24" iMac 2.8 cpu fan full blast then shut down

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This is what happens every time I restart my iMac: The CPU fan will start ramping up speed slowly until it seems to be maxed out around 4000 rpm (noisy). After running full speed for anywhere from 5 minutes to 30 minutes, the computer screen goes black. It appears to shut itself off in an attempt to not self destruct. But it's not completely shut down. It's almost like it goes into sleep mode. You have to press and hold the power button to shut down completely and then start over. I took it to apple 2 hours away and they ran a little diagnostic and told me what I already know and then brought to my attention that they WILL NOT work on anything over 6 years old. Begging and pleading didn't help. I was willing to spend a significant amount of dollars but no way, no how. I have a local place that says they work on Apples but I am pretty sure they've worked on 1 or 2 and didn't give me much confidence. They essentially told me they could try a power supply and see what happens. I decided to try that myself. Just replaced the power supply and it did not fix the problem. I can't find this exact issue anywhere on forums. Has anyone else experienced something like this? Would you recommend moving on to replacing the logic board? I'd hate to have $200 in the power supply plus $500-600 in a logic board and still have the issue. I would love to keep this thing going, it has worked so flawlessly otherwise. Please, any advice much appreciated.

Thank you,

Tyler

24" iMac 2.8 Dual Core Extreme, Mac OS X (10.5.6)
 

chscag

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This could be caused by a bad thermal sensor on the logic board. The iMac until the 2012 model had three fans and thermal sensors. I don't know for sure if that's what's causing the CPU fan to go full speed but it's suspect because that same CPU thermal sensor will also cause the machine to shut down so as to prevent damage to the CPU.

Go to the ifixit web site and look up your model iMac and see if you can locate the thermal sensors. It may be possible to change them out. www.ifixit.com

There is also a special iMac fan controlling utility that may help: LINK There are other apps that can do the same thing but some are not free.
 
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This could be caused by a bad thermal sensor on the logic board. The iMac until the 2012 model had three fans and thermal sensors. I don't know for sure if that's what's causing the CPU fan to go full speed but it's suspect because that same CPU thermal sensor will also cause the machine to shut down so as to prevent damage to the CPU.

Go to the ifixit web site and look up your model iMac and see if you can locate the thermal sensors. It may be possible to change them out. www.ifixit.com

There is also a special iMac fan controlling utility that may help: LINK There are other apps that can do the same thing but some are not free.

Wow, thanks for the quick and helpful information! I will try these suggestions and report back.
 
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This could be caused by a bad thermal sensor on the logic board. The iMac until the 2012 model had three fans and thermal sensors. I don't know for sure if that's what's causing the CPU fan to go full speed but it's suspect because that same CPU thermal sensor will also cause the machine to shut down so as to prevent damage to the CPU.

Go to the ifixit web site and look up your model iMac and see if you can locate the thermal sensors. It may be possible to change them out. www.ifixit.com

There is also a special iMac fan controlling utility that may help: LINK There are other apps that can do the same thing but some are not free.

Well I tried ifixit.com, awesome website, but, I can't find any guides on thermal sensors. I agree that that could be the problem. It makes sense. I feel like a sensor is bad, especially since the temperature inside the computer is well within acceptable (30-45C). Note: this happens every single time, without opening any applications.
 

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That's too bad about the ifixit site not having a guide to change out the thermal sensor. I ran across a link in Europe that you may be interested in. He's selling a thermal sensor board for the 24" iMac. Take a look at the site as he also has some compatibility info included. LINK

You might also want to try that free fan controlling program I linked in my previous reply. Not sure it will help but worth a try.
 

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Well I tried ifixit.com, awesome website, but, I can't find any guides on thermal sensors. I agree that that could be the problem. It makes sense. I feel like a sensor is bad, especially since the temperature inside the computer is well within acceptable (30-45C). Note: this happens every single time, without opening any applications.

I don't think that you're going to find a guide specifically written for temperature sensor replacement. Many of the ifixit guides are mostly written for the "major" parts of the computer…and parts that frequently need replacing.

As an example the procedure I linked below for the hard drive replacement...mentions the LCD temp sensor and the hard drive sensor. So you basically have to look at a procedure like this…and "glean" what you can regarding temp. sensors.:)

https://www.ifixit.com/Guide/iMac+Intel+24-Inch+EMC+2134+and+2211+Hard+Drive+Replacement/8968

- Nick
 
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Thanks chscag. Will certainly try the fan program when I get home. Right on pigoo3. "Glean". Nice, I'm going to use that from now on. :)

So, I'm curious if there are cpu thermal sensors and logic board thermal sensors, or are these one in the same? (I'm capable of tinkering, but a novice with computer service) If so, would a pretty sure fire way to fix my problem just be to replace the logic board? Best price I'm seeing is about $400 and based on an old G5 guide at ifixit.com, logic board replacement is a bit intensive.
 

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"Glean". Nice, I'm going to use that from now on. :)

Hey…how often to we get to use the word "glean" in everyday life…and not look like a nerd/geek!;) Ain't going to pick up too many chicks/birds/sheila's at the pub this way!;);)lol

So, I'm curious if there are cpu thermal sensors and logic board thermal sensors, or are these one in the same?

They tend to be more integrated…not something that is user replaceable.

Best price I'm seeing is about $400 and based on an old G5 guide at ifixit.com, logic board replacement is a bit intensive.

In many cases…replacing a logic board on an older computer is not worth it. Too large of a proportion of the value of the computer. I find this breakpoint (to replace or not replace)…is usually around the 3-4 year point of a computer. But if someone can find a good working used logic board for the right price…and can do the install themselves…this always opens more possibilities.:)

As far as this temp sensor discussion. It could be a failing temp sensor, or a temp sensor that is not mounted in the correct place (came loose), a temp sensor that is not getting a correct reading, or a temp sensor that got disconnected some how.

- Nick
 
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I'd also highly recommend the particular Fan Control that chscag mentioned and provided a ink to, and I've used it for years on our mid-late 2007 24" iMac.

As least it will also provide the temps and fan speeds to let you know what's happening inside your iMac.

I would have thought that replacing a power supply might have been overkill and excessive, but seems typical for most Windows PC techs to do.
 
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Ok, I don't want to jump the gun here but holy freaking ****, the second the fan control program finished installing, my fan slowed to 1400 rpm and is holding! I was nervous and working fast because the moment the computer is finished booting up, the fan speed was consistently ramping up beyond 4000 rpm and then inevitable shut down usually within about 5 minutes. I have had it up and running now for about 20 minutes. I was skeptical because I had already installed a fan controller called smcFanControl. But it only had values for minimum fan speed, not maximum. This Macs Fan Control is proving to be a life saver to my iMac! I can't thank you enough for recommending. The strange thing is I didn't set a maximum fan speed, but the second it was installed, it brought the cpu fan speed back to the normal 1400 rpm. This turned into a 30 second fix. Free too! (definitely have to donate if this keeps on chugging along) Thanks chscag for bringing it to my attention and pm-r for reiterating it.
 
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I'm glad to hear that Fan Control is working and I'd suggest you get rid of all traces of the smcFanControl if you haven't done so already.

PS: If you ever get rid of the Fan Control, or even if you have any trouble with it, just do a SMC reset to get things back to a default state. But Iv'e never had any problem with it on our older iMac.
 
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Well, I hate to say it but I did jump the gun. The computer just stayed on a little longer than usual by chance. Lasted 35 minutes and then the screen went black. Unresponsive like before and had to restart. Fortunately, this Macs Fan Control program has helped me further pinpoint the issue. The GPU temp is going thru the roof. The Celsius temperature starts climbing the moment the computer is booted up and maxes around 128. Thus, why the fan would max out trying to cool it off. Some research tells me that GPU overheating is a common issue due to many things. Since the temperature starts out fairly normal and increases consistently, I feel the temp sensors are doing their job (unless they're somehow incorrectly multiplying their readings). I am going to tear back into it tonight and just make sure there isn't some simple reason why it's not getting enough circulation but my guess is the GPU is just shot. I will report back the findings.
 
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See if you can test that the fans are actually running when you look into its guts.

The fan controller app may be showing the speed but that may just be due to the voltage that its sensing that's being applied to the fans - no guarantee that the fan(s) are actually running and doing what they're supposed to do. :D
 
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Just for a reality check, have you tried booting up disabling extensions (shift key) or from an install disk or something? Might be good to make sure there isn't a misbehaving extension wreaking havoc with the GPU.
 

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I have a late 2007 iMac with very similar specs. The Video CARD IS a separate card that can be changed. If for sure the GPU is shot, it might be able to be changed on your iMac. Give me the exact specs and model identifier and I will check on it.
 

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...but my guess is the GPU is just shot.

I think that if the GPU was shot…the computer wouldn't work (you wouldn't see anything on the display or it would look "weird" in some way). Sounds like you have an overheating problem.

- Fan not working properly
- computer dirty inside
- clogged cool air pathways blocked
- plugged up heat sinks
- temp sensor not working correctly
- etc.

Earlier in the thread you mentioned something about using a Fan app to slow the fan rpms down. In an overheating situation…this is exactly the wrong thing to do!

Yes I know…we were talking about a bad temp sensor at the time. But the high gpu temps most likely were there all the time. By forcibly reducing the fan temps (against what the computer wanted to do by design…higher fan rpms)…you seriously could have damaged the computer.

- Nick
 
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I'd agree Nick with your thoughts of the GPU being shot.

Also, AFAIK, no Fan Control app will allow any temp settings to be set lower than the Mac's OS X default settings, but I'm not sure that's true with smcFanControl, but regardless, an SMC reset would have reset any settings it might have made.

I'll stick to my thought that at least of of the fans just isn't running. Maybe dead or partially disconnected somehow. And I don't know how else to check them other than with a visual inspection. ;)

And I don't know if Apple Hardware Test would be of any help.
 

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An iMac (up until the 2012 models) has three fans: CPU/GPU, Hard Drive, and Optical Drive. If any fan is inoperative, overheating will occur. But the fan for you to check is the one mounted close to the large heat sink for CPU/GPU. As member pm-r mentioned, it's best to open the machine up and observe it. Instructions and tools are available from the ifixit web site. www.ifixit.com
 
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Here's an unlikely possibility: airflow to the CPU has been blocked by dust or dirt. Even better, you could be experiencing a repeat of the very first computer problem, discovered by Grace Mary Hopper in 1947 (see First Computer Bug) A moth had gotten caught in a relay. Subsequently, any operational problem with a computer was called a bug.
Check to see if there isn't a moth on top of your CPU.
 
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No worries with a moth or bug in an iMac, as all contacts and relays are buried in the chips, and the CPU would cook any very well if they landed on it or it's cooling fins. **** :p :Smirk:
 

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