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iMac 2011 shuts down 10 seconds after startup

T

Tom.Tom

macrumors member
Original poster
Jul 26, 2020
85
32
Hello,

Recently a friend on mine brought me his 2011 iMac for diagnostics. It boots up, starts loading the system and after 10 seconds of booting the system it shuts down.

I removed the hdd and booted it with option/alt pressed into boot selector. Same result - so it's not the osx issue. Removed the LCD, disconnected everything from the board except one ram module and one speaker - same result.
Then I removed the gpu and this time it did not shut down. After 10 seconds the fans started too spin at full speed - standard when one of the temp sensors is not connected. Put different gpu - same shut down after 10 seconds - just when the fans should start to spin up to the full speed. No such problem with different motherboard and same other components from the same iMac.
Examined the original motherboard for any knocked off components around gpu slot - nothing suspicious.
Tried smc and pram reset - didn't help.

Anybody had similar issue? Any suggestions?
 
uhm...something about psu part? tried a different psu? Maybe it can't handle the full charge of the imac?
 
If you're able to boot off the original hard drive using the new motherboard, and it doesn't shut down, are you able to access the console logs and see if there's any messages that might give you a clue as to why it shut down with the old motherboard?
 
Well, I do not suspect the psu, as it works fine with different mobo. Although I can do the test with another one.

I thought maybe the gpu temp sensor or the circuit responsible for reading from it may be faulty, but I tried with the gpu temp sensor disconnected and it also shuts down.

The problem with the logs is that it shuts down too soon to get any... Is there any other way to find out the reason of shutdown?

It's interesting that it happens at the same time where the fans starts spinning at max when there is no gpu or there is another mobo. I guess the smc is responsible for that, right? Is there any way to somehow log/debug the smc logic? Maybe I should check the voltage levels coming to smc? What are the possible causes of such shut down - a false overheating signal?
 
I would suspect a failed GPU, the most popular errors in those iMacs 2011.
Try booting without the GPU (or LCD) and check on the IP address management page of your router. If you iMac 2011 shows its IP there, then you can conclude the failing GPU.
 
@Nguyen Duc Hieu I replaced many gpus in these iMacs in the past - it's not the GPU as I wrote in the first post - it shuts down with different GPU as well. Basically whenever there is any GPU in the MXM slot the mobo shuts down after 10-12 seconds.
Before it shuts down, there is the video on the LCD and the systems starts booting.

I have no clue how to debug this - looks like maybe SMC receiving some signal telling it to shout down the system. Is it possible to find this out somehow?
 
Tom.Tom said:
@Nguyen Duc Hieu I replaced many gpus in these iMacs in the past - it's not the GPU as I wrote in the first post - it shuts down with different GPU as well. Basically whenever there is any GPU in the MXM slot the mobo shuts down after 10-12 seconds.
Before it shuts down, there is the video on the LCD and the systems starts booting.

I have no clue how to debug this - looks like maybe SMC receiving some signal telling it to shout down the system. Is it possible to find this out somehow?
Click to expand...

From your description, the iMac is booting well without the GPU installed.
The shutdown only occurs when there is the GPU in the slot.
So I suspect the GPU.
If the GPU is good, then the MXM slot maybe the issue.
You have already tried NPRAM reset and SMC reset without any good results.
 
Yeah, PRAM and SMC resets didn't help.

I'll try to clean the slot with compressed air, and I'm going to try to boot it up with plugging in the powercord while having power button pressed to see if it shuts down as well or keep working with fans at max speed.
 
Ok, so I did the following test: pressed and held the power button and unplugged the power cord while still holding the power button pressed. Kept the button pressed for couple more seconds, then released it. Then plugged back the power cord. When pressed the power button to turn on the iMac, the fans immediately started spinning at max revs. No shutdown. Was able to run the AHT and it reported no problems. However I do not think this test is relevant in that "high fans revs" mode, because I had the DVD temp sensor disconnected, which should have been reported by the test.

I'm not sure what is this mode - maybe bypassing sensors checking by the SMC...? The iMac runs fine in that mode, with fans spinning at full speed all the time, until I unplug the power cord. When I plug it back and start the computer again, the fans work at normal speed, and iMac shuts down after 10 seconds again...

Anybody knows what is this "high fans speed" mode, and what does it mean that this iMac does not shut down while in this mode, but it does in a normal mode?

Any other clues than trashing the mobo ?
 
@Nguyen Duc Hieu it's not PSU nor GPU. All components work fine with other motherboard. This is something to do with the motherboard - either some sensor or the SMC itself. I just need more information about how to debug this and why it works in that different mode.

The steps to enter this mode are to press and hold the power button and unplug the power cord while still keeping the power button and release it only after several seconds. Then plug the cord back and use the power button to start the iMac. The fans will start spinning at full speed.

Here I found the exact same problem described (except I have no errors reported by AHT), but also no real solution: https://www.ifixit.com/Answers/View/382922/2011+iMac+shuts+down+few+seconds+after+power+on
 
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