Lock the cMP's fan speed

Discussion in 'Mac Pro' started by h9826790, Sep 7, 2015.

  1. h9826790, Sep 7, 2015
    Last edited: Sep 8, 2015

    h9826790 macrumors G4


    Apr 3, 2014
    Hong Kong
    I just use my cMP in Windows 10 to play some games, and test the crossfire setup. Since I only use the internal mini 6 pin to power dual 7950. The PCIe fan will spin up to around 2000 RPM under load (I guess that would because of the total current draw from the 6pin is high.

    To get rig of that fan noise, I use MacFanControl (of course, the Windows version) to lock all the fan speed as follow, which is virtually silent for me ( I am sitting 6'+ away from the machine when gaming) and provide decent cooling for both CPU and GPU.

    Booster 1100
    PCIe 1300
    PSU 800
    Exhaust 800
    Intake 900

    Interestingly, now when I boot back to OSX. All fan speed still locked at those speed, no matter how I stress the CPU / GPU, all the fan won't go up. And regardless what I set in iStat, the fan won't go back to the normal idle either.

    Screen Shot 2015-09-08 at 16.10.56.jpg
    (No matter what I set, the fan stay there)

    TBO, I know a SMC reset can fix this issue. I just want to share this experience, in case someone that feeling the fan spin up and down is annoying (e.g. the PCIe fan going crazy after reboot). And they have Windows partition, they may try to fix the fan speed at Windows, which may actually lock the fan speed in OSX as well and give them a nice constant low noise machine. However, I recommend to lock the fan at a higher speed then the true idle, especially if the OAT is high. The cMP may lost the ability to adjust the fan even though the CPU approaching overheat.

    In my test, I can only push my CPU to 82C with those constant fan speed. So, I don't know what will happen if it goes above the native Apple threshold 85C. And as expected, the GPU's temperature basically won't affected by lowering the PCIe fan. They are mainly cooled by their own fan, I increase the PCIe fan to 1300 simply because that's the max speed which I still can't hear it, and will provide better cooling to the HDD.

    This is how I run my test (for 10min). As you can see, OAT 34C (aircon off), the CPU stay at 100%, and both GPU is stressed by Luxmark, the current draw from those 6pin is almost at it's max rated current (75W / 12V = 6.25A), but the fan just stay at those lock speed.
    Constant fan speed stress test.jpg
    Before this happen. I always think that I can only manually set the minimum fan speed in OSX. And no way to stop the system to spin up the fan. However, it seems I am totally wrong, we can in fact keep all fans at constant speed and disable the system to control it. I don't know if this is a bug or what, but now I am happy with this result.
  2. Lucas Godfrey macrumors 6502

    Lucas Godfrey

    Jul 12, 2010
    Somewhere between Here and There
    Try this using macfancontrol in both OS's, ditch istat and see if you still can't control min fan speeds using macfancontrol instead. personally i prefer mfc over smcfancontrol and istat.
  3. h9826790 thread starter macrumors G4


    Apr 3, 2014
    Hong Kong
    Thanks for the suggestion. Just tested. MacsFanControl in OSX able to revert the fan back to auto control. It seems MacsFanControl actually change the target fan speed value in the system, but not like other fan control software just change the minimum value, which still allow the system protection to kick in.

    Also, MacsFanControl will leave the target value at there even though you boot into another OS.

    So, again, the same pros and cons, MacsFanControl give the user total control of the fans. However, it will disable the system protection, should only be use with care.

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