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  1. #1
    Established Member Two Rings mrismail's Avatar
    Join Date
    Oct 27 2014
    AZ Member #
    291240
    Location
    Brooklyn, NY

    Knocking noise when AC is on...HELP plz!!

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    I just can't catch a break. I fix one thing and something else happens. I spent a lot of money trying to figure out the knock on acceleration and deceleration (which turned out to be the driver side engine mount), I certainly don't plan on doing the same on this new knock that started yesterday.

    Symptom:
    I turn on the AC and hear a loud constant knock from the engine bay. The knock is not always present. It comes and goes, no specific triggers. No noise in cabin. When I turn off the AC it fades away with the fans whining down. No issues with cold air cooling the cabin. No issues with car overheating. There is no hissing or metal on metal squealing/grinding. No belt squeal.

    Possible Diagnosis:
    Fan clutch, AC compressor bearing, blower fan. I'm leaning more towards fan clutch assembly.

    Plan:
    Disconnect the fan power harness when I hear the knock to see if it goes away while the AC compressor is still engaged. This way I'll at least know if it's coming from fan or compressor. Question is, will I be doing any harm to anything if I did this? I only plan on doing it for 10-20 seconds for the fans to come to rest then I'll turn off the car and reconnect the harness.

    Anyone have any thoughts? I'd really appreciate it.
    Thanks!! (sorry for the whole doctor approach, I'm trying to tackle the issue like I would with my patients )
    2011 A6 S-Line Premium Plus
    Current mods:
    ST Coilovers | G6 3000K LED Fog Lights | Blacked out with PlastiDip: Chrome trim, front grill, rear emblem, corner lights | Quad tips | 25% 3M tint all around | 4000K LED license plate | Matte black OEM rims wrapped with Toyo Extensa HP tires | Black vinyl wrapped roof | Rear trunk spoiler | Rear window spoiler | Tinted rear lights | SLine steering wheel with paddle shifters |

  2. #2
    Established Member Two Rings
    Join Date
    May 23 2016
    AZ Member #
    373689
    Location
    Edmond, OK

    Nothing wrong with the doctor approach. Damn good way to present a scenario!

    Our cars do not have fan clutches (that's for engines with mechanical fans that are constantly on). We have two electric puller fans.

    I'm not sure how it is on your C6.5, but on my C6 the radiator fan control module is wired into the front-end harness, so can't really be unplugged. You CAN pull the fuse/ relay for it. But something to keep in mind (and someone with more knowledge please jump in), if you disconnect power to the fans, it's POSSIBLE that the ECU will pick this up and in an effort to save the engine this may shut off the AC compressor (since there would be no fans to cool down the hot AC condenser.... similar to how if enough pressure in the AC system builds up [heat], the AC compressor gets shut off). So this could add complexity to your testing.

    Recommendation- Since we are in the spirit of doctor-hood... I think this would be a perfect case to use a stethoscope! Most auto parts stores and even Harbor Freight have these mechanic's stethoscopes for cheap and you can just put the probe against any solid surface (like the AC compressor's casing, or the fan shroud close to either of the 2 fan motors). Where ever the sound is strongest is your culprit. These are actually the only areas that CAN make additional noise during AC cycling operation- the cooling fans, and the compressor itself.

    Good luck!
    07 A6 3.2- Out & In- Morimoto 5500k & Nokya 2500k, DeAutoLED Reverse, LP, & Footwells, Ziza Full Interior, 10" lo-pro Pioneer, Audi Aluminum Pedals
    Drivetrain & Suspension- Stoptech X-Drilled & Sport Pads, ST X-Coilovers, Hacked Airbox with aFe, Resonator Delete

    "What is best in life? -Kill your enemies. See them driven before you. And hear the lamentation of their women" Conan the Barbarian

  3. #3
    Established Member Two Rings mrismail's Avatar
    Join Date
    Oct 27 2014
    AZ Member #
    291240
    Location
    Brooklyn, NY

    Quote Originally Posted by 95blkmax View Post
    Nothing wrong with the doctor approach. Damn good way to present a scenario!

    Our cars do not have fan clutches (that's for engines with mechanical fans that are constantly on). We have two electric puller fans.

    I'm not sure how it is on your C6.5, but on my C6 the radiator fan control module is wired into the front-end harness, so can't really be unplugged. You CAN pull the fuse/ relay for it. But something to keep in mind (and someone with more knowledge please jump in), if you disconnect power to the fans, it's POSSIBLE that the ECU will pick this up and in an effort to save the engine this may shut off the AC compressor (since there would be no fans to cool down the hot AC condenser.... similar to how if enough pressure in the AC system builds up [heat], the AC compressor gets shut off). So this could add complexity to your testing.

    Recommendation- Since we are in the spirit of doctor-hood... I think this would be a perfect case to use a stethoscope! Most auto parts stores and even Harbor Freight have these mechanic's stethoscopes for cheap and you can just put the probe against any solid surface (like the AC compressor's casing, or the fan shroud close to either of the 2 fan motors). Where ever the sound is strongest is your culprit. These are actually the only areas that CAN make additional noise during AC cycling operation- the cooling fans, and the compressor itself.

    Good luck!
    Thanks for the info.
    I see a plug on the back of my fan assembly. It's a whole control module with a wire harness and wires going to both fans. I unplugged it with the battery disconnected cause I didnt wanna cause any harm. (Do no harm).
    I thought you were being sarcastic about the stethoscope 'til I looked it up! Kinda cool! And lucky for me, it's covered by my insurance (budget to spend on this fix).

    Sent from my Samsung Galaxy S9+
    2011 A6 S-Line Premium Plus
    Current mods:
    ST Coilovers | G6 3000K LED Fog Lights | Blacked out with PlastiDip: Chrome trim, front grill, rear emblem, corner lights | Quad tips | 25% 3M tint all around | 4000K LED license plate | Matte black OEM rims wrapped with Toyo Extensa HP tires | Black vinyl wrapped roof | Rear trunk spoiler | Rear window spoiler | Tinted rear lights | SLine steering wheel with paddle shifters |

  4. #4
    Established Member Two Rings
    Join Date
    May 23 2016
    AZ Member #
    373689
    Location
    Edmond, OK

    oh sweet! and LOL no sarcasm at all, lol. I've had mine for maybe 12yrs now. It's great for finding random knocks/ pings/ grinding sounds around the engine bay. I think the best use for it is figuring out "which accessory pulley bearing is making the noise", lol. Since you can just put the probe end right up to the accesory or the mounting bolt and it's like day and night through the stethoscope, so no guess work there.
    07 A6 3.2- Out & In- Morimoto 5500k & Nokya 2500k, DeAutoLED Reverse, LP, & Footwells, Ziza Full Interior, 10" lo-pro Pioneer, Audi Aluminum Pedals
    Drivetrain & Suspension- Stoptech X-Drilled & Sport Pads, ST X-Coilovers, Hacked Airbox with aFe, Resonator Delete

    "What is best in life? -Kill your enemies. See them driven before you. And hear the lamentation of their women" Conan the Barbarian



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