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  1. #1
    Join Date
    Jul 2005
    Automotive A/C. Charging R-134a via manifold guage...

    Car has poor, but some, cooling. Hooked and and purged the gauge set and let it run for 20 mins with the a/c on high and windows open. Noted a slightly low suction (25psi) and discharge (130psi) pressures, ambient is 70*. The condenser is clear and the fins are straight, so I figure the charge is low ('96 Nissan, never been recharged)

    I start to charge it by cracking open the avlve in the can of 134 and opened the low side valve until the low side read to 40psi. Within a few seconds the suction guage drops back to 31-32 psi and the discharge slowly cimbs to 140 - that's better...

    However, as I continue to charge the pressure no longer climbing. The compressor is running, R134a can upright (and cold, so I know it's flowing). There's still liquid refrigerant in the can.

    I put in almost the entire can and the pressures did not climb any more. I figure at this point I'm just filling the receiver and that the pressure won't rise again until the receiver is full and the system becomes overcharged...

    Just want to make sure you guys agree with that assessment...?

  2. #2
    Join Date
    Aug 2003
    Fort Worth, TX
    Just want to make sure you guys agree with that assessment...?
    Can't say I do outright. More likely as the can gets cold due to gas entering system the pressure of the gas approaches the pressure of the more flow.

    Charging methods may vary by car maker, but what I understand (I'm not an auto a/c mechanic, but have worked on them in the past) is the system is charged with windows up, fan on hi, set on recirculation. Stick a good thermo in the dash outlet nearest the evap.

    The suction and discharge pressures will vary with engine RPM. Higher compressor RPM's equate to lower suction pressure/higher discharge pressure.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.

    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    Jul 2005
    So I guess I could warm up the can a little...

    I'll have to look up the temp/press chart for r134 and see how hot the can would have to be to exceed 32psi and allow refrigerant transfer.

    If the pressure equalizes like that, and warming up the can allows more refrigerant to flow... Then could you look up the R134 temp for the desired suction pressure, and heat up the can to that amount and allow to equalize again?

    Example... Charign in 70 deg ambient. Manual calls for ~45psi suction and ~150psi discharge. Find the r134a temp for 45psi (50*) As long as I keep the can at 50deg, it should equalise at the correct pressure, right? Just sit back and watch it charge, keeping an eye on the gauges...

  4. #4
    Join Date
    Jan 2004
    set the can where it can be warm... Or crack you manifold valve and turn the can over, what is your temp drop at the center vent? sometimes it is just easier to weigh in a fresh charge, my experience with my vehicles has been windows open, upper vent, no recirc, that is the kicker with auto ac it is all manufacture specific, I know a prof. mechanic who is happy if they can get a 60 degree temp out of the vent!!-
    Still learning opinions welcome.

  5. #5
    Join Date
    Jun 2003
    Chicago, IL
    many auto manufactures recommend the pressures be checked with the engine at 1500 to 2000 RPM. try increasing the engine speed and see what happens.

    what is the temperature of the suction line like? with the fan on high, the suction line should be cool, but not very cold. if its very cold, the a/c is probably working fine and you have a problem with the blend doors that is causing the heater core to reheat the air that has been cooled by the A/C. a good way to check this is to turrn on the A/C, and pull the fuse for it or disconnect the wire on the compressor. if the air coming from the vents gets warmer than ambient temperature, you need to look at the heater blend door.

  6. #6
    Join Date
    Apr 2005
    Inland Empire,CA
    how about weighing in the charge? if data is available

  7. #7
    Join Date
    Dec 2002
    davenport, iowa
    set the can on the engine while it is running and charge, engine heat will help transfer the juice.
    Da Threadkilla

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