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Fans going crazy, CPU 100% idle


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#1 .Neo

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Posted 27 September 2012 - 16:51

I'm going insane over the behavior of my 27-inch, medio 2010 iMac. The CPU fan continues to spin up even though the iMac doesn't do a thing. I've reinstalled OS X Lion multiple times, I reinstalled OS X Mountain Lion several times. I did PRAM-resets more times I can count, same goes for SMC resets. Today I even disconnected the iMac, laid it down on my bed and cleared all the vents with the vacuum cleaner on the weakest setting. The other day a friend of mine walked in and immediately asked why my iMac was making so much noise.

The first picture shows the CPU Fan right after startup. The second less than two hours later. I haven't done a thing with the iMac except play music and browse the web a bit.

How on Earth is it possible the fans spin up, sometimes over 4000 rpm, when the CPU sits next to idle most of the time? The worst part is they won't power down once the iMac cooled off. There are no optical drives in the SuperDrive either.

The measurements are in Celsius. The room temperature averaging at 20,5 degrees throughout my apartment. It's only 14 degrees here in Amsterdam.

Attached Images

  • Screen Shot 2012-09-27 at 16.52.02.png
  • Screen Shot 2012-09-27 at 18.38.10.png
  • Screen Shot 2012-09-27 at 18.55.22.png



#2 +LogicalApex

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Posted 27 September 2012 - 16:56

assuming your temps are in Celsius it is spinning up because the chip is hot. Is there anything to justify the chip being hot? Dust on the HSF, high ambient temp, etc?

#3 OP .Neo

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Posted 27 September 2012 - 16:58

I can't really say for sure how much dust has accumulated inside the iMac itself, but judging by how little there was directly underneath the grill I seriously doubt it's a lot. I tried looking inside with a flashlight but everything seemed clear.

#4 f0rk_b0mb

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Posted 27 September 2012 - 16:58

Did you recently install an SSD?

http://forums.macrum...d.php?t=1235234

#5 +jamesyfx

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Posted 27 September 2012 - 17:00

If I leave my iMac on for long periods of time the fans will gradually get faster and faster until they are 100% on, and they will not go back to normal speeds unless I shut down and power back on. Rebooting just keeps them running at full tilt.

It's strange. It's like the driver/sensor controlling it has failed to do it's job properly. :/

#6 Farchord

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Posted 27 September 2012 - 17:02

Yeah if it's THAT hot at idle, you have a problem on your hands.... either your system has absolutely 0 airflow or the heatsink aint applied correctly, if so I'd take the computer and bring it to a store.

#7 Elliott

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Posted 27 September 2012 - 17:06

Did you recently install an SSD?

http://forums.macrum...d.php?t=1235234


That actually affects the hard drive fan, not the CPU fan (at least it did in my Late 2010 iMac). Also, it happens a lot sooner than 2 hours after boot. :p

Maybe your GPU is being taxed by something? The CPU fan cools both, so...

#8 OP .Neo

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Posted 27 September 2012 - 17:15

Yeah if it's THAT hot at idle, you have a problem on your hands.... either your system has absolutely 0 airflow or the heatsink aint applied correctly, if so I'd take the computer and bring it to a store.

It's an iMac. It's normal for things to run a little bit hotter.

Did you recently install an SSD?

Everything is stock. No modifications whatsoever have been made to the iMac after I unpacked it. Also, the SSD issue wouldn't affect the CPU fan, rather the HDD fan.

Maybe your GPU is being taxed by something? The CPU fan cools both, so...

Okay, this is something I can work with. Is there any way of checking what the GPU load is?

If I leave my iMac on for long periods of time the fans will gradually get faster and faster until they are 100% on, and they will not go back to normal speeds unless I shut down and power back on. Rebooting just keeps them running at full tilt.

It's strange. It's like the driver/sensor controlling it has failed to do it's job properly. :/

That's EXACTLY what ****es me off. The fans won't slow down once things are cooling off.

#9 OP .Neo

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Posted 27 September 2012 - 18:15

PS The thing is I vaguely remember my fans spinning up faster as early as during the OS X Lion Developer Previews. When starting up using Mac OS X Snow Leopard the issue was gone.

#10 OP .Neo

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Posted 27 September 2012 - 23:50

UPDATE:

Okay this is incredibly interesting. I'm now 100% sure this is indeed a software issue, rather than a hardware related one. For whatever reason OS X Mountain Lion is telling my iMac to increase CPU fan speed in order to cool off. But apparently this is a one-way street. As time progresses the fan speed just continues to go up until it hits a maximum of about 4150 rpm. Like jamesyfx noticed they it will continue running at its maximum until I do a full shutdown. So for whatever reason OS X Mountain Lion won't tell my iMac to reduce fan speeds once it cooled off.

After a lot of Googling I noticed other people having the same issue with OS X Mountain Lion. Not just iMacs but MacBook Pros too. One person came with the suggestion to temporarily download and install iStat Menus 2, set Fan speed control to "Medium" (it's on of the default "rules") and then return to "Default". In literally an instant my iMac's CPU fan would drop from its maximum of ±4150 to a quiet 1940 and then continued to drop.

Check out the below screen shot and note the uptime: Over 6 hours and a regular CPU fan speed of 1885 (it's still dropping as I type).

So what's going on here people?

Attached Images

  • Screen Shot 2012-09-28 at 01.55.56.png


#11 +jamesyfx

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Posted 28 September 2012 - 10:03

It does a similar thing on my system, CPU fan speed drops down to a steady 1190rpm when I set to default.

Seems like it kicks the default behaviour back in once you set it as such.

My iMac is a completely different model to yours, so the issue must be software related.. and I think using this utility has proven that.

#12 OP .Neo

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Posted 28 September 2012 - 12:13

Yeah that's exactly what seems to happen. iStat basically slaps OS X on the wrist telling it do proper fan control. After a while CPU fans would indeed drop to an average of 1190 rpm on my model as well. Here I was thinking my CPU/GPU fan was constantly triggered because it couldn't handle the new OS X Lion/Mountain Lion graphics effects. But that's not it either.

Anyway, it seems like you can uninstall iStats Menu from your system until you do a SMC reset.

#13 LaP

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Posted 28 September 2012 - 14:21

I'm not familiar with Mac since i never owned one but why does OSX has to control the fan ?

I would never ever trust any OS to control the fan speed of my computer. Right now i have a fan controller but if i would not have one i would let my Motherboard control the fan speed or at worst install my MB software (or a 3rd party alternative like speedfan) to control them.

#14 OP .Neo

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Posted 28 September 2012 - 14:45

Honestly I have no idea if OS X is fully responsible, but I do know the OS can trigger different fan speeds.

#15 LaP

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Posted 28 September 2012 - 15:21

Here's an interesting blog post about the issue. You probably have already read it since it was in the first 10 results of a google search but i still post it just in case.

http://gigaom.com/ap...lion-heres-why/