iMac fan runs full speed on shut down

[jonofuf]

Hi Everyone, this has probably been discussed before, but I may have some new information to add that could interest the tech savvy people out there.

My iMac runs perfectly fine; more or less as it did when it was new. In OS X, when I shut down the computer, the fan runs at full speed indefinitely, unless I unplug it or boot back up. After following the SMC reset procedure (http://support.apple.com/kb/HT3964) with no luck, I searched a bit and noticed there are one or two cases where the problem was believed to be a motherboard issue, and therefore a costly fix.

However, it may interest someone to know, that when booted into Windows 8.1, shutting down the iMac does not cause this issue to occur. It stays silent, as it should, even if I disconnect/reconnect the power. This leads me to believe it's unlikely to be a motherboard issue.

So then, my questions are these:
-What is the difference between shutting down from OS X and shutting down from Windows?
-Is an operating system allowed/able to touch BIOS settings?
-Is it possible to still be a hardware problem, given this difference?

My default OS is OS X, and it makes no difference to play with that setting.

I'd certainly like to avoid a costly repair, if it's just something that a future software update will fix.

Thanks in advance!

-Jon
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iMac 27", Late 2012, 3.4GHz i7, 16 GB, OS X 10.9.4

 

[Detrius]

You mentioned doing a PRAM reset. That's not likely to make a bit of difference. PRAM resets were a big deal before OS X. Since then, there's very little stored in PRAM. The right thing to do is an SMC reset, which is what manages power and fans. If I remember correctly, that's done by unplugging the iMac and holding the power button down for about five seconds while it's unplugged. You'll want to check the procedure for your specific model, though.

A better check than booting into Windows would be to boot into a known-good copy of OS X and see if you still have the problem shutting down from there. You could even install a clean copy on a USB thumb drive, if necessary.

 

[h9826790]

Agree that you should try SMC reset but not PRAM reset.

Anyway, you can perform the SMC reset, and then boot into recovery mode. Do the permission and disk repair. Shutdown the computer to check if the fan is normal.

If that's OK, boot into OSX and try shutdown again.

 

[maflynn]

Another vote for the SMC reset. Report back and let us know if that corrects the issue.

 

[jonofuf]

Another vote for the SMC reset. Report back and let us know if that corrects the issue.

Hey guys, I got them mixed up, sorry, but the procedure I followed was in fact the SMC reset (see updated section in original post). I did do PRAM as well after it didn't help anything... because, I dunno, why not...

Jon

P.S. I appreciate the idea of the SMC reset, but according to the Apple support page, it only affects problems where the fan runs at high speeds when the computer is ON. Is it any surprise that the reset didn't fix anything?

 

[jonofuf]

Anyway, you can perform the SMC reset, and then boot into recovery mode. Do the permission and disk repair. Shutdown the computer to check if the fan is normal.

Did exactly that again just now; no change.

Any other ideas, anyone? I get that this site is all Mac / OS X lovers, but I was hoping someone might be able to shed some light on why the fan doesn't go nuts when shutting down from Windows... And how it could possibly be a motherboard failure if this is the case...

Thanks.