ac high side fluctuates rapidly

Discussion in 'Domestics' started by MikeP, Jul 31, 2009.

  1. MikeP

    MikeP Sr. Member

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    '93 bonny resto. AC system fully disassembled, new accumulator, new condenser, lines, evap, painstakingly flushed. Not-very-old compressor flushed repeatedly with correct oil until clean. Reassembled with proper oil in proper amounts in proper locations. (I am being vague because this was 2 years ago, very long term project) Last night I pulled a vacuum, 160 microns, kept vacuum pump on for 3 hours, charged with R12. Works great - 38 degrees at the vents. BUT, at idle, the high side guage fluctuates very rapidly. Pick it up off idle and it abruptly smooths out. Let it idle and it may or may not fluctuate. But when it does, pick it up off idle and it stops again. Sounds like the valves are bad in the compressor? Thoughts? Thanks, Mike
     
  2. crunch

    crunch wrench

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    Give us the low and High side readings at idle and at 2K with compresser running?
    Is the ac condenser fan running good and blowing good with ac on?
     
  3. NickD

    NickD wrench

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    Piston compressors are nothing but average, a rapid series of sharp pressure spikes and far more pronounce at idled. Just a shot in the dark, but the gauges use a restrictor to only show average readings, better gauges use a screw in type restrictor, has a very tiny hole in the center of it where cheaper gauges use a piece of stamped brass pressed in. Possible your restrictor fell out that would also cause your symptoms. Try a different set of gauges or try your gauges on a different vehicle.

    Regarding the reed valves, if you can get your system to cycle, dead give away would be the high side pressure will drop to nothing quick of the valves are bad, should see more like 20-40 seconds, also your high side pressures will be low with that refrigerant flowing back. Can't assume anything, if everything else appears to be normal, your test equipment can be defective.
     
  4. MikeP

    MikeP Sr. Member

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    Have to go by memory.. Both condenser fans running great on high speed. HVAC fan blower runs fine, great air flow. Idle, when guage is not fluctuating - low: about 35 or so, high side about 120. At 2K rpm, high side 175, low side at about 28. When the load on the AC is reduced (like lower the fan speed) the low pressure switch will cycle, at which time the low side slowly rises and high side slowly falls, about 20 - 30 seconds before it kicks on again. I think the low pressure switch is cycling at 22 and 40. These readings are all well within the charts in the factory service manual. Really, all seems great, except for the vibration of the high side at idle. Lots of great cold air and tons of condensation. By the way, ambiant temp was 75, humidity at about 75%. Gauges are not top quality.
    FWIW, gotta love R12. Nice and cold.
     
  5. NickD

    NickD wrench

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    R-134a isn't bad either, in a good R-134a designed system. When you say your high side gauge is fluctuating, what s the width of the needle? And does the rate increase as when you slowly increase the engine speed?
     
  6. MikeP

    MikeP Sr. Member

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    true on the R134a Nick. The newer cars are awesome. I'd say +/- 30psi, and yes the speed and amplitude of fluctuation increases with the rpm, then it suddenly gets completely steady when the revs get up a bit.
     
  7. NickD

    NickD wrench

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    I'm going back about 30 years with this, about a half inch of flutter is what you are seeing, feel it's your restrictor, can you hear a slight rattle? But it could have blown away. Ran into this problem a long time ago if you remove your high side gauge, can peek in. Believe it was in military gauges we used the threaded restrictor.
     
  8. MikeP

    MikeP Sr. Member

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    Yes, that is about the amount of fluctuation. I'm not going to worry about it, to be honest with you. If it blows, I get the fun of taking it apart and fixing it. That's the point of this whole project. Thanks!
     
  9. NickD

    NickD wrench

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    For a 93, you should have a HPCO in your compressor, so hopefully it will just shut down rather than blow.

    If fixing stuff is fun, I must be having way too much fun right now with four vehicles of my own and six kids with twelve more. Talking to my wife about that government clunker program to get just two good vehicles, what a farce. Only way to get $4,500.00 is to buy something like an Aveo, then the dealers want full MSRP for their vehicles, that includes Ford, Chrysler, and GM even on a cash deal. Wife wants a Toyota 4 Runner, we would get zero on that vehicle. Anyway, heard on the news this program, that clunker deal is off, government ran out of money. But sure they will be back at the end of this year with their hands in our pocket, maybe even with a gun pointed at our heads.

    Then they even added more spy gear to these new vehicles, but it is for our protection.
     

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