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  1. #11
    Join Date
    Oct 2013
    Location
    NovaScotia Canada
    Posts
    12

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    One year later on repair job work well with car now at 225000km. However the AC compressor vibration - whining noise returned last fall as the weather got colder. I did not use A/C during winter here in Canada and when I turned on in spring the whining noise had returned. I connected A/C gauges and everything looked OK but LP side was around 30 psi on a 40F (5C) day with car running. Thinking that the desiccant bag was blocking again I decided to lower the pressure while car was running. Interesting find in that as the pressure got lower (LP25psi, HP100psi, 40F) the noise got louder. So I decide to add gas to the system. Once the LP pressure got to 40psi the whining went away. I charged it to LP-45psi, HP-160psi and 40F. No noise now and blowing cold air.

    I am not sure why this whining occurs at both high and low pressures. At a high LP pressure I assumed that there was too much gas and I was getting liquid carry over from the evaporator. This is also the reason that Volvo has a software change to lower the evaporator level. At low LP pressure I assumed that the compressor is going to full stroke to maintain HP pressure constant. Either this full stroke or too low gas level now causes whining to occur. Finally the new style variable displacement compressor used on the newer 2007+ P3 series cars tries to maintain the gas pressure constant for various engine speed over a range of temperatures. This probably requires a tight tolerance on gas charge (.8 kg R-134 XC70) which implies a certain operating pressure.

    In summary there seems to be a "sweat spot" for LP pressure that will prevent whining on the 2008+ variable displacement AC compressor on P3 series cars as stated below:

    LP-30psi - Too Low. Whining appears
    LP-45psi - Good.
    LP-70psi - Too High. Whining appears

  2. #12
    Join Date
    Dec 2012
    Location
    Canada
    Posts
    61

    Default

    So finally got ready to fix my A/C. Since my last post the A/C stopped blowing cold almost completely and that was the summer of 2015. Luckily was the end of the summer, so I had the winter to get prepared to fix the system again. Went to US for couple of days of vacation and on the way back to CAN bought 3 canisters of R134a from WalMart, because I didn't want to mix the existing even low level of gas in my system with the R12. Finally by June this year I plugged in the gauges and started filling up the system. I was a hot day, 30 C and I was following your advice to reach LP 45 psi, but I couldn't go over 35-38 psi...I emptied one whole canister of gas and the A/C start working again - nice and cold. But the compressor noise was still there - this got me ambitious. So I hooked the second canister and started filling up again. This time I was monitoring the HP site and when it reached some 200 psi I stopped. Then released a but of gas to lower it to 190 psi or so and boom - the noise was gone!
    Apparently R134a behaves differently then the R12 and this is to be expected since it is a different gas altogether. The long story short is that the noise in the compressor was obviously due to the low level of gas in the system. I still have to determine where I have a leak (probably the condenser), but this whole summer the car A/C ran flawlessly and super cold too. My take is that HP has to be close to 200 psi to have quiet compressor run.

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