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Modèle A1312 / Mid-2011 / processeur 2.7 & 3.1 GHz Core i5 ou 3.4 GHz Core i7, ID iMac12,2

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Fan is going berserk!

Yesterday I baked a GPU on an iMac (27" mid 2011).

After assembling all together again, it works as is should except – The CPU fan is going berserk!

I have downloaded Macs Fan Control, and I have put the fan to "sensor based", and thus it behaves, but I would like for it to behave without this software. Something is obviuosly wrong.

I should also mention that the fan starts to max out as soon as I power it on.

My guess is that I have forgotten a cable connection when I assembled it, the problem is which one?

My question is: Where can I find out which cables go where? I have googled for this, but can't find any descent pics.

Help would be appreciated.

Update (01/18/2023)

Thanks for the input! 👍

I just removed the display to see if I could see any missing cables. I have two empy ports, but according to internet these should be empty, AFAIK (see red arrows on attachment).

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However, I realize that I might have reinserted the SATA cables from the drive and the Optical unit in wrong ports. Apparently there are different ports doing different things. These are located on the backside of the board though, so I want to be sure that is the error before disassembly. What should go where?

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I should also mention that I upgraded this iMac with an SSD last year. At that time I installed SSD Fan Control and it has worked flawlessly until now. If I open SSD Fan Control now it only sees two fans.

Furthermore, the fans starts to max out not directly after restart as I thought, but when the login dialogue is present.

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Andreas - Any luck getting the diagnostics or get the TG-Pro output so we can discover why SMC is pushing the fans.


Alright, I have messed a lot with the machine, here's an update since last post:

- I took the time and disassembled the motherboard just to verify all connections, but all cables was in the correct place.

- I don't have the diagnostic DVD.

- Starting up with D took extremely long time. After 30 minutes of "Starting up from internet recovery" I aborted.

- No extra thermal paste but the factory applied one.

- I tried TG-Pro, but I couldn't change the rpm on the fans in any way. It showed the correct data though? Since this is a friends iMac I never bothered to buy a license.

- The only app that actually seems to regulate the fans correctly is Macs Fans Control ( The app have a setting called "Sensor Based" where you can set which sensor temperature, or an average temperature from all sensors, that you base the fan speed on. This setting didn't work with TG-Pro buy some reason. (I have some screen shots, but I can't find any way to add them in this post.)


This tells me that the sensor actually work as they should, but something is effed up with the SMC communication. Maybe something got broken in the baking process...?


The machine works as it should, and with the app Macs Fans Control the fans behave as they should. My friend is happy, and with a little bit of luck she can use the machine one or two more years.

We will probably never know what's actually broken with this machine.

Thank you guys for the help!


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Do you still have the Grey CD/DVD’s that came with the system? If you do one will be ID’d as a diagnostic disk (in fine print along the edge). When you restart quickly slide it in to get it going. If you have Sierra or High Sierra installed then the newer onboard diagnostics is again able but Apple killed it in the newer OS’s with firmware updates.

The last option which I personally think makes it a better tool over Macs Fan Control is TG-Pro within the paid version offers a useful diagnostics to isolate which sensor is causing SMC to push the fans up.

Post a snapshot of the main window, you will need to either slide the window to get all of the sensors, or reside the window so they are within the one snap.

Update (01/19/2023)

@Andreas Carlsson - Well I can tell you the fans will race after you swap out the original HDD to another HDD or SSD as Apple leverages a sensor within the drive for thermal management of the system. While many drives have a thermal sensor the connection Apple uses is not present in these off the shelf drives. While one can use a software solution you do loose the full performance the system could offer you as SMC also lowers the CPU’s clocking as it wants to protect it for excessive heat.

The way to fix this is using A special sensor unit made by OWC iMac Intel 21.5" and 27" (Mid 2011) SSD Temperature Sensor while expensive it’s the easiest way. Some people have fashioned their own solution emulating what OWC did. As this point in time with fewer of this series still in use I would just get the sensor. At which point you don’t need any software.

As you haven’t run any diagnostics we don’t know if you still have something deeper going on, like a failed fan unit. Maybe you can find the needed CD online for a good price of just break down and get the full TG-Pro to check things. Even still, with the free version you should be able to see if all three fans are even running.

Image iMac Intel 21.5" and 27" (Mid 2011) SSD Temperature Sensor


iMac Intel 21.5" and 27" (Mid 2011) SSD Temperature Sensor


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Where did you connect the fan?

If you put thermal paste on the processor, one theory is that it starts up like crazy because it detects overheating.

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Andreas Carlsson sera éternellement reconnaissant.
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