iMac 2009 i7 CPU fan on 100% but reading 0rpm

Discussion in 'iMac' started by Uncle Pinny, Nov 27, 2011.

  1. Uncle Pinny macrumors regular

    Uncle Pinny

    Jul 15, 2008
    Any pointers here?

    I took apart my 2009 i7 iMac (out of warranty) to replace the right hand speaker which was broken. Was easy enough. Speaker now works.

    However the CPU fan seems to be running 100% continuously form the moment of boot up. My iStat readings list it as running at 0rpm although that's clearly not the case. All other fans are working fine.

    Temperatures are normal.

    I know there's issues people have had when adding SSD's but all I've done is take apart and put back together. I didn't even touch the CPU fan. All connectors are back in place including the HD temperature cable. Hardware test gives me the error:


    which leads me to think that it's something to do with the HD temp. Interestingly if I install Fan Control it tells me the Internal HD temp is zero. Yet iStat reads the temperature correctly at around 29 degrees.


    - looks like all temps are being read correctly.
    - CPU fan is running 100% yet reading 0rpm
    - have reset NVRAM/PRAM and SMC etc

    Any suggestions? Like I said everything is stock and all connectors connected.
  2. jenseng macrumors newbie

    Jan 30, 2012
    You ever find a fix for this? I'm experiencing the same thing (different back story though)... I added an SSD, and the CPU fan started going at 100% all the time, but it registers 0 rpm. All temperature readings are normal.

    I have since fully reverted the SSD upgrade, but the crazy CPU fan behavior persists.
  3. Uncle Pinny thread starter macrumors regular

    Uncle Pinny

    Jul 15, 2008
    Wow. A reply. No. Never worked out what the problem is. Really weird. I just sent it off for repair yesterday having bought a new machine to ride me over. Will see what the diagnosis is then get back to you. Such a strange issue and it sounds like you're having exactl the same problem
  4. jenseng macrumors newbie

    Jan 30, 2012
    Yeah I'm curious what it turns out to be. I'm dropping mine off for repair later today. Hopefully it's just the fan that needs replacing (e.g. rpm sensor may have crapped out), though I suppose it could be a problem with the logic board :-/
  5. GGJstudios macrumors Westmere


    May 16, 2008
    How do you know the fan is at 100% if it shows 0 rpm? I'm sure it's spinning faster than it should, but without accurate rpm readings, there's no way to tell exactly how fast. Try resetting the SMC. PRAM/NVRAM has nothing to do with fan issues. Resetting it will not help.

    If resetting the SMC doesn't help, my guess is that you disconnected a sensor while installing the SSD.
  6. jenseng macrumors newbie

    Jan 30, 2012
    Valid point, but given that it sounded like a jet engine (whereas normally it was whisper quiet), I have to assume somewhere near 100% ;). Apple tech seemed to think the SMC was just maxing out the fan to be safe, since it couldn't get any speed readings from it.

    Did that three times, but no dice. Also fully disassembled/reassembled it twice after the fan started acting up to ensure I hadn't forgotten to plug something in or had bent a pin.

    Apple techs initially thought it might be an issue with the fan, and replaced it. When that didn't fix it, they ran a more thorough hardware diagnostic, and the logic board had to be replaced. My best guess is some static discharge fried something on the SMC when I first installed the SSD. You can never be too careful I guess :(

    So I'm out about $600, but I was able to put the SSD in after I got it back, and everything is working great now.
  7. GGJstudios macrumors Westmere


    May 16, 2008
    Glad you got it worked out, even though it was an expensive fix.
  8. jenseng macrumors newbie

    Jan 30, 2012
    Thanks, yeah, I think I learned two important lessons here: 1. be really really really really careful and 2. buy AppleCare just in case ... I was only one month out of warranty, making this that much more painful
  9. Uncle Pinny thread starter macrumors regular

    Uncle Pinny

    Jul 15, 2008
    I'm the OP.

    Fan running 100% - 0rpm read - all temps normal and being read correctly. Every sort of reset tried as well as opening up and re-inserting everything and double/triple checking.

    The diagnosis for my iMac should be back this week from the repair company. Will post what I find. Tried resetting all the SMC bits etc and no joy. I suspect my issue is like yours jenseng, something faulty on the logic board caused by me removing it. Odd it's the very same problem as you. Doesn't sound like you even removed the logic board.

    I sent my iMac to a place that specialises in repairing logic boards rather than back to Apple who would most likely tell me it's screwed and offer an expensive logic board replacement. This place has told me they do lots of small logic board repairs so fingers crossed.

    My lesson learnt so far is don't to more than the absolute minimum when opening up iMacs - they're clearly very sensitive.

    Will post an update when I get the diagnosis back.
  10. rdupuy macrumors newbie

    Jul 27, 2010
    iMac CPU fan

    I just wanted to share my own story - with a semi-happy ending.

    I bought an iMac off e-bay an iMac 11,3 - 27" i7 2.93Ghz with 256GB SSD drive and 16GB of ram.

    Well, I'm not sure what happened, if it was poorly described or damaged in shipping, but it had a huge dent in it and a broken superdrive.

    I was upset and I wanted my money back, and I mentioned to the seller that it would take me at least $500 to fix this mess. He asked me if I wanted to get my money back or $500 partial refund - my choice.

    Well, I took the partial refund, because I figured I might be able to fix it for that cost.

    So, I bought a new iMac case for $99 from somewhere and a new superdrive for $50 - and set off to repair the thing.

    WOW, first let me just state - WOW was that a lot of screws, small parts, wires, and not very, very incomplete take-apart guides that left out just tons of screws in their pictures.

    After getting very frustrated and spending about 4 hours - I finally had the iMac back together in a new case.

    Turn it on - no hard drive. Fan running full blast.

    I was so frustrated I waited a week. Next week, take it apart again. 1st issue fixed, I had left the backside hard drive data cable unplugged.

    Turn it on - it boots. Fan running full blast. DVD drive takes a disk in - but won't spit it out.


    Wait another week.

    I read your thread and several others and realized - I have no idea why the CPU fan reports 0 rpm and runs full blast. I decide to go the 'resistor' route.

    I reset the DVD drive and make sure its aligned. I put a 100 ohm resistor on the CPU fan. turn it out. Fan spinning, but not very loud at all, nice.

    Yes, I know some people are aghast right now. My CPU fan runs at the same speed all the time, but instead of sounding like a wind tunnel, it's a very gentle hum, that frankly I can't even hear most of the time.

    CPU temps - normal. I know I can't blast the CPU fan if I ever needed it, I'd have to just power down the machine instead.

    But I have to say at this point I'm just happy, I have a machine that works - the DVD drive ejects DVDs now, the fan isn't too noisy, the dent is fixed - and I more or less got a bargain. I might not try that again, but it worked out OK.
  11. rdupuy macrumors newbie

    Jul 27, 2010
    one other thing

    Oh let me add one other thing - the thought has occurred to me too, that in a pinch you can blow air through the air intake.

    What I'm talking about is if someone, by any weird chance is reading this thread and has a non-working fan, but are loathe to take the machine apart.

    The intake for the cpu fan - in a pinch I imagine you could force air into it, and then air is going to flow through the fan area and eventually over the cpu heatsink - which is better than trying to passively cool the machine anyway.

    I used to have a B&W G3 tower, that had a G4/550mhz 3rd party accelerator - it never wanted to run at full speed without over heating, but I'd open the case, and run air into the case with a regular house fan - and that would work, that was my setup for over a year.

    Eh - strictly for the adventurous and super cheap, of course.
  12. Uncle Pinny thread starter macrumors regular

    Uncle Pinny

    Jul 15, 2008
    Bit late with the reply but it may help someone in the future. I'm the OP. The faults found on the iMac by the technician were:

    - damage to the LCD backlight sync cable – this very likely damaged the logic board causing the current issue with CPU fan and Bluetooth.

    - damaged component on CPU fan circuit on the logic board

    - faulty component on the Bluetooth circuit also on the logic board which is preventing the Bluetooth module from being detected.

    So in short - by opening up the iMac and removing the logic board I succeeded in damaging it in 3 ways. They are more sensitive than I expected and I would think twice about attempting anything inside them again bar maybe replacing the HD/OD etc.

    Cost me a pretty penny to repair as well. Kids be warned!
  13. Bacon01, Sep 3, 2012
    Last edited: Sep 3, 2012

    Bacon01 macrumors newbie

    Sep 3, 2012
    What wire

    What wire did you connect the resistor too? I'm not sure which on to pic.
  14. nouxtywe macrumors newbie

    Jan 6, 2014
    Hi guys, can you give us a little more description in how you added a 100 ohm resistance to the fan?
    I'm really curious and I have the exact same problem of yours.
    So if I'm courageous enough to open my iMac and get to the fan (which will take me at least 1 or 2 hours) I'd like to have your technique in mind before that :)

    If you can give us some tips about that it would be awesome... thanks a lot :)
  15. nouxtywe macrumors newbie

    Jan 6, 2014
    on this picture, could you please telle me which number is what?
    Thanks for your help :)
  16. nouxtywe macrumors newbie

    Jan 6, 2014
    just did it, everything went fine... I'll upload the youtube video soon just in case someone needs help on this.
    Bye all and thanks for the help


    here is the youtube video:
  17. serdiez macrumors newbie

    Jun 20, 2014
    100 ohm 22w resistor

    Please confirm 100 ohm 22w resistor,Is this correct? Do not be 100 ohm 2w resistor?

  18. MichaelJohnston macrumors member


    Jul 18, 2007
    San Luis Obispo
    If you have a CPU fan which reads 0 rpm but seems to be going full blast, there's likely a problem with the circuitry on the logic board. What a few people have found over on iFixit is that there's a 47KΩ resistor which is missing near the power button connector. If you have one missing from there, all you need is a new resistor and your Mac will function CORRECTLY because it knows the fan speed. Link:
  19. datanutz, Oct 5, 2015
    Last edited: Oct 5, 2015

    datanutz macrumors newbie

    Oct 5, 2015
    I wanted to share my experience.
    I bought an early 2009 imac core2duo with the updated ddr3 memory support so I could get 8gig max. I go it on ebay for $200 without a HDD or Memory. Once I put in fresh ones, I had the dreaded jet engine noise. I scoured the threads to find the answer and tried replacing all the sensor cables which did nothing. I tried the SMC reset, pram reset, nothing.... Tried SMCfancontrol and two others, nothing.... The CPU fan is spinning at 6000RPM and the temp is 76degrees. I wanted to try the 100ohm resistor, but I would like to know if a different resister would allow for a faster fan speed if required and what resistor would you recommend?
  20. datanutz macrumors newbie

    Oct 5, 2015
    Thanks for the story, I too bought an iMac off ebay. It is an early 2009 core 2 duo. It is the upgraded DDR3 model so it will support 8gig. I got it for $200 marked as "not working". It was missing a drive and memory which I didn't mind, but the guy selling it said it worked and seemed to know what he was talking about. It did work after a new drive and memory, but it had the old jet engine noise. I had a late 2007 model that was malfunctioning, so after reading all the posts about fan noise, I yanked the HDD heat sensor from the older machine and added it to the 2009. Nothing.... The only thing now was, instead of waking from sleep properly, the screen would blink on for a moment and when you started typing or clicking on something, it would black out again. If you waited long enough, it would eventually come back on, but due to the impatience in my house, it usually was quicker to force reboot. I solved this problem by just setting the energy saver to only shut the screen off and listen to the fan forever. After tearing it down to fix a broken pin on the lcd temp sensor that broke when taking it apart so much, I discovered that it still made no difference and the CPU fan was the culprit. I did the whole SMCfancontrol app and 2 other apps that claimed to adjust the fan, but nothing worked. I could see the temp and fan speed, but it would never drop below 6000rpm. The CPU temp was a cool 76 degrees, so I'm going to try the resistor method. The only question I have is, would different ohm resistors allow the fan to spin faster if required? If so, can you give me the right ones for the right speed?

  21. Renovation macrumors newbie

    Aug 13, 2017
    Some years later, I replaced a hard drive in a mid-2007 iMac with an SSD and got the same symptoms: fans running at 100%, sensors reporting 0 rpm. However, in my case, the culprit was the CPU Fan connector being loose. Reconnected it, problem vanished. Replaced the CPU a few days later, still no problem.

    That can cause the problem, too.

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