13 Replies Latest reply: Jan 14, 2011 6:42 PM by Edward Boghosian
Jez DK Level 1 (5 points)
My iMac suffers from excessive noise from the CPU fan.

The noise usually starts within the first 10 minutes of usage but sometimes it can go noiseless a bit longer.

Activity Monitor shows no heavy processes running to justify the noise (no Spotlight indexing or anything) and the temperatures are as low as can be.

Usually putting the Mac to sleep and waking it up slows the CPU fan down, but it will go nuts at 3900 rpms soon again.

Once the CPU fan has spun up to its max it never spins down again unless the iMac is reset or put to sleep. Even after hours of useless fan spinning has left it freezing cold.

iMac (Early 2008) Intel Core 2 Duo 2.66 GHz, 4 GB RAM

Main OS: Snow Leopard (fully updated)
Boot Camp partition running Windows 7

iStat Pro stats:
Optical Drive: 999 rpm
Hard Drive: 1272 rpm
CPU Fan: 3899 rpm <-- culprit

HD : 50 (degrees celcius)
CPU: 33
GPU: 40

CPU 97-98% idle

I have already tried the following to no avail:
Resetting SMC (Numerous times)
Vacuming the air vents
Reinstalling the system

Also SMCFancontrol won't help as it can't slow down fans only make them go even faster (I definitely don't want that).

There's numerous threads about racing and noisy fans on the boards but none with a definite solution to the problem.

I have been a very happy switcher for more than 2 years now, but with this being the second iMac of mine to show this behavior, I'm starting to losing faith in the iMac cooling fan system.

My first iMac had the exact same problem (Racing CPU Fan) and spent most of its short life in the repairs. Finally after two failed attempts to repair it Apple replaced it with my current model that shows the same annoying behavior.

I'm ready to try creative solutions as this is my last resort and the next step is 'retiring' the iMac. The ideal solution for me would be a way to limit the CPU Fan to the usual 1000 rpms and prevent it from going higher. I am totally willing to risk it.

Any help is much appreciated.

Cheers, Jez.

iMac (Early 2008) Intel Core 2 Duo 2.66 GHz, 4 GB RAM, Mac OS X (10.6.4), Boot Camp Partition with Windows 7, 64 Bit
  • WZZZ Level 6 (12,875 points)
    Sounds like it could be a faulty sensor. I'd bring it in and have it checked out by Apple. I don't think there is a charge for diagnosis at an Apple Store, but call ahead to be sure. Maybe you have Applecare?
  • WZZZ Level 6 (12,875 points)
    Just saw you're in Denmark, so maybe an Apple Store is not an easy option.

    Can you try running the Hardware Test?

  • Jez DK Level 1 (5 points)
    The normal hardware test came up with no problems. I instigated the extensive test before I left the house this morning, so I'll post the result later.

    You're right about Apple Stores not being a possibility. In Denmark repairs and service are left to third party partners and none of them are any good at it. As I mentioned my previous iMac (same model) had the exact same problem and it went to two different repairers who couldn't fix it.
  • Jez DK Level 1 (5 points)
    The extended AHT also gave no errors as result.
  • Jez DK Level 1 (5 points)
    Hooray, I think I found a solution.

    Using terminal commands there actually is a way to force smcFanControl to set a tolerable max rpm for the CPU fan.

    Be aware that this solution should only be used if you know what you're doing and you are absolutely sure the CPU fan on your iMac is indeed out of control.

    Forcing a low max speed on a healthy CPU Fan with fully functioning sensors can seriously damage your Mac. You have been warned.

    The solution:

    Install smcFanControl

    Go to the terminal and use these commands to regain control of your noisy iMac:

    /Applications/smcFanControl.app/Contents/Resources/smc -k F2Mx -w 12c0

    This will set the CPU Fan to max at 1200 rpm: (Oh yes!)

    To revert to standard settings use:

    /Applications/smcFanControl.app/Contents/Resources/smc -k F2Mx -w 3cf0

    This will set CPU Fan to max at the default 3900 rpms

    You might have noticed that the last 4 digits are the fan speed (in hexidecimal)

    Other fan speeds you can try:
    1100rpm = 1130
    1500rpm = 1770
    3900rpm = 3cf0

    Now all I need is a way to make the setting permanent. Anyone know how to run the above terminal command automatically on bootup?
  • Jez DK Level 1 (5 points)
    Well, the SMCFanControl terminal command works, but must be fired off every time the fan kicks in.

    Also it doesn't help you when in Windows mode.

    This seems to be the only (partial) solution to the 20" iMac noisy fan problem out there.

    If you're unfortunate enough to own such a model I wish you good luck.

  • bushikempo Level 1 (0 points)
    Hi, good post, I've just used the terminal command to give me some piece and quite for a few minutes! Did you ever get to the bottom of this? I've got the same issue. SMC/ PRAM reset used to do the trick, but not any more. Been searching for days for a solution! Even opened my mac to give it a good clean, but still issues remain. Ran a hardware test and no issues there. If you have any other further tips that would be great as have come to a dead end. My fan issues seemed to start when I had a new java download, but maybe just coincidence!
  • Keith Nickas Level 1 (25 points)
    I've been having the same issue. Just recently it started to run at 3600rpm and under less than 1% load. I tried the smcFan option with success, however, this isn't a long term solution.

    Does anyone have an idea of the possible cause?

    Attempted solutions: Complete clean of iMac, reinstall of Snow Leopard, Permissions and cache, reset of SMC etc. No change.
  • Keith Nickas Level 1 (25 points)
    Well ... everything I was ready lead to installing a new CPU fan. It seemed like it was the solution, however, just about 10 minutes after the fan started to rev up again! CPU temp hovers around 35C when it revs up.

    The only thing I can think of is that it must be either hardware related (i.e. the sensor) or Mac OS 10.6.5--but I doubt that it is Mac OS as Bootcamp does the same thing.

    Does anyone have a solution or some guidance of what to do next?

  • Keith Nickas Level 1 (25 points)
    I figured it out ... I had to add some CPU compound to the heat sinks, so far so good.
  • James778 Level 1 (0 points)
    I have the same problem on my early 2008 iMac. I tracked the problem down to a malfunctioning power supply temperature sensor. If you get a temperature monitoring program you check if this is your problem as well. "Temperature Monitor" allowed me to check the power supply sensor ("iStat Pro" didn't have that sensor listed). When the fan was operating normally, the power supply temp reading would be normal - less than 40C. When the fan started acting up, I would check the power supply temperature - now it wouldn't show a reading. Apparently the CPU fan is not just dependent on the CPU temperature.

    After searching around, I don't think would be cheap or easy to fix (if anyone fixes it, please post here). This script is a great "temporary" solution. Since none of my temperatures ever go above 50C in my iMac, I figure it is safe.
  • Edward Boghosian Level 4 (1,395 points)
    This may be helpful. I had a problem and Apple sent a technician to my home who changed the HD, HD fan, and the temperature sensor. After he left and while using the computer the HD fan went up to a very high rpm. Nothing would slow it down except putting the computer to sleep or shutting it off and restarting or reawakening it. But, the HD fan would slowly creep up and be as loud as before and very fast.
    This symptom sounds familiar. Took it into the Apple store and was told it needed a sensor cable but when it came back the work order showed the logic board was the only thing changed.
    The original problem was not an overactive HD fan.
    The problem could be due to a faulty temperature sensor or a faulty logic board. Bring it to Apple, if you can, and have it checked out. Hopefully, everyone has the extended warranty if owned for more then a year.
  • Edward Boghosian Level 4 (1,395 points)
    I forgot one thing. Before taking it to Apple, did check the computer using the hardware testing disc that came with the computer and an error code came up. But when I took it in the Genius was not interested in the code. She did a test with some software Apple has and it showed certain items not operating properly.