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A6 compressor freon capacity R12

92470 on Tue August 21, 2007 10:46 PM User is offline

Year: 1971
Make: Chevrolet
Model: C10
Engine Size: 350
Refrigerant Type: R134
Country of Origin: United States

Can anyone verify what the original freon capacity was for a 1971 Chevy truck w/350 and with the stock A6 compressor? I bought a rebuilt one, and it didn't have a capacity sticker. I bought one of the repo original looking Harrison stickers to apply to it that I thought was period correct. It stated on it 3.75 lbs. I have had numerous problems with my conversion, and I think I found out why. I based everything off of this amount.

I have since checked with 3 shops who all told me that for that year of truck with an A6, the capacity was originally 3.25 lbs. of R12. Thus, I am overcharging my system with the 80% R134 rule. I was basing 80% off of 3.75, when it looks like I should have been 80% of 3.25.

Can anyone verify this? I have no A/C shop manuals or any data to support, but 3 shops told me the same thing. If anyone has a reference off this site that proves this to be true, I would like to know. I hope they were right because this might be the key to fixing my A/C. Thanks. This site is great!

oznznut on Tue August 21, 2007 11:19 PM User is offline

For what its worth. I have a 1971 factory manual. However it doesn't list trucks. Anyway, all car models, Impala,Camaro, Chevelle, etc, all list for 3 lbs 12 oz. Only the Corvette lists at 3 lbs 4 oz. Remember the Corvette used a smaller A6 compressor. 10.8 cubic inches, while all the others were 12.6 cubic inches. Whatever, they ALL used 11 oz of oil.

I would guess that you bought the bigger compressor as most outfits don't carry (or know about the smaller one). And anyway 80% of the extra 8 ounces, if in fact it really did use 12/4, is only 6.4 oz. The size of the receiver/drier would take care of that.

The 1971 cars, and I would have to believe the trucks also, had a poa valve for evap freeze protection. If you converted to 134, you must reset it to about 26psig. If not, you will always have rotten cooling.

My 02 cents. Go back to 12. Not that expensive any more.

Dave

92470 on Wed August 22, 2007 10:31 AM User is offline

This might be a dumb question, but when freon capacity is discussed, is it the capacity of the compressor, or the "system" itself? I guess I'm saying if my truck originally had a smaller compressor and the book listed capacity of the system was 3.25 lbs, would my putting a larger compressor on it increase my available freon capacity for the sytem and require me to put more in?

I think I'm answering my own question here by typing through this, but I guess the amount of freon required by your system is dependent on the size of compressor you have and what the capacity of that compressor is, right?

GM Tech on Wed August 22, 2007 10:49 AM User is offline

The capacity is for the system-- it just so happens that in the old days- the label where the capacity is stated was put on the compressor- now it is underhood elsewhere- so the capacity describes the system and is not tied to any given compressor.........

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

92470 on Wed August 22, 2007 11:48 AM User is offline

So if I understand you correctly, if my original system capacity from the factory was 3.25 lbs, then it will always be 3.25 lbs. regardless of whether my compressor is a little larger? Evidently from a previous post, there were 2 sizes of A6 Harrison compressors.

I don't know whether the one I have is the stock size or not. I thought it was. I just bought the rebuilt one from the parts store that was specified to my truck. All I know is that it looks like every A6 I have ever seen. If it is larger than what my truck was originally manufactured with, I just want to know if my system capacity is still 3.25 lbs. Sorry if I'm confusing the issue, but I'm still not straight on this. This is important because obviously this is what I am basing the percentage of R134 I am putting into the system. And a few ounces here or there do make a differerence in my experiance. And if I'm basing my percentage off of a larger cpacity size, then I am overcharging. Thanks!

oznznut on Wed August 22, 2007 12:17 PM User is offline

Im sorry if I threw in something that maybe I shouldn't have. What I was trying to point out, was that ALL 1971 Chevy cars except the Corvette used 3 lbs 12 oz of refrigerant. Also ALL cars except the Corvette used the larger compressor. For clarification, the PHYSICAL SIZE of the compressors are the same. The difference is the piston/cylinder bore size. Just like 283/327/350. Same outside size, different bore sizes.

I am just guessing here now, but I THINK the Corvette used the smaller A6 because it used a smaller evaporator than the other cars, had a smaller cabin size to cool,and much shorter refrigerant hose lengths, hence used less refrigerant, so would not need the capacity of the big compressor. Again my guess, I've never seen anything in writing from GM as to why the smaller Corvette compressor.

Don't know the system refrigerant capacity of the trucks, but I would guess it would be smaller, because of less volume to cool. And using a larger capacity compressor will not do anything bad on a smaller system.

But, as I said earlier, you must change the setting of your poa valve, if still there, to get good cooling with 134a. And don't try to "clear the sight glass" as we did on the r12 systems. If you do, you will be way overcharged.

Dave

92470 on Wed August 22, 2007 12:36 PM User is offline

Thanks for the clarification, everyone is being helpful. I bought a rebuilt 134 calibrated POA and I'm getting conflicting stories as to what it was calibrated to. One person said 26, the other said 20. I'm going to try and rig up a test unit and see where it pops for myself. If it isn't 26, I'm going to try and recalibrate with instructions off this board. This weekend, I'm going to try the "blower disconnect and read your low side pressure, should be 26 test!" I can see the capacity in the trucks being smaller. They had larger condensors and of course a much smaller area to cool that lets say a 70's era Cadillac!

I would love to stick with R12 and was very reluctant to switch, but at DIY prices of $25 a can, I can't afford to make mistakes when I have no reclaim unit. After seperate instances of a slow leaking evaporator, POA, and drier cost me about $200 worth of lost freon, I gave in. Plus running to Auto Zone and buying R134 for $6.00 a can is a heck of a lot easier and cheaper.

dw77 on Wed August 22, 2007 4:42 PM User is offline

Thanks for the great info. I am doing a changeover from 12 to 134 on my 71 cutlass. replacing compressor, receiver, and TXV. I am confused about the reasoning of using 80% of the original R-12 specs for 134. I guess I never asked the question. I have the original GM service manual which shows a charge of 4.5 lbs in the system. It also says the POA cannot be adjusted. Does this mean I will have to replace the POA also?
One other thought from my days in home air conditioning systems....As long as the original charge is correct, and the subcooling is correct, why can't you use a clear sight glass to indicate a low charge?
Sorry bout the 5 million questions, but I want to do this once, and right.
Thanks

oznznut on Wed August 22, 2007 8:29 PM User is offline

dw77

As we learned from the late Mitch (rip) the POA is adjustable. Many posts in the archive on this. Search for "poa". There is also a long thread talking about 134a and sight glasses. Search for, ta-da, "sight glasses". I would put in the links, but haven't figured out how to do that.

Is that for real? 4.5 lbs r12 in a cutlass! Yikes !! Weird, when you consider a Caprice station wagon of the same year only used 3 lbs 12 oz. I would have guessed that the Cutlass and Chevelle would be nearly identical.

I work in the HVAC trade. I understand what you are talking about with subcooling, superheat, and that stuff, but most of it just doesn't apply to automotive a/c. They dont have an adjustable txv ( heck, most don't have ANY txv). Without that adjustability, the concern of subcooling and superheat goes away. And the accumulator serves to prevent any liquid from getting back to the compressor.

Good luck with your system

Dave

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