iMac fan runs full speed on shut down

hi everyone, has been discussed before, may have new information add interest tech savvy people out there.

imac runs fine; more or less did when new. in os x, when shut down computer, fan runs @ full speed indefinitely, unless unplug or boot up. after following smc reset procedure (http://support.apple.com/kb/ht3964) no luck, searched bit , noticed there 1 or 2 cases problem believed motherboard issue, , therefore costly fix.

however, may interest know, when booted windows 8.1, shutting down imac not cause issue occur. stays silent, should, if disconnect/reconnect power. leads me believe it's unlikely motherboard issue.

then, questions these:
-what difference between shutting down os x , shutting down windows?
-is operating system allowed/able touch bios settings?
-is possible still hardware problem, given difference?

default os os x, , makes no difference play setting.

i'd avoid costly repair, if it's future software update fix.

in advance!

-jon
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imac 27", late 2012, 3.4ghz i7, 16 gb, os x 10.9.4

 

you mentioned doing pram reset. that's not make bit of difference. pram resets big deal before os x. since then, there's little stored in pram. right thing smc reset, manages power , fans. if remember correctly, that's done unplugging imac , holding power button down 5 seconds while it's unplugged. you'll want check procedure specific model, though.

better check booting windows boot known-good copy of os x , see if still have problem shutting down there. install clean copy on usb thumb drive, if necessary.

 

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