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High Pressure on Low and High Sides

69-er on Sat March 23, 2013 10:04 AM User is offlineView users profile

Year: 1968
Make: Pontiac
Model: Firebird
Engine Size: 400
Refrigerant Type: R12
Ambient Temp: 70
Pressure Low: 40
Pressure High: 325
Country of Origin: United States

This AC work can be very frustrating for us guys that don't do it often enough! I thought there were some troubleshooting guides somewhere here based on pressures, didn't see it. Hoping somebody can give me a tip or two what might be the problem here:

68 Firebird, TXV/POA system, R12.

Background: Had intermittent cooling problems last year so I changed the TXV last night. Evacuated for an hour, no indication of leaks for several minutes after closing valves. As before, it will only take a few ounces of refrigerant, have to operate system and then it takes several minutes to charge the entire 3 3/4 lbs. (This has always worried me running the system with such a low charge at the beginning.)
I couldn't get the last 2 oz in.

Pressures: 40/325 at 1500-2000ish RPM. Ambient temps: 70 degrees.

I would imaging the pressures are going to get higher as the ambients get higher. Didn't have time to try a water test on the condenser. Also didn't check vent temps. At 70 degrees ambient, it will always feel cold anyway.

Any ideas?

69-er on Sat March 23, 2013 10:11 AM User is offlineView users profile

I also changed out the receiver drier during the TXV change. And I didn't think of looking at the sight glass during operation.

Dougflas on Sat March 23, 2013 1:48 PM User is offline

70* ambient is just a little too cold to test accurately. You have already stated you did not test by misting water on the condenser. High side to too high thus low side will read high also. Disconnect the blower motor, low side should drop to 30psi. Rpm's 1200 to 1500. If that happens, try misting the condenser after reconnecting blower motor. High side should be 170ish at 70*ambient. Don't forget to place a 20inch window fan in front of the vehicle.

69-er on Sun March 31, 2013 4:13 PM User is offlineView users profile

I forgot about the blower test. I didn't think a small fan like that would make a difference. I'll give a try.

So, with an extra fan and/or misting the condenser, if the pressures come down that indicates an air flow problem?

Dougflas on Sun March 31, 2013 8:22 PM User is offline

Quote
Originally posted by: 69-er
I forgot about the blower test. I didn't think a small fan like that would make a difference. I'll give a try.



So, with an extra fan and/or misting the condenser, if the pressures come down that indicates an air flow problem?


correct

WyrTwister on Mon April 01, 2013 7:54 AM User is offlineView users profile

Quote
Originally posted by: 69-er
I also changed out the receiver drier during the TXV change. And I didn't think of looking at the sight glass during operation.



Please excuse my ignorance , but what is TXV ? Throttle valve ?

God bless
Wyr

AutoCool on Mon April 01, 2013 8:19 AM User is offline

Thermostatic
eXpansion
Valve

NickD on Mon April 01, 2013 10:54 AM User is offline

Trying to think of the last time I worked on a POA system, how does around 1974 sound? POA compressors, was that an A-6? Have a sump in them, run all the time and with very cool weather, pump very little refrigerant. Ha, what is R-12?

Extremely difficult to charge via pressures, the earlier ones had a sight glass, needed an 85*F + day, engine at 1,500 RPM, doors open, blower at max, would charge to a clear sight glass, than add a couple of ounces more. GM marketing got rid of that so you had to buy a charging station. POA pressure should show around 28.5 PSI, these and should show that even at very high ambients. Maintained that constant pressure above normal idle so would cool well even in a 130*F ambient. This fed the TXV valve that you already got a definition of. Has a capillary tube on it that opens this variable orifice valve on demand, when more cooling is required.

When they quick using the sight glass, guys didn't understand only worked right on hot days, switched to a charging station, but had an electronic device on the high side line that was suppose to measure the bubble content, those didn't work well either, could do a better job with a fine feel with your hand. After you sandpapered off all those calluses. Like a safe cracker.

Best bet is to use a charging station and measure in the exact quantity. When I see pressures like yours, air in the system comes to mind, a charging station gets rid of that air first without having to change hose connections. Air in system also results in long term charging.

Typing all this from memory and feel I am losing it at times, photographic memory has been well over exposed with all this current BS. Other old timers on this board, they can add more. Do you even have a good POA valve in your system? Doesn't sound like it.

In my neck of the woods, even after 4-6 years, these would be a pile of rust back then, so didn't keep them very long. Recall others trying to convert these to R-134a, a real nightmare. Least you are sticking with R-12.

Dougflas on Mon April 01, 2013 5:31 PM User is offline

>

I have a photographic memory also. It's just sometimes I run out of film.


Orignal poster, Make sure you test the airflow across the condenser before you start chasing your tail. All shields and baffles must be in place. Clutch fan must work also. Mist the condenser and let us know if the high side drops like a rock. CLUE!!!!

NickD on Tue April 02, 2013 7:21 AM User is offline

Have to spend at least a couple of hours on my computer each week cleaning up all this spam and junk for more memory space, not easy to do that with my own brain. Like when GM switched from that POA to that CCOT system, cast iron compressor to aluminum, ceramic seal to that single lip seal, great Scharder valves to these quick couplers that cant' be repaired.

But when I see these posted pressures, sure looks more like a CCOT system than a POA system.

Edited: Tue April 02, 2013 at 7:21 AM by NickD

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