faulty iMac CPU heat sensor?

Discussion in 'iMac' started by Zortrium, Jun 7, 2013.

  1. Zortrium macrumors 6502

    Joined:
    Jun 23, 2003
    #1
    Sorry in advance for the verbosity...this is a pretty odd problem.

    Last night I fired up Handbrake to rip a DVD for the first time in many months on my 27" 2009 i7 iMac. As expected, Handbrake basically maxed out my CPU using all eight threads (4 physical cores, plus 4 'virtual' cores with hyperthreading). The CPU fan speed quickly went to max (4200 RPM), which is way higher than I ever recall it going -- I know what it sounds like when the fans spin up during normal 'heavy load', and this was way more than I'd ever heard it spin up before, including many past Handbrake rips.

    Opened up Temperature Monitor and saw that several of the 8 CPU sensors were giving readings in the mid 90's C...i.e. too freaking hot. I quickly shut off Handbrake, but it took quite awhile (several minutes at least) for the CPU fan to stop running at max, and the temperature readouts by two of the cores stayed at mid 90's even though the machine was 99% idle the entire time.

    I know the machine only has four cores, so I'm a bit confused why there seem to be eight CPU temperature sensors. It seems like what's going on is that two of the sensors are faulty and always report mid 90's temps, thus causing the CPU fan to run at max, but those (virtual?) cores are often shut off when the iMac isn't running at full load, which would explain why the fan only went crazy when I started Handbrake.

    Evidence:

    After a few minutes at idle (post Handbrake), CPUs 6 and 8 still showing mid 90's, other cores back to normal temps (cores 2 and 4 deactivated, which I think are virtual cores along with 6 and 8), CPU fan still at max 4200 RPM:
    temperature readout 1

    After a long time at idle (5+ minutes), CPU fans have returned to normal and all virtual cores (2, 4, 6, 8) are inactive:
    temperature readout 2

    I noticed that very occasionally, the sensors for cores 2, 4, 6, and 8 report values for a split second even when the machine is idle (maybe once every 1-2 minutes) before going blank again. I captured one of these transient readings and yet again, cores 6 and 8 give mid 90's readings, while everything else is normal:
    temperature readout 3

    Do people agree with my assessment/guess that what's going on is one or more faulty temperature sensors, or are there any other ideas?
     
  2. Apple fanboy macrumors P6

    Apple fanboy

    Joined:
    Feb 21, 2012
    Location:
    Behind the Lens, UK
    #2
    Could well be faulty. But as its an older machine when did you last hoover out the air vents? If it's all clogged up might be worth a check. Just remember to shut it down and unplug first.
     
  3. Zortrium thread starter macrumors 6502

    Joined:
    Jun 23, 2003
    #3
    Just stick a handvac to the top air vent, you mean? Haven't done that recently, but I'm not sure how that would explain cores 6 and 8 being ridiculously hot, while everything else in the machine is cool.

    MTA: Unless you're suggesting that dust could be interfering with the sensors, which I think I might've read before.
     
  4. Apple fanboy macrumors P6

    Apple fanboy

    Joined:
    Feb 21, 2012
    Location:
    Behind the Lens, UK
    #4
    Yes I'm sure I read something about dust causing issues with the sensor. It's worth a try.

    The vent should always be cleaned out regularly to enable air flow. Which reminds me I better do mine tomorrow!
     

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