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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iMac CPU fan periodically runs at max speed.

Hello amazing people! :)

I've got this great iMac 27 inch that troubles me.

Model: MB953 / A1312 / EMC 2374

Issue: CPU fan is running at MAX speed. Sometimes for a few seconds, a minute and sometimes until I power it down.

When: Whenever it likes. It doesn't matter wether I'm running Final Cut, rendering HD movies, browsing the web for funny cats or if it's idle in standby mode.

Safe to say, it's incredible annoying.

I've got this monitor program, and here's what it tells me, when my iMac er running normally.

CPU heat: 38 degress (celcius)

CPU fan: 1500ish RPM.

Now, when the issue happens, the readings are:

CPU Heat: 38 degrees (still celcius)

CPU fan: 0 RPM (<-- what!?)

Yes, 0 RPM is making enough noise to wake up my entire building (Or at least me...)

What is wrong with my iMac?

My own logic sense, would say; There's something wrong with the CPU RPM reader, making the system to believe, that it's actually running at 0 RPM. Therefor opening up everything to blast air trough. Might also be the heat reader? But my CPU heat reader seem to work fine. (Various temperature readings)

Note: It's ONLY the CPU fan. HDD and ODD fan functions as intended.

Update (09/10/2015)

So, as suspected, my iMac would make some noise again.

The iMac was idle/standby mode, when it happen. Proximately 15-20 mins. after I last used it.

The fan went to "slow" speed after 5 mins.

Here are the readings from TG Pro and iStat.

Block Image

Block Image

CPU speed:

TG: 1520 RPM

iStat: 0 RPM

The noise is a lot louder than this. (Probably max speed)

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Download this app: Temperature Gauge Pro post a screen shot of it when the fans are running wild so we can see what's up.

It sounds like SMC or one of the fans is having a problem.

Update (09/10/2015)

First, I don't like running two SMC monitoring apps at the same time as they can interfere with each other.

In any case both appear to show the fans are not running wild which is a good thing. Clearly, one of the fans bearings are gone here which is why you are hearing the noise.

I should point out the Hub Contoller temp reading is on the high side. For a system which is at idle this should be lower. I suspect you have some dust buildup and you may need to clean off the old thermal paste and apply a fresh coat.

Get a can of can'ed air as well as a small paint brush to carefully scrub down the fan's, heat exchangers and logic board of the dust buildup then use the can'ed air to blow it away. Then review things again.

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I'll post a screenshot, as soon as it happens again. The 3 day trial should be enough for it to happens I'd say :)

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John Shin Hansen will be eternally grateful.
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