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A/C Pressure Question

Discussion in 'The Garage' started by cbbr, Jul 10, 2005.

  1. cbbr

    cbbr 1 ton status GMOTM Winner

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    I just charged my A/C - it's converted from R-12 to 134. I only used 5 12oz. cans of freon. 350 with front and rear air.


    High side pressure is 250-260 with the engine and compressor running (A/C on max and rear A/C on). Low side pressure is running 90 with the engine off. The low side presure seems a little high to me.

    Anyone know what the correct readings are and why mine may be off?
     
  2. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    What is the low side pressure with it running ?

    it will equalize with it off .
     
    Last edited: Jul 10, 2005
  3. BIG*RED

    BIG*RED 1/2 ton status

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    well looking at my ac pressure guage and charts it says:

    "LOW SIDE..SET AC FOR MAX COOLING AND ALLOW TO RUN FOR 5 MIN,
    CHECK LOW SIDE WHILE RUNNING AND COMPAE READING TO CHART"

    0-23 = LOW ..REQUIRES ADDITIONAL REFRIGERANT

    24-43 = FILLED..PROPER AMOUNT OF REFRIGERANT

    44-64 = ALERT..DO NOT ADD REFRIGERANT

    65-200 = DANGER..OVERCHARGED, SEEK PROFESSIONAL HELP TO RECOVER EXCESSIVE REFRIGERANT


    i hope that helps you
     
  4. FWP

    FWP CRS

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    And what is the outside temp and RH?

    There's a chart for 134A, depends on the above.
     
  5. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Hi side with r134a is 165-270 at 90-100 ambient temp and 210-310 at 100-110 degrees ambient temp .

    And low side is 25-43 with evap temps at 40-65 , 37-51 with evap temps at 48-65 .
     
  6. cbbr

    cbbr 1 ton status GMOTM Winner

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    Like everything, I'm somewhat new to this so bear with me. The ambiant temperature was about 95 and the truck was running for 45 mins at idle when I turned it off after charging the AC. I did not take a high side reading with the engine off.

    I will take the readings again this afternoon - both sides with the engine off - and then with the engine running and post those numbers. Thanks for the relpies so far.
     
  7. k5guyva

    k5guyva 1/2 ton status

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    AC pressures

    you want to run the AC on max with doors open at 1500-2000 rpm. High side should be 2 1/2 times the ambient (read it in front of the condenser). Low side will vary some, but from 20-45 is normal. Refrigerant pressure varies with temperature. Also what are your vent temps?

    Normal charge of a R12 -> R134a retro is 80% - 90% of the original charge. What does the label under the hood say?
     
  8. cbbr

    cbbr 1 ton status GMOTM Winner

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    Just to make it interesting, the low side pressure is now 0. Apparently there is a very small leak somewhere. I put dye in it with the freon, but don't see any leaks.

    Anyone use leak stop or is that not a good thing to put in the system?
     
  9. k5guyva

    k5guyva 1/2 ton status

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    Stop Leak

    Stop leak is BAD! Don't use it. Use only refrigerant and the correct oil in an AC system. Here is an article on leak sealants. http://www.motor.com/MAGAZINE/Pdf/042003_04.pdf

    A high or low side pressure of 0 usually means that you didn't get a good connection with the manifold gauges. Do both sides read 0?
     
  10. cbbr

    cbbr 1 ton status GMOTM Winner

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    Yep. Looks like zero it is.
     
  11. k5guyva

    k5guyva 1/2 ton status

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    leak detection

    If you installed freon with dye, then you need a set of UV glasses and a UV light. You didn't mention if you have these. I picked up a set at Autozone last month for around 20 bucks. Your other option is to buy a R134a sniffer (maybe you can borrow one?), install a test charge (like 3-4 oz) and use the sniffer to see where its leaking. When you locate the leak, look around and see if you find oil there. If so, you'll need to put a few oz of oil back in.

    This is what you want if you don't have one. http://www.id-usa.com/product.asp?CID=30&PID=34
     
  12. HarryH3

    HarryH3 1 ton status Author

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    Keeping this thread alive... :doah:

    The a/c on our '91 S-Jimmy suddenly just decided to blow cool air, instead of cold air. It was converted from R-12 to R-134a 6 or 7 years ago and has been working fine (at least as good as R-134a seems to be able to do) ever since. On Monday that suddenly changed. I just checked the pressures with the a/c set on max, fan on high, engine running at 1,500-2,000 RPM. Here are the results:

    Low side: 48 PSI
    High side: 175 PSI
    Ambient Temp: 84 degrees
    Humidity: 25%

    Any ideas what would cause such a wacky reading? Is a valve clogged or is the compressor saying that it's had enough? :eek1:

    Just for giggles I also checked our '94 Suburban with dual a/c. It has had R-134a from day one. Here are the results from the Sub:

    Low side: 35 PSI
    High side: 195 PSI
    Ambient Temp: 84 degrees
    Humidity: 25%

    This one cools decent, but I've always felt that it could do better. When the temps are over 90 and it's sunny out, we run both units on high continuously, just to keep it comfortable. Does it want more gas, or less?

    Thanks!
     
    Last edited: Jul 14, 2005
  13. HarryH3

    HarryH3 1 ton status Author

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    Where did the a/c gurus go? :(
     
  14. dave_90_blazer

    dave_90_blazer 1/2 ton status

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    harry, is the compressor clutch cycling often? what is your pressure with the engine off and the reading equalized? you might have a small leak
     
  15. k5guyva

    k5guyva 1/2 ton status

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    jimmy

    Yeah, I think the compressor is on its way out. Make sure you check the belt and ensure its not slipping. The high side pressure seems a bit on the low end, and there is a corresponding increase on the low pressure, indicating to me a decrease in the effectiveness of the compressor. I wouldn't wait long to replace it, or else don't use it. You don't want a black death on your hands. Thats big money to fix. If you don't want to replace it right now, unplug the connector on top of the compressor. Even with the AC system off, some GM systems turn the AC on for defrost.

    On the 'burban, I'd replace the fan clutch out. Those are known for causing marginal cooling. Your pressures look good to me there. One test to verify the fan clutch is to spray down the condenser with a garden hose, and see if the cooling improves. If it does, then I'd consider replacing the fan clutch.

    One other thing while I'm thinking about it....try measuring the temperatures at the vent. There a low pressure cutoff/cycling switch that is adjustable. Sometimes getting the compressor to stay on a tad longer by adjusting the switch helps.

    One purpose of this switch is to prevent the evaporator from freezing and icing over. (LOL done that been there) So you want to get the compressor to shut off somewhere in 20-25 PSI range while looking at the low side pressure.
     
    Last edited: Jul 14, 2005
  16. HarryH3

    HarryH3 1 ton status Author

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    Nope. The compressor runs all the time. I did notice yesterday that it's louder than the one in the Suburban. It's a 6 or 7 year old rebuild, so it may be time to just replace it. :(
     
  17. HarryH3

    HarryH3 1 ton status Author

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    I hear ya on the Black Death problem! I had to unplug the compressor on the wifes Suburban a couple of years ago so that she wouldn't "keep trying" until it blew apart. Cleaning out a dual a/c system would be WAY too much hassle. :eek1:

    I think that the fan clutch on the 'burb is working properly. I can hear the fan kick in while sitting at a stop light. The drag on the engine is also quite noticeable on acceleration.

    Where might I find the adjustment on the cycling switch? :D What's the lowest vent temp that I should shoot for? Cooling this thing is like trying to cool a barn, so the more BTU's I can get, the better! :wink1:
     
  18. k5guyva

    k5guyva 1/2 ton status

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    It should be attached to the accumulator. You should see 2 wires coming from a switch. If you unplug the connector, you'll see a screw in the middle. Give it one or two rotations CCW. If you measure the vent temps, (AC on, fan on low, doors and windows closed) you want the compressor to turn off around 35F. You can also set it by pressure. Don't go much below 18 psi.
     
  19. HarryH3

    HarryH3 1 ton status Author

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    Sweet! I never looked in there for the screw before. I'll have to go try that out! :cool1:
     
  20. opfor2

    opfor2 1/2 ton status

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