Fan will not work on late 2015 27" iMac

[lferg8]

Fan was noisy then stopped working. Installed a new fan and it would not work either. Only thing I could figure was the fan circuit on the motherboard, so replaced the motherboard. Now I have a new motherboard and new fan and the fan still doesn't work. CPU temps are 190F to 200F and fan just won't run. Only other replaceable hardware is the power supply, but that doesn't make any sense that the power supply would ONLY affect the fan when everything else works fine. Any suggestions are greatly appreciated.

 

[jav6454]

Have you checked the voltage of the fan header?

 

[lferg8]

So far I've not found any diagrams on which of the four terminals should have what voltage. I'm on my second motherboard (2nd one was pulled from a functioning machine) and third fan (2nd is new, third works in another machine). If the voltages were wrong, wouldn't this be a motherboard issue?

 

[Street28]

Bit of a thread necro, but did you ever get to the bottom of this? I have exactly the same problem with a 27" late 2015 iMac.

 

[Longplays]

I really wish Apple would release a larger iMac M2 2-4 months from now so for those of us using unsupported iMac 27" can replace it to an Apple Silicon.

 

[lancedragons]

Bit of a thread necro, but did you ever get to the bottom of this? I have exactly the same problem with a 27" late 2015 iMac.

I did an SSD upgrade with an iFixit bundle on my Late 2014 iMac, and also had issues with the fan. I tried spraying it with compressed air, but when that didn't work, I bought a 3rd party fan off of Amazon, which worked

 

[Street28]

I tried two other fans and had the same result. I ended up checking the voltage on the header on the board and it was only about 8V whereas it should be 12. The fan was badly choked with dust so I'm assuming it's stressed something on the board and blown a component somewhere.

 

[kk123]

I never encountered the fan problem but I did get a new fan from Amazon that's working okay.
Odyson - Cooling Fan Replacement for Apple iMac 27" A1419 (Late 2012-Mid 2017)
just in case you want to try another fan.