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1988 A/C problem..

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by linksvu, Aug 6, 2006.

  1. linksvu

    linksvu Registered Member

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    I know these A/C systems can be a pain, but I am not sure why mine is not working. I have a gauge and when the a/c is off, it shows 45psi (which is in the good zone). However, when I turn it on, the compressor turns on for less than a second then shuts off. So the gauges shows that when the compressor turns on, the PSI dropps to 20 or so and then shuts off, as it should. It should shut off when there is not enough pressure in the system.

    Does anyone know why the system looses pressure when the compressor turns on and then it builts back up?

    So what I have is a compressor going on and off every couple of mins while the systems looses pressure and they builds back up...

    Any thoguhts?
     
  2. rick88blaze

    rick88blaze 1/2 ton status

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    Sounds like it's a little low on freon. That or your low pressure shutoff switch might be going bad.

    Rick
     
  3. linksvu

    linksvu Registered Member

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    I do not think it is low as it seems to be full enough to turn on, and they it drops pressure and shuts down.... hard to figure it out.
     
  4. mikras1

    mikras1 Registered Member

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    To understand this you have to follow the path of the Freon. In front of the compressor (the low side) you have low pressure gas. The gas gets compressed into liquid that is at high pressure and very hot. It then goes thru the condenser in front of the radiator to shed the heat. The high pressure liquid then goes thru an expansion valve and changes to low pressure gas. This change from liquid to gas cools the evaporator (really it’s pulling the heat out of the air) which cools the air in the cabin.

    The low side normally cycles between about 25 and 40 psi. like you said, but usually after several seconds. The rapid cycling is happening because there's not enough fluid coming out of the evap to then get re-drawn into the compressor, so the compressor shuts off.

    When the AC is off, the high and low sides of the systems equalize. If you put your gauge on the high side you will also see 45 psi. The pressure you read is the vapor pressure of the gas whether there is 1 Ounce or 3 Lbs of Freon in the system. The Vapor Pressure changes with the outside temperature and should be about 70 psi on an 80 Degree F. day. I suspect your system is very low.

    The only way to use a gauge to measure the Freon is on the High side with the system running. There should be between 175 – 220 psi on the High side when the system is working. I suspect you probably have around 100 psi. If you attempt to charge the system yourself, BE CAREFUL and make sure you are connecting to the right ports. You will be working with high pressures. Also, do not go over 220 psi on the high side or it will blow the pressure relief safety valve.
     
  5. linksvu

    linksvu Registered Member

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    I had always been filling on the low side (the port on the tank) but it seemed as if the tank was full because it no longer accepted the freon. You suggest filling on the high-side now? Thank you for you help, that was very informative...:bow:
     
  6. kd7kmp

    kd7kmp Registered Member

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    You should NOT fill on the high side of the compressor! That could burst the can. Always fill from the low side of the compressor.

    If you have a large cylinder of R-12 (stock refrigerant in a 1988) then there may be two different colored knobs. The red draws the R-12 out as a gas and blue draws the R-12 out as liquid. Filling from the red is much slower than from the blue. The proper way to fill from such a cylinder is with a scale so you can measure, by weight, how much refrigerant is going in. It's all to easy to over charge the system. That can lead to problems as well. The principle is the same for R-134a refrigerant. By the way...R-12 and R-134a are NOT compatible. Bad things happen that require lots of expensive parts when you combine the two.

    When you say that you have a gauge it sounds like your gauge only connects to the low side. You can't get a good, accurate reading with only one gauge. My recommendation is to take it to an A/C professtional--someone you know or to a shop--to have it looked at. It's all to easy to mess it up even worse (not that you messed it up to begin with :))

    Kevin
     
  7. K5er4Life

    K5er4Life 1/2 ton status

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    dont mean to highjack the thread but kd7kmp seems to be pretty knowledgeable about a/c systems. Do you knwo where I could get the tool that draws a vacumm on the system? I know one uses vacumm pumps but they are expensive, and when I was reading in my a/c book they talked about a venturi type deal that works off of an air compressor to draw a vacumm on the a/c system. I havent seen one and I would like to purchase one and see if it works very well. If not Ill take it to a shop after I do the neccessary repairs and let the shop draw it and fill it.

    To linksvu, I highly suggest if you want to work on your a/c that you purchase atleast one good a/c book. I bought the haynes manual a/c and it helped me alot in understanding, diagnosing and repairing air conditining systems. I still have yet to fix the a/c in my crew cab but atleast when I get in there Ill know what Im doing. Heres the book I bought, helped quite a bit. Its not all inclusive but should allow you to do just about anything you would need or want to do. http://www.amazon.com/gp/product/1563923815/sr=8-1/qid=1155651810/ref=pd_bbs_1/104-3421686-0304737?ie=UTF8
     
  8. mikras1

    mikras1 Registered Member

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    Linksvu - always charge from the low side. The High side gauge is just for measuring so that you don't overfill.
    "seemed as if the tank was full because it no longer accepted the freon" - If you are using the small cans, the last one is always hard to get in when the system is almost full. As the can empties, the pressure drops in the can. When the can pressure is the same as the system pressure, there is nothing to "push" the Freon out of the can.
    Here's a trick - fill a pot with warm water and dip the can in and out of the water. You can tell by feel when the can gets cold, dip it in again. Warm tap water works fine, you don't need really hot water.
    Hopes this helps,
    Mike
     
  9. kd7kmp

    kd7kmp Registered Member

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    K5er4Life,

    The venturi type is really a very mediocre way to draw a vacuum. It should be avoided as they can only draw around 14" to 15" of vacuum. Plus, you have to have a pretty big compressor to get them to function. The system should be drawn down to 29" (adjusted for altutude). I can only recommend using an actual vacuum pump. Vacuum pumps will run around $250 and up for a decent one. If you can afford that take it to a shop and they may draw a vacuum for you for a nominal charge.

    Kevin
     

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