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iMac doesn't adjust fan speed, and dies...
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Mac Elite
Join Date: Oct 1999
Location: Montréal, Québec (Canada)
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In 2008, I posted a thread regarding this problem : http://forums.macnn.com/90/mac-os-x/...t-down-itself/ . I installed smcFanControl and that took care of the issue for a while. It's usually fine during winter as it's colder in my appartment, but it started again this summer. The temperature will slowly rise and all of the sudden, the computer will just shut down, without any message. I had upped the minimum fan speed to alleviate the problem, but sometimes it's not enough and the CPU will rise to about 50° C and then die. Shouldn't the computer adjust fan speed in order to avoid such situation? Seems stupid to just let the temp rise and then die.. I have a fresh install of mountain lion and a fresh user (created a new user then dragged files manually), so it shouldn't be a software issue. My sensors seem to be working fine. What could be the problem? The top-left corner is a bit hot but not that much. The problem is happening more frequently while doing CPU-intensive work like iMovie rendering, but sometimes just by browsing the web. I get no warning whatsoever. I can hear the fan very well so they are working fine. I recently opened my computer and there's no dust inside or anything that seems out of order.
This is a mid-2007 iMac with 6 GB of RAM.
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Moderator  Join Date: Apr 2000
Location: Gothenburg, Sweden
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The CPU should not die at 50C. In fact, the fan speed controller probably does what it should, not revving too high when the temp is only 50C. The Tjunction on that CPU is 100C - it should not be hitting the catastrophic thermal shutdown until then, and anyway should start throttling (makes the computer feel slow) long before that.
I suspect that either something else in the computer is dying due to heat. Do you have any bulging capacitors, perhaps? I would sniff around the PSU. Also, the memory controller on that model is separate, and the North Bridge can generate quite a bit of heat - check that its heat sink is on.
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The new Mac Pro has up to 30 MB of cache inside the processor itself. That's more than the HD in my first Mac. Somehow I'm still running out of space.
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Mac Elite
Join Date: Oct 1999
Location: Montréal, Québec (Canada)
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Someone mentionned the power supply earlier (it's normally 75° C, which was said to be hot) but the problem seems to happen when CPU hit 50°. If I prevent it to reach that temp, it doesn't shut down. Also, it happens mostly when using CPU-intensive work. It's an iMac so it's difficult to see all the parts, but everything looked fine when I opened it. I didn't think of removing the PSU unfortunately.
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Moderator Join Date: Apr 2000
Location: Gothenburg, Sweden
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I don't know what the PSU usually is in that model. It shouldn't fail at 75C, IMO, but... anyone with the same model willing to chime in?
CPU-intensive usually means memory intensive, so bad cooling on the Northbridge is a possibility - and bad caps on motherboard or PSU is still an option. If your CPU is truly failing at 50C, without even touching the throttle options, then your CPU is bad and needs to be replaced.
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The new Mac Pro has up to 30 MB of cache inside the processor itself. That's more than the HD in my first Mac. Somehow I'm still running out of space.
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Fresh-Faced Recruit
Join Date: Jun 2003
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Clinically Insane
Join Date: Nov 1999
Location: 888500128, C3, 2nd soft.
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Originally Posted by bigpoppa206 
Original post, second sentence.
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Moderator Join Date: Apr 2000
Location: Gothenburg, Sweden
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Perhaps it might be time to repeat what SMCFanControl does and what it is designed for...
Intel mobile CPUs - which Apple also used in older iMacs and still uses in the Mac mini - can run very hot without shutting down to prevent damage. It's 100C for the old "Merom" CPUs in the OP's Mac, and has varied between that and even higher ever since. Cooling is more effective if the temperature difference between the cooling medium (the air) and the thing being cooled (the heatsink, and by extension the CPU) is large. By letting the CPU get hot before cooling, you don't have to run the fans so hard. Apple exploits this to keep the laptops nice and quiet - but hot. In some cases, scorching hot. If you like to keep your laptop on your lap, as it were, you might prefer a cooler but noisier laptop. In that situation, you can use SMCFanControl to tune up the fans a bit. That's the reason the software was made.
If your Mac crashes and SMCFanControl makes it crash less, then something is wrong with your Mac. If you want to go ahead and rely on the bandaid that SMCFanControl is, then go right ahead, but the problem is not with the cooling system or Apple's programming of it. The problem is that something is failing at a temperature that it should still be working fine at.
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The new Mac Pro has up to 30 MB of cache inside the processor itself. That's more than the HD in my first Mac. Somehow I'm still running out of space.
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