Automotive Air Conditioning Information Forum (Archives)

Provided by

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

Pressure for 1970 Chevy Pickup on R12 Pages: 12

Bob B on Tue July 31, 2007 4:59 PM User is offline

Year: 1970
Make: Chevy
Model: C10
Engine Size: 350
Refrigerant Type: R12
Ambient Temp: 100
Country of Origin: United States

What should normal low and high side pressures be running at 1500 rpm, 100F ambient, R12 for
1970 Chevy pickup C10 - 350 cu in?


oznznut on Tue July 31, 2007 6:38 PM User is offline

30 psig suction----200 psig discharge


Bob B on Wed August 01, 2007 11:48 AM User is offline

At 30 suction and 200 pressure the line up to and including the expansion valve and sensor are frozen and no flow is getting to the evaporator.

Any ideas?


mk378 on Wed August 01, 2007 11:52 AM User is offline

Your evaporator will also be frosted then, so there's no air flow. Refrigeration system is working good -- too good. If it doesn't have a POA valve there should be a thermal switch on the evaporator to cycle off the compressor before frosting occurs.

Bob B on Wed August 01, 2007 5:18 PM User is offline


I not too familiar with all of the acronyms (POA ?) but there is a small (approx. 1.5 " dia) diaphragm type valve or switch with a capillary tube running to the evaporator housing which is also freezes up. Is that the control device for the compressor? Or should I be looking for a switch inside the evaporator housing itself?


Dougflas on Wed August 01, 2007 5:41 PM User is offline

That is probably the tXV. There may still be a screen in the inlet. They were known to plug up causing a restriction thus a freeze-up. There should be a sensing bulb at the end of it secured to the evap tube. Remove the clamp and put the sensing tube in your hand of hot water. You should see your pressures change if it's working correctly.

Mk378 they did not cycle the compressor back then. They used A6 compressors and tXV and POA ( or suction throttling valves)

Edited: Wed August 01, 2007 at 5:43 PM by Dougflas

Bob B on Thu August 02, 2007 5:04 PM User is offline


I did some research and the system does have a POA and a expansion valve with a bulb attached.
I will have to wait for the weekend to check it out. What is the easiest way to check out the POA?

Bob B

mk378 on Thu August 02, 2007 5:12 PM User is offline

Ah I read your post again. The lines between the condenser and TXV should not freeze in any case. It should all contain warm liquid refrigerant at condenser outlet temperature and pressure. There must be an unwanted restriction where the line gets cold.

Dougflas on Thu August 02, 2007 5:13 PM User is offline

To check the POA on the vehicle, you need to be correctly charged. THen unplug the blower motor lead, enginf at 1000 rpm or so, the low side should be 30 lbs with R12 at sea level.

Bob B on Thu August 02, 2007 6:04 PM User is offline


There was on thing that happened that I forgot to mention:
With the system at 35 psi and 200 psi and rpm at 1500, the high side pressure crept up slowly about 20 psi every 2-3 minutes until it hit 300+ psi. A that point the frozen components thawed and the system seemed to be working. Shut down the system and let the system stabalize and the problem was there all over again.


Dougflas on Thu August 02, 2007 9:03 PM User is offline

It is important that you zero your gages before you start. The low side needs to be 28 to 32 lbs with 30 lbs optimum. Make sure your cooling fan is working. If the pressure is 300, mist the condenser with a water hose. If the pressure drops dramatically, look for a problem there. If it drops. check the low side pressure. Also, test the POA as I described earlier. Don't rule out two problems.. 1) the cooling fan clutch is bad 2) you may be undercharged at the same time causing the POA to frost up. I wouldn't think it wouldn't frost at 100º but who knows. Other things that come to mind with POA systems are blockage of the oil bleed line and the TXV screen plugged.

I don't usually throw parts into a system but since we are looking a 37 years here, why not put a new TXV in if you open the system? And how about a thorough flushing? And you could test your POA on the bench.

Edited: Thu August 02, 2007 at 9:06 PM by Dougflas

Bob B on Mon August 06, 2007 1:46 PM User is offline

I didn't realize that an under charge would cause the POA to freeze up. Everything seem to work ok (several months back) until I had to replace a leaking hose assembly from the compressor. After a few months, I had lost the a/c charge completely. I replaced the hose assembly, vaccumed down the system and recharged from a 30 lb cyclinder. I wasn't able to measure the pressure charge so I just worked with the low and high side pressure until the were correct (L =30; H=200).
Some observations that I made this weekend were that at 1500 rpm and the POA is frozen up, the line from the expansion valve into the lower part of the evaporator is good and cold buy not frozen. the return line from the evaporator is warm. If I drop the rpm down the idle speed, everthing warms up as if the a/c isn't on. But even at Idle speed the pressures are still as stated above.


Dougflas on Mon August 06, 2007 5:11 PM User is offline

Is the sight glass clear or does it show bubbles? The POA is supposed to keep the low side at 30lbs so it appears it is working. I feel that you are under charged. Even by using a 30 lb cylinder, you could have put it on a digital bathroom scale or a ups postage scale. Did you test the POa as I described earlier?

Bob B on Mon August 06, 2007 6:02 PM User is offline

The sight glass does not show any bubbles, not even if you shut it down for a few minutes and then restart. I didn't have a chance to do the POA test yet but will do so when I get back to the desert where my truck is garaged.


Bob B on Tue August 28, 2007 2:07 PM User is offline

I finally got to check ou the POA by unplugging the fan motor as suggested. The LP side was 30 psig but everything else stayed the same. The POA was still frozen with the line from the evaporator housing to the POA being warm. The 3/8" line from what appears to be an expansion valve (has a small capillary tube running from it and going into the evaporator housing) which goes into the lower portion of the evaporator housing is also very cold. Even the suction line from the compressor to the POA is cold. The cooling fan is working well as spraying the condenser doesn't change the high side pressures. Any other suggestions before I tare down and replace the parts?

Bob B

Dougflas on Thu August 30, 2007 8:02 PM User is offline

Did you test the TXV? Remove the sensing bulb, warm it up in your hand and then place it in a cup of ice water. You should be the pressures change. One of your posts states that if you shut the syatem down you con't see any bubbles in the sight glass. You HAVE to see them when you shut the system down. The pressures equalize. You may have a restriction. Verify that the pressures equalize.

Bob B on Fri August 31, 2007 10:36 AM User is offline

When I checked this past weekend, I was able to see very slight bubbles at first during start then eventually disappearing. The flow looked normal. I will check the bulb out this weekend. The one this that's odd is that the 3/8" line going from the expansion valve to the evaporator box is very cold (not iced) which should indicate that the sensing element is working and the freon is being released to the evaporator from the valve. Is this a correct assumption? If so, a restriction between the expansion valve and the evaporator might exist. ??
In any case, I will open the evaporator box this weekend and check out the bulb and any thing else I can see.


Dougflas on Fri August 31, 2007 11:20 AM User is offline

Photos would be a big help. Your TXV has 4 lines,right? One in, one out, one for the sensing bulb, and one equalizer line if we're thinking the same way. That is why we need some photos. These systems were not that hard to work and diagnose as long as you knew how the system worked.

Bob B on Tue September 04, 2007 10:16 AM User is offline

I did take some photos but have to download them and figure out how to get them on this website.
For the time being the TXV does have 4 lines. While running the a/c, the 3/8 "in" line is hot, the 3/8
"out" line to the evap is very cold, the "equalizer" line to the POA is frozen and the "bulb' line is warm.
Hope this info helps.


TRB on Tue September 04, 2007 11:06 AM User is offlineView users profile

There is an upload image icon on the top right. After uploading the image or images copy the link. Use said link in a new post and insert the image using the image icon in the posting menu.


When considering your next auto A/C purchase, please consider the site that supports you:

Bob B on Thu September 06, 2007 4:07 PM User is offline

Will not be able to post photos until next week. However, in the meantime, any comments on my
post of 9/4?


Bob B on Mon September 17, 2007 10:30 AM User is offline

I finally got the photos of the POA and TXV but cannot seem to get them transferred to this reply.
I can email them to anyone who wants to comment on them.


Dougflas on Mon September 17, 2007 11:25 PM User is offline

email them to meMy email

Bob B on Tue September 18, 2007 11:39 AM User is offline


Have sent photos to your email.


Dougflas on Tue September 18, 2007 6:17 PM User is offline

I looked at your pics... The oil bleed line is frozen, output of POA, and the external equalizer is frozen. There is also frost at the TXV power head. (diaphram) I would go for the TXV. Did you try to test the TXV?

BY the way...the line you labeled equalizer is actually the oil bleed line from the evap. The external equalizer line is the small line from the TXV connecting to the POA. There is no frost at the low side connection...frost stars afterward. My gut feeling points to the TXV.

Edited: Tue September 18, 2007 at 10:16 PM by Dougflas

Back to Automotive Air Conditioning Forum

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.