Late 2009 iMac desktop - CPU FAN - 0 RPM

Discussion in 'iMac' started by kwikslip, May 17, 2017.

  1. kwikslip macrumors newbie

    May 17, 2017
    I know there are several threads about this, but none which I have been able to use to help with my problem. I recently removed my screen from my late 2009 imac desktop and accidentally ripped off my LVDS display port from the main system board. I tried to order a replacement and solder it back on, but it was too small to work on, so I ended up buying a new system board off of ebay. When I had the old system board in, the CPU fan was spinning up to a high level as soon as the machine turned on. After replacing the system board, it is still doing this. Is it possible this is just a problem with the CPU fan itself ?
    I know many people have talked about putting in a resistor to slow the fan speed, but I find it hard to believe that both system boards have this issue. There seems to only be one connector for the CPU fan, but where does it get the reading to spin up or down? I assume this is from the system board. Is there a way to measure (the voltage?) this from the pins on the CPU fan connector ? Is there a way to test this fan ?
    Other people have talked about a resistor missing from the system board (near the cpu fan connector - R5797) but I looked at mine on both boards and it appears to be there and looks intact. (see attached).
    Is there any other way to troubleshoot this or correct the issue ? I have tried all of the software based fan speed control apps out there, and it does not allow me to adjust the CPU fan anymore. (app is showing the RPM @ 0 now for CPU fan). The replacement fan is not a ton of money, I just want to make sure before I buy another component for this system that it is going to work.

    Any help would be greatly appreciated.

    Attached Files:

  2. DeltaMac macrumors G3


    Jul 30, 2003
    Is the CPU fan actually running?
    Sounds like the CPU fan (if you can hear it running at high speed) is working just fine!
    But, running at high speed usually means that the temp sensor that controls that fan is doing one of several things.
    The temperature is too high, so the fan is running at high speed.
    Or, there's something wrong with that temp sensor (which is the ambient temp sensor),
    Or that temp sensor is simply disconnected.
    (I am pretty sure there is also a CPU temp sensor, but is integrated into the logic board, with no sensor connector, as it is permanently attached.)
    The ambient temp sensor connector is several inches from the CPU fan connector, near the top edge of the logic board.
    It's actually the connector farthest up the logic board, near the hard drive.

    If you run your hardware test, do you get any errors?
    If rebooting while holding the D key on your keyboard does not start the hardware test, then you will need the set of 2 DVDs that came in the box with a 2009 iMac.
    If you still have the original DVDs that came with your iMac when new, the hardware test will be on the #2 grey DVD.
  3. ryguy92000 macrumors regular


    Sep 20, 2014
    Salt Lake City, UT

    Just to be clear, your fan is NOT spinning at all or it IS spinning but like mad? If it is spinning like mad then the resulting repair is simple but SMALL. Im going to assume your working on a 27" 2009 iMac. There are TWO small resistors in the area shown. I don't have the exact machine infront of me but they ARE there. These two items are inductors which provide a filter to the ambient air sensor located under the CPU fan. It is VERY common for these two inductors to be broken off after repeated removal of an LCD. If you run a small bead of solder closing the circuit then your fan will behave as normal. again the inductors are tiny like the size of a pencil lead tip. I will continue to try to find an image of the inductors. Don't pay attention to the orange line as this is a screen shot from another article. The inductors are located near the board connector for the ambient air sensor on the fan

    Attached Files:

  4. kwikslip thread starter macrumors newbie

    May 17, 2017
    The CPU fan is indeed running - I can turn it on without the display attached, and it immediately goes to full speed.
    So are you saying that CPU fan is controlled by the ambient temperature sensor ? I have the technicians guide for this model and can see the ambient Temp. sensor on the top of the board between the bluetooth and LCD temp. sensor.
    I will double check that one tonight to see if there are any problems with it or if it is maybe plugged in backwards.
    I bought the desktop used years ago and never got the discs, so I wonder if I can download the hardware test disc to test it ? Is there a way to check the ambient temp sensor to see if it is working. Would VOM give me a reading on this ?
  5. kwikslip thread starter macrumors newbie

    May 17, 2017
    Are they the ones in the picture that I post above ? I think they are labeled R5700 and R5797.
    I looked closer tonight, and the R5797 looks to be completely missing. While the system was powered on, I tried to short the contact to see if the fan would slow down, but it didnt appear to have any affect. The contacts on this are smaller than the LVDS connector that I tried to solder. I dont know if I can get a bead that small - is there another way to slow it down ? Some people have mentioned adding a resistor to one of the wires on the CPU fan connector. Would this work ?
    --- Post Merged, May 17, 2017 ---
    Just to be clear - this is a 27" late 2009 model and the fan is spinning at what seems to be full speed.
  6. kwikslip thread starter macrumors newbie

    May 17, 2017
    Will this allow the system to read the RPM again from the fan if I bond these two points ?
  7. kwikslip thread starter macrumors newbie

    May 17, 2017
    Well the inductor/resistor or whatever it is on R5797 is certainly gone.
    I have tried to hold a resistor in place but it doesnt seem to slow the fan. Wasn't there a fix that allowed you to use a resistor on the cpu fan connector? Does anyone have a link to that post? I remember reading it, but cant seem to find it anymore.

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