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71 Corvette Help Needed

Roger3 on Tue August 20, 2013 9:53 PM User is offline

Year: 1971
Make: Chevy
Model: Corvette
Engine Size: 454
Refrigerant Type: R12
Ambient Temp: 80
Pressure Low: 45
Pressure High: 255
Country of Origin: United States

AC wasn't blowing very cold with these readings

Low side 37 psi
High side 190 psi
Ambient temp 90f
Vent temp 60f
No bubbles in sight glass

Added couple lbs r12 freon

Low side 35 psi
High side 225 psi
Vent temp 65f

That was a couple weeks ago

Checked it again tonight

When I first cranked engine got these readings
Low side 35
High side 200
Ambient 80f
Vent temp 53f
Engine temp 100f

After running engine for 15 minutes
Low side 39
High side 210
Vent temp 50

Both pressure readings high - thought I have too much freon so I let some out, still no bubbles in sight glass, and got these readings
Low side 45
High side 255
Vent temp 55f
Engine temp 180f

Any tips, ideas, etc


Dougflas on Tue August 20, 2013 10:27 PM User is offline

With a fan in front of the grill, 1200 to 1500 rpm, after 10 minutes running, what are the temps and pressures? Don't forget to give ambient. Then disconnect blower motor , low side should hold around 30 psi.

Olds442 on Wed August 21, 2013 1:30 AM User is offline

You need to insure that you have correct number of oz of freon. Then when you run it , high pressure should be about 2 times ambient. If it's too high you may need to replace the fan clutch to get better air flow over the condensor if the fan clutch is not fairly new and high quality.

Dougflas on Wed August 21, 2013 4:47 PM User is offline

He is using R12 with a clear sigh glas. At 80* he shuld be getting text book readings. POA and TXV system, even a few oz low should be ok with a clear sight glass. He needs to verify 30psi with blower disabled to check POA.

Roger3 on Wed August 21, 2013 6:54 PM User is offline

New test data with fan on grill at 1500 rpm

After 6 minutes of running
Amb 79f
High side 200psig
Low side 37psig
Vent temp 48f

After 10 minutes of running
Amb 79f
High side 255psig
Low side 43psig
Vent 53f

Unplugged Blower Motor
Amb 79f
High side 220psig
Low side 36psig

Sight glass is clear all the time. Sight glass is mounted in top of drier if that matters.

This is an r12 system. Fan clutch, radiator, shroud, etc all in great shape. Fan clutch is heavy duty unit with less than 5000 miles.

Thanks for help.


an clutch is new (less than 5000 miles, heavy duty unit)

With fan on front of ca

Roger3 on Wed August 21, 2013 7:03 PM User is offline

Since both high side and low side pressures are high do I still have too much r12 in system? Sight glass is perfectly clear.


Dougflas on Thu August 22, 2013 2:19 AM User is offline

looks to me that the POA is not working as it should. It should be at 30PSI with the blower disconnected. Are you sure your low side guage is zeroed out? I used to be happy on the POA systems if the vents were 48*. By any chance, have you tried to clamp off the heater hoses?

My POA tests were done with the vehicle closed up, vehicle in shaded area, fan in front od grill, and 1200 rpm. After ten minutes, right hand vent 48* and I was satisfied with high speed blower on max.

Edited: Thu August 22, 2013 at 2:25 AM by Dougflas

Roger3 on Thu August 22, 2013 7:46 PM User is offline

Yes the low side guage is zeroed out.

There are no moving parts in POA and I checked its calibration when the system was apart a couple years ago.

Maybe it has eroded the orifice opening so that its not throttling the freon as it should.

If the vent temps would stay at 48 f I would be happy but after 15 minutes of running it he vent temps are 55 to 60 f.

When I first put the system together a couple years ago the pressure readings were correct and vent temps were 48f.


Dougflas on Thu August 22, 2013 8:02 PM User is offline

only thing I can think of would be the oil bleed line partially restricted. You would have to recover the refrigerant and there is a special schrader at the line. Do not mix it with a standard schrader. You are checking the system in the max air mode, correct?

Roger3 on Fri August 23, 2013 6:20 PM User is offline

Yes, I'm checking the system with it a maximum settings.

I am looking for someone in my area that can reclaim the R-12. No luck yet, but i'll keep looking.

I was reading an AC book I have that shows the diaphragm in the throttling valve not working correctly. The book listed three possibilities when you have higher than normal pressure on both low and high side:

1. Too much Freon in system - I may have too much Freon in system but I started having the problem before adding a little extra Freon. One day vent temps were at 48F, the next day they were at 55 to 60F. It was an instant change.

2. Malfunctioning (partially plugged condenser, or blocked air flow). I know the outside of the condenser is very clean. Fan clutch are in good working shape so I don't believe anything is wrong externally with the condenser or air flow. I have trouble believing my condenser just instantly got plugged up. The site glass perfectly clear so I don't think I have any contaminants floating around.

3. Malfunctioning TXV valve. Possible - its the original 1971 valve. Probably out of warranty by now.

I have had this car about 5 years. I'm the second owner. The original owner lived in New Jersey and had disconnected the ac hoses (open to atmosphere) but didn't know for how many years.

I removed the flushed the compressor and condenser.

Replaced the hoses, evaporator, and drier.

Cleaned the POA and TXV really good. Checked the POA calibration.

Put it all together, pulled a vacuum, charged it, and it worked great for the last two years so I am thinking of removing the POA and TXV and sending them out to have them restored, hopefully they will last another 20 years, especially if they are not exposed to the weather like they were for the last several years.

Thanks for the help.


Dougflas on Fri August 23, 2013 7:51 PM User is offline

Before you pull it apart, wrap a rag with hot water around the TXV. You may have to keep pouring water on it. See if the vent temps stay lower. Also, test the TXV. Look in the tips section. I posted an article on Chrysler EPR systems that step by steps test the TXV. If you do open the system, just replace the TXV. They are inexpensive . Make sure the bulb is positioned correcrly and it is insulated. I'm still suspect of the POA because it is above 30psi. What is the altitude of your location?

Edited: Fri August 23, 2013 at 7:53 PM by Dougflas

Roger3 on Fri August 23, 2013 9:32 PM User is offline

Roger3 on Fri August 23, 2013 9:33 PM User is offline

I'll test the TXV tomorrow.

I'm in Denham Springs LA. About 100ft or less above sea level.


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