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  1. #1

    Default HVAC question - Any issue using vehicle with HVAC at -27Hg?

    For my 04 Sierra 1500 5.3L I had to replace the tube assembly with the high and low side ports (and orifice tube in between) because of a stuck, non-serviceable schrader valve on the high side port. I'm doing some work that will require me to open the system up, again, so there's no sense charging the system with R134a (since I know I'll need to replace it in a few weeks).

    It's cool/cold out, so I see no need to use or engage the AC. As I understand it the compressor should be prevented from engaging by the low pressure sensor in the system, anyway -- which may be relevant as I have climate control (I tend to manually make sure there's no snowflake showing, though).

    So, is there any reason not to leave the system at -27Hg for the next few weeks? Please advise.

  2. #2

    Join Date
    Sep 2010
    everett, massachusetts


    The system should be pumped down with a vacuum pump. see if it holds . then add some refrigerant just to pressurize the system and no more. this will keep the air out. air is water and water and PAG oil creates an acid that will damage all your work. you must pump down for a longer time to suck out the moisture from the compressor oil , this takes some time like an hour to be sure it is all boiled off. remove the low pressure cut out connector then the ac compressor will not be commanded on .

    if you leave the system with air in it then this is not good for any period of time.

    any time the system is opened with the work you did replacing the filter dryer is usually done.

    if the orifice tube is with debris this can mean a compressor failure. flushing the system is recommended and then hope that the compressor holds up.

  3. #3


    -27 inches Hg is actually under vacuum but not quite evacuated which should be around -29 to 30 Hg.
    I will not leave it under vacuum as it will let in air and moisture. Charge a little R134A to get it the positive pressure like 5-10 psi on the gauge. That way the system is pressurized.
    As jcat suggested, either pull the compressor fuse or clutch connector and the pressure switch to keep the compressor from engaging.
    2011 Chevy Silverado LT 4WD 30k miles
    2001 Suburban 2WD, 5.3, V8 185k miles
    1993 964
    1996 318TI

  4. #4


    Thanks, guys. It's still under vacuum, at the moment. I won't get a chance to refill until this weekend but will do it first thing Saturday morning.

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