iMac Intel 27". EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz), EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz)

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My CPU fan *sometimes* indicates 0 rpm with fan at full speed

I have checked other threads regarding this issue and they suggest replacing the resistor with the "47K 1/16W 5% in a 0402 package" on digikey.

However, the issue described in other threads are about the fan constantly at full speed .

My issue is random and only happens about once a day, for about 30 seconds.

This is what happens: For no apparent reason (ie I'm not using more processing power than usual), the fan starts to rev up, a bit like car racers rev up their engines just before a race. It's goes a bit like this:

VR.... VRRRRRR.... VRR.. VRR.. VRR.. VRRRRRRRRRRRRRRRRRRRRRR

The pattern is not the same every time but it's a bit like that. (smcFanControl indicates 0 rpm during that time)

The thing is, it only happens once a day, so the resistor must still be in place (since the fan functions normally 99% of the time).

How come the issue happens only sometimes? What could be wrong?

Answered! View the answer I have this problem too

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Not sure what your problem could be, I just wanted to say I love the fact that you gave an example of what the revving sounds like XD

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Sounds like the fan unit its self is failing here. What is happening is SMC is going into failsafe mode as it is trying to keep the system cool but since the fans tachometer (glitchy) is not working its pushes the fan to the max.

I'm not a lover of smcFanControl software as it has messed me up a few times. While I understand why people use it I prefer this app instead: TG Pro between its better information panel it also allows you to turbo the fan when the system gets to a set threshold which is better than what smcFanControl offers.

I would also strongly recommend you do a good dusting out of the system and look at re-pasting the CPU & GPU's as the thermal paste in this series was not very good.

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Hi Alex,

I had a similar problem with my late 2009 27" iMac. I tried resetting the SMC controller and followed all the advice on Apple support forums etc. without joy.

When I had PCs, taking the side off the tower and giving the whole thing a good clean with compressed air was a breeze (forgive the pun) but I was worried about doing this with my all in one iMac. In the end, I bit the bullet and disassembled it for cleaning. It was full of dust and the cleaning alone took around 30mins. You'll have to remove the coverglass which is attached with strong magnets (little suction cups from eBay were the route I took) as well as the display panel itself to get to the fans and other components. The panel removal is the most delicate part as there are a couple of fiddly ribbon cables and a temperature probe that must be disconnected/reconnected very carefully. All in all I'd give yourself an hour to do this. There are some good guides on youtube that helped me. If you don't have an air compressor you can buy compressed air in cans from a photography shop or similar, if you've got a compressor just make sure you go easy on the pressure and don't squirt the fans up to serious speeds as you'll damage the bearings.

So long story short, I agree with Dan. Give her a good clean. I would be very surprised if the fault was caused by the resistor you mentioned.

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Was the hard drive was changed recently?

Try installing this fan utility, and use the hard drive's internal SMART thermal monitoring instead of the one inside the iMac. Doesn't matter if its a regular hard drive or SSD, it works for both.

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@avanteguarde - He already has a fan control app smcFanControl it basically does the same thing.

Sadly, these SMART services are slow in response and people tend to misuse them too. Lastly, they can mess up the OS as some are fitted with a Trojan on some sites (beware!) Which is another reason why I don't like these. TG Pro is a bit different (yes, you can also use SMART within it - not recommended). You can manage the systems heat just by SMC services which is more responsive and won't kill your system (overheat or burnout the fan).

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I would agree for everything regarding smcFanControl except when it comes to hard drives. Apple specifically designed 2009-2011 iMacs to prevent a customer from simply swapping out a hard drive. Even iFixit's own hard drive sales mention this in some way. I can vouch for the software fix for the fan, as I've repaired a dozen of these 2009-2011 iMacs, none of them using this specific program has had a trojan. Avira and Norton can attest. On a side note, even in the latest MacBook Pros, one cannot simply change out the SSD to any other brand.

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Well almost ;-}

I've put in quite a few HD's in these iMac systems without any problems and not using any special software!

The trick at the beginning was to match the brand of drive the system had (i.e. Seagate for Seagate or WD for WD). Later on OWC came out with a special adapter cable which is what I use: OWC In-line Digital Thermal Sensor for iMac 2009-2010 Hard Drive Upgrade for either a HD or SSD upgrade. A different unit is needed for the 2011 and 2012 and newer systems which have a 3.5" drive. The newer (2012 onward) 21.5" iMacs use a 2.5" drive that doesn't need this adapter,

The blade SSD used in the newer 'Thin Series' iMacs is a different subject which I also have better solutions for but lets not get distracted here.

Using any software solution you are defeating Apples design while there was some cases it made sense (heavy gaming) we have learned over time it shortens the systems lifespan and often the fan's wear out prematurely. What has happened is the gaming software outran the design of the older systems so people pushed the systems just to hard for what they where designed offer. Think of it this way a street car is not a race car and would not hold up on the race track in a race. So using fan over-rides just is not smart as you likely are pushing the system beyond what it was designed to do. And you likely have other issues within the system if you still think you need it which needs addressing not masking.

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Alex222 will be eternally grateful.
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