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Very strange issue with iMac fans

R

retta283

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Original poster
Jun 8, 2018
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I recently found a maxed out 21.5" 2010 iMac for a good price. Got it home, and everything seems to work fine. But for some reason the fans run really loudly upon startup. I put the computer to sleep, and when it woke up, it was silent. Ran for about 30 seconds, then got loud again. Went to sleep and woke up again. Now there is no fan noise at all. I tried watching HD videos, playing games on maxed out settings, and doing stress tests and the fan is very quiet, nearly silent during operation. So I restarted the computer. Did the same thing. Put to sleep and woke up a few times and it was fine.

At this point I decided it would be a good idea to reset the SMC. I did that, and still nothing. This really makes no sense to me, it's silent during heavy operations, and doesn't get burning hot, but the fans rev up to 100% when I start it up.
 
When you hear the fans, open your Activity Monitor, and check for high CPU usage (or even high memory load)
You reset SMC, and you said that did not help.
Did you try an NVRAM reset? That's not the same reset, and can help with system control issues.

Each fan (out of three) can run independently. Did you run an app that will show you which fans are running at high speed?

Have you done an erase/install of the macOS system? If not, you should do that next. You have no idea what the previous owner did (or did not do) to the system. Better to operate from a system that you know has been installed properly.
 
DeltaMac said:
When you hear the fans, open your Activity Monitor, and check for high CPU usage (or even high memory load)
You reset SMC, and you said that did not help.
Did you try an NVRAM reset? That's not the same reset, and can help with system control issues.

Each fan (out of three) can run independently. Did you run an app that will show you which fans are running at high speed?

Have you done an erase/install of the macOS system? If not, you should do that next. You have no idea what the previous owner did (or did not do) to the system. Better to operate from a system that you know has been installed properly.
Click to expand...
I completely wiped the install of OS X, everything is fresh. Can't be a problem with anything related to the OS, as it still does this even when in the option boot menu, which is onboard. I will try next to reset the NVRAM. Didn't know it could be related to any sort of fan control issues.
 
retta283 said:
I completely wiped the install of OS X, everything is fresh. Can't be a problem with anything related to the OS, as it still does this even when in the option boot menu, which is onboard. I will try next to reset the NVRAM. Didn't know it could be related to any sort of fan control issues.
Click to expand...
The hard drives in 2010 iMacs need to have a thermal sensor. If the drive was replaced, the sensor may be missing, leading to the fans running at full speed.
https://eshop.macsales.com/item/OWC/DIDIMACHDD09/
 
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chrfr said:
The hard drives in 2010 iMacs need to have a thermal sensor. If the drive was replaced, the sensor may be missing, leading to the fans running at full speed.
https://eshop.macsales.com/item/OWC/DIDIMACHDD09/
Click to expand...
That would explain it. I'd assume in the short term I could use a fan control program to stop them from spinning up the whole way. I'll be sure to watch my temps. NVRAM did not fix it, so I am almost certain that's the issue.
 
retta283 said:
That would explain it. I'd assume in the short term I could use a fan control program to stop them from spinning up the whole way. I'll be sure to watch my temps. NVRAM did not fix it, so I am almost certain that's the issue.
Click to expand...
If you can run the diagnostics for the computer, it will probably report the missing sensor, if that’s the case.
 
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retta283 said:
That would explain it. I'd assume in the short term I could use a fan control program to stop them from spinning up the whole way. I'll be sure to watch my temps. NVRAM did not fix it, so I am almost certain that's the issue.
Click to expand...

Yep you will want smcfancontrol or similar. I fixed up a similar old Mac and replaced the HDD with an SSD, the thermal sensor was embedded in the HDD so I had to set up the fan controller to manage the same problem. There is no worry using software to manage it.
 
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chrfr said:
If you can run the diagnostics for the computer, it will probably report the missing sensor, if that’s the case.
Click to expand...
Where exactly should I go for this? I'm running Sierra on it right now.
 
I've downloaded smcfancontrol and the software seems to do nothing. I have everything on the lowest rpm and it still is blaring like a jet engine. Temps don't go above 91 F. Am I doing something wrong with this software?
 
What is the fan speed, according to SMCFancontrol?
Run speed to the maximum setting. What RPM does the software report?
Then, change speed to minimum possible. What RPM is reported then?
Which of the three fans is making all the noise (you should be able to hear the difference when you change the settings in SMCFancontrol)?
 
DeltaMac said:
What is the fan speed, according to SMCFancontrol?
Run speed to the maximum setting. What RPM does the software report?
Then, change speed to minimum possible. What RPM is reported then?
Which of the three fans is making all the noise (you should be able to hear the difference when you change the settings in SMCFancontrol)?
Click to expand...
SMC fan control seems to make 0 difference on fan speed. The RPM refuses to change in the menubar even cranked up/down, and there is absolutely no sound difference. I downloaded MacsFanControl instead, and it seems to working good. I'm keeping it at about 2000 RPM on ODD and CPU fan, and it is fairly quiet. Normal operation temp is about 97-100 F, and when doing gaming 100-105. Not sure if these are good temps or not.
[doublepost=1564593496][/doublepost]Actually a follow-up. SMCFan Control shows 100 F temp when using my custom MacsFanControl settings, and MacsFanControl shows only 82 F. Not sure which is true
 
You are running hot. That's not related to a missing sensor on the hard drive.
Your fans are running high, trying to cool your iMac. A missing sensor would NOT drive the temps up, just the fans.
Do an NVRAM reset, restart, then check temps.
Monitor your Activity Monitor to find out if any processes are loading your system and causing processor, etc to heat up.
(Make sure that Activity Monitor is set to show "All Processes", and not just "My Processes"
Watch the CPU window in Activity Monitor. You can sort the processes so high CPU is at the top of the list.
I think you need to open up that iMac, and inspect for clogged cooling vents, etc.
Finally, if it is mostly clean inside, and nothing impeding airflow (and Activity Monitor doesn't show anything helpful), then you may want to re-do the thermal grease on the CPU.
 
DeltaMac said:
You are running hot. That's not related to a missing sensor on the hard drive.
Your fans are running high, trying to cool your iMac. A missing sensor would NOT drive the temps up, just the fans.
Do an NVRAM reset, restart, then check temps.
Monitor your Activity Monitor to find out if any processes are loading your system and causing processor, etc to heat up.
(Make sure that Activity Monitor is set to show "All Processes", and not just "My Processes"
Watch the CPU window in Activity Monitor. You can sort the processes so high CPU is at the top of the list.
I think you need to open up that iMac, and inspect for clogged cooling vents, etc.
Finally, if it is mostly clean inside, and nothing impeding airflow (and Activity Monitor doesn't show anything helpful), then you may want to re-do the thermal grease on the CPU.
Click to expand...
After looking into this, my real temp is between 84-88 F, and it doesn't seem to go up at all. When I let the fan go all the way up, the temps don't change at all. The iMac does not physically feel hot, nor does it shoot out very warm air, and the flow of air can be quite strong when cranked up all the way. There could be some weak paste on the CPU though.
[doublepost=1564613165][/doublepost]31 Celsius average temp for everyone else
 
As a second note, I tried playing taxing games at full resolution and maxed out settings, and the temps had a small increase, about 1 degree F, with only the GPU really going up at all. After an extensive search, I pulled multiple sheets taken from others 2007-2012 iMacs and saw temps very similar if not higher than what I am seeing. I really don't think it's a major heating issue, I will clean it the best I can though and will report if my temps are noticeably lower.
 
Ah, snap! ... I usually notice those "little" details, and I completely missed C and F.
Less than 100 F. is quite cool, and nothing to worry about. I take back my statements in post #15.

Do have still have problems with the fans?
On your iMac, even the hard drive sensor, if it was missing, would not cause ALL 3 fans to go up, just the hard drive fan.
You should be able to determine which fan is really causing your issue by spinning up each fan individually, followed by adjustment to minimum speed for each, noting if any one fan seems louder than the others.

Did you note anything after watching your Activity Monitor?
 
DeltaMac said:
Ah, snap! ... I usually notice those "little" details, and I completely missed C and F.
Less than 100 F. is quite cool, and nothing to worry about. I take back my statements in post #15.

Do have still have problems with the fans?
On your iMac, even the hard drive sensor, if it was missing, would not cause ALL 3 fans to go up, just the hard drive fan.
You should be able to determine which fan is really causing your issue by spinning up each fan individually, followed by adjustment to minimum speed for each, noting if any one fan seems louder than the others.

Did you note anything after watching your Activity Monitor?
Click to expand...
Activity monitor is fine. It seems that the hard drive fan monitor is broken, because my temps are actually excellent, but the hard drive temp is static, it never changes, and it wants to go to max setting when I turn off my custom controls. When using my Macsfancontrol settings, it's very quiet, and it runs cool. Luckily the program is not obtrusive at all, so it looks like I can just leave it be. I will still give it a cleaning, just to be safe. Never a bad idea. Thanks everyone for helping
 
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