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Air Conditioning Receiver/Drier

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Marc RobertsonMarc Robertson
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    If the system wasn’t properly vacuumed after it was opened and before it was recharged, the refrigerant may be contaminated with air on top of moisture. The same thing applies to the manifold set, if you open it to add oil or dye as this will also introduce air into the system.

    A small amount of air won’t change the performance much if any but a lot of it will. It basically holds the place of the refrigerant yet contributes no refrigeration action. Air can be hard to diagnose on expansion valve systems such as the Civic. With a simple orifice system that is charged to the proper pressures, the refrigerant flow will be discontinuous between vapor and liquid when it should be continuous liquid.

    Both moisture and contaminated refrigerant will be fixed by properly recovering the refrigerant, vacuuming the system and recharging . The receiver/drier should be replaced after the system is fixed. For the purpose of diagnosis, a good long vacuum will be enough but not enough for the long term. You cant recharge the desiccant because it usually require heat to release the moisture and not just a vacuum. Heat will probably damage the filter element inside. If the refrigerant is contaminated, it can give bad diagnosis readings. With this potential thorn in diagnosing the system, it may be best to have the refrigerant tested for air and other contamination or have the system recovered then vacuum and recharge the system with good refrigerant. Then re-diagnose the system for other issues.

    With a manifold, you should be able to see other issues such as a low high side and a high low side pressure indicates either a worn compressor or a stuck open expansion valve. A high, high side pressure and a low, low side pressure indicates a blocked orifice or stuck closed expansion valve.

    The tricky thing about diagnosing with the pressures is that it is very dependent on the ambient temperature, airflow past the condenser and the load placed on the evaporator. The load is the airflow and temperature change of the air through the evaporator. The load can range from a few hundred watts with a low fan setting on a cold day to the system’s maximum output which can be 10k+ watts on a hot day with the fan on high. The high side pressure is proportional to the heat load on the evaporator. Very high demand will put a high pressure on the high side. The other factor that determines the high side pressure is the condenser’s ability to remove the heat. This is because the refrigerant condenses as the temperature drops. Condensed refrigerant has a smaller volume than vapor which tends to lower the pressure when condensing.

    If the expansion valve is stuck in the middle, it will act like it is stuck open for light loads and then act like it is stuck closed for heavy loads.

    A thermostatic expansion valve is what the 2004 Civic has. It has the valve on the inlet to the evaporator and connects to a “bulb” located on the evaporator’s outlet with a thin tube. These valves are designed to maintain a specific amount of superheat, It is 7.5 degrees for the civic. The jargon superheat simply means temperature above the pressure/temperature chart for r134a. This ensures that there is no liquid present in the line on the way back to the compressor. A pressure temperature chart for 134a is located in the link.
    http://autoforums.carjunky.com/Automoti … 4a_P59651/

    The way to diagnose this valve is to measure the temperature of the outlet of the evaporator for a properly charged system. This won’t work if there is much air/moisture in the system or the charge isn’t correct. Use a moderate load such as the fan on low to medium. Then measure the pressure on the low side. In the pressure temperature chart, find your pressure and look up the associated temperature. The temperature of the line near the bulb on the evaporator’s outlet should be about 7.5 degrees above the temperature in the pressure temperature chart. If it is not correct, then there is either a bad valve or another issue preventing the expansion valve from being useful. An example of the valve unable to be useful is an undercharged system. The valve can’t do anything if it’s wide open and there isn’t enough refrigerant to properly condense. If the thermometer is in question, test it against a cup of ice water.

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