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Overpressure valve on AC compressor? 900 1993

posted by  mila on  Mon Jun 11 21:13 CST 2007
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last visit: Fri Sep 5 06:26 CST 2014
[RELATED]

Yesterday there was a new sound when I turned on the AC. Kind of one distinct bang. When I looked under the hood the left motor mount was sprayed with oil.
This is right behind the AC compressor.
The AC started up, no noise from the compressor and it was cooling.

Is it possible the oil is from the compressor?
Will I ruin something if I run it or is service needed asap?

Thank you for any suggestions.
mila








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    Overpressure valve on AC compressor? 900 1993

    posted by  DanR on  Tue Jun 12 14:57 CST 2007
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    last visit: Thu Oct 23 15:25 CST 2014
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    You don't state what year you have, but my '94 came with a blow off valve on the back of the compressor. The first warm day and bang, most of the freon was evacuated. The dealer struggled a long time with that one, but it turned out to be the high pressure switch had failed, the coolant fan never went on unless the coolant sensor signaled it. When the high pressure switch failed, not only did the coolant fan not run, but the compressor never got a stop signal. So the compressor was running flat out, the pressure builds, then the valve on the back of the compressor opened and dumped about 2 lbs. of freon. Better than rupturing a hose, the first time I had a problem the hose blew, after that about a half dozen times the valve opened.

    Check your high pressure switch.

    DanR '94 964 330,000 miles (96,000 on the new engine)
    --
    DanR








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      Overpressure valve on AC compressor? 900 1993

      posted by  mila on  Tue Jun 12 22:15 CST 2007
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      last visit: Fri Sep 5 06:26 CST 2014
      [RELATED]

      Thank you for verifying there is a blow off valve on the compressor.

      It's a 1993 with 446000 km on the clock (277 kmiles).
      I don't think there is a problem with the high pressure switch though, the electric fan starts when idling.
      I suspect the valve is flaky and dumps at a to low pressure.
      Some months ago I had the system topped up with R-134A since the compressor was cycling and the system was not cooling very well.
      The mechanic couldn't find any leaks although he had a sniffer, but if my theory is right it would explain why the system was low on gas.

      If the valve is bad, would you AC-gurus out there recommend a new compressor at this mileage, or should I just try to get a new valve and hope for the best?

      Thank you
      mila








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        Overpressure valve on AC compressor? 900 1993

        uid 18083's pic
        posted by  lucid on  Wed Jun 13 01:51 CST 2007
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        last visit: Fri Oct 24 12:31 CST 2014
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        "Some months ago I had the system topped up with R-134A since the compressor was cycling and the system was not cooling very well."

        My US 940 may not be to the same specs as yours, but the compressor is supposed to cycle. The generic term for that system is CCOT, meaning "Compressor (or Clutch) Cycling Orifice Tube".

        My first 940 summer was in 2004, when I got the AC working by replacing the Orifice Tube and the hard line leading to it. I also bought the Climate Control Greenbook and Wiring Diagrams for the '93 940. From those documents it appears that your fan control is not operating as intended either (not that would cause excessive pressure). I described what the book and diagrams say about fan control in this post a few days ago.

        I also wonder about the "topped off" R134. The book method is to evacuate the system and weigh the amount of R134 that comes out. Only if that amount is below spec should any topping off be done.

        For what it's worth, my compressor has NOT cycled since the second season in 2005. It runs all the time because the Low Side pressure does not get down into the cycling range that is controlled by the pressostat on the accumulator. Yet I still get vent temps as low as 40°F and even lower at times! And the amount of R134 is correct, as measured my my local shop (with me watching).

        My guess is that the compressor is at fault, and I'll just wait and see what happens this summer.

        --
        Bruce Young
        '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.








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          Overpressure valve on AC compressor? 900 1993

          posted by  mila on  Wed Jun 13 09:00 CST 2007
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          last visit: Fri Sep 5 06:26 CST 2014
          [RELATED]

          You are correct that a top-up is not by the book.
          Since R-134 is only available to professionals in Sweden, and the repair prices at the AC shop is accordingly, a friend who works with AC had a look at it. After connecting his dials and gauges he decided it was low on gas.

          If I understood the tech correctly, the cycling of the compressor at the surrounding temperature when he looked at it, was a sign of gas shortage.
          The compressor runs for a while, compressing some gas to fluid and then there is not enough gas left to condensate. The fluid goes through the evaporator, loses pressure and goes into the gas phase which gives the compressor some more gas to compress.

          Other posts confirm that it is an acceptable, although not quite correct method, to top up until no cycling if you don't want to evacuate the system.
          Here's what Chris Herbst says on the subject.
          He's an AC-authority who used to be active on this forum.

          If you mean the big fan for the condensor, it seems to work. After a short while idling it kicks in.
          The noise was instantaneous when I activated the AC and it dumped some gas (at least I think so) and oil.

          Let's hope your compressor makes it through the summer!

          Good luck,
          mila







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