few hours? Really?

The evaporator should not ice. The POA valve is set way too low @ 20 psig. Or it is stuck due to some sort of debris from an un-flushed component.

Right back to square one.

System teardown, flushing. New dryer - it is full of whatever else came from the condensor. Inspect the TXV, it may be full of something in the inlet. Bench test the POA valve as shown on this site. If it is set at the correct pressure, make sure it is clean. Cycle it a few time to make sure it works well.


"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.

Dougflas on Thu June 24, 2010 5:10 PM User is offline

engine running at 1200 to 1500 rpm, 20inch window box fan in front of grill, test POA while on vehicle by disconnecting blower motor lead. Low side should not fall below 26psi for R134 or 30psi for R12.
If you are using r134, the OEM condenser is inadequate; especially if there is any garbage stopped up in it. Air blown thru it will not clean it out. Use a pulsator by Hecat inc to clean out evaporator. Back in the yrs 1972 thru 1974 Chev had a bunch of bad evaps. You may want to replace yours. The placement to the tXV sensing bulb is also critical so make sure you have that correct. It should be at the 2 to4 o'clock posion of the tubing: definitely not on top nor bottom. Make sure the oil bleed line from the evap (small line connecting to POA) is clear. The schrader for this line is special--Do not just put any schrader in there. Start with 80% of the R12 charge. Do not rely on the sight glass for R134.

I recommend staying with R12. These systems were bullet proof in their day. They usually put out 48º in the driveway with the above rpm and fan.

70monte on Thu June 24, 2010 6:32 PM User is offline

I'm interested in how this turns out since I have a 1970 Monte Carlo that I have had for 23 years and the air has not worked for pretty much all of that time. Since I have my 609 certificate and can buy R12, I plan on sticking with R12 but I don't have a clue about how the POA valve works.

I do have a new POA, dryer, compressor, and condensor that I bought a few years ago from another Monte owner but I have never installed any of it and don't know the quality of any of the parts. Good luck.


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