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Loud bang when i start air conditioning

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    Loud bang when i start air conditioning

    96 328i sedan Air Conditioner has loud bang at startup, then cools very well.
    My AC wasn't cooling too well so i had a neighborhood shop do a recharge.

    Afterward recharge, system blew quite cold BUT a very loud bang when I start the AC. I don't even start the AC while driving right now. I leave it on and only comes on when cranking the car. Much quieter noise during engine and ac startup at the same time.

    Mechanic troubleshot then replaced compressor and accumulator. After he pulled a vacuum and recharged, had warm air. Turned out he put 2/3 air and 1/3 freon for the recharge. Told me the gauge set he had was bad. He then took it to another shop and ran a vacuum on it and refilled freon by weight. AC is blowing cold again BUT the loud bang at startup is still there. Hi and lo gauges read normal range.

    Any ideas what could be causing this loud bang at startup?
    Could it have been overcharged both times and this is the culprit?
    Is there a high pressure relief valve on the compressor that is releasing?
    Could air still be in the system causing this?
    Could the accumulator have gotten messed up? First time on the initial charge when the bang started; and then the NEW accumulator being messed up after install because he recharged the system with faulty equipment.
    Should i have another vacuum and recharge done by weight hoping this eliminates the bang at start up.


    Gus in Vegas
    USN retired

    Something is moving under load, likely a hose but could be the compressor mount. A relief valve will sound like a machine gun, if over pressured. Someone is going to have to look while it is being turned on.
    Best guess is something wasn't installed correctly when the compressor or accumulator was replaced. I look to hoses hitting the hood, or fender when the compressor is called for.


      Thanks for the quick reply. I'll have the mechanic do a visual inspection. I know he has done that before, especially since this noise starting AFTER the initial recharge of the AC unit AND before anything was replaced. Still could be something loose when the AC is placed under load.


        After visual inspection, Sin City BMW determined the loud bang was coming from the compressor clutch when I start the AC. They said it was common if the PAG oil is off in the compressor. I asked about the tensioner, belt, mounts and other and they couldn't see any issues with those.

        I will be sending back the new compressor for an exchange. After the install i will post back.


          Mechanic swapped out compressor, pulled vacuum and recharged. Blowing cold. Still have loud engagement bang at start up.

          I'm thinking about swapping out the ac hydraulic tensioner, pulley and belt? Any thoughts?


          Gus in Vegas


            Swapped out compressor.......but used old Clutch ??

            I can't thhink of anything else that can do that so replacing the tensioner would be a good next more.


              My bad; compressor with the magnetic clutch was returned. New one's put in. Evac'd and recharge new unit. Still have the loud bang at start up.


                Is it possible that what you are considering a "loud bang" is just your perception?
                I do know that some compressors and some clutches do make noise when engaging.

                Could you record the sound and post a video of the noise?


                  According to shop I ended up taking it to, it was because of too much PAG oil in the system. They evac'd and recharged system again, no more problem.....


                    Originally posted by sdgusman View Post
                    According to shop I ended up taking it to, it was because of too much PAG oil in the system. They evac'd and recharged system again, no more problem.....
                    Great, it's fixed !!