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mid 2011 27" won't power up and only 1 diagnostic LED

I

Iwan Reddevil

macrumors newbie
Original poster
Nov 12, 2021
9
1
Warsaw, Poland
Hi,
I come to you with a known problem - after replacing the CPU and GPU at the same time, imac does not start (previously it worked fine). After connecting the power cable, only LED1 lights up on the MB. Pressing the switch starts the fans for 0.5 seconds but iMac doesn't. LED1 is on all the time. There is no blinking or flash of another LED (even for second).
I have already checked the power switch - it is not the cause. I also put in the old CPU - to result.
The next step is to connect to the PSU and MB with only the CPU connected and check if the next LED lights up (maybe the GPU is the cause of the problem?).
Is there any method to determine what may be defective in my case - MB or PSU? In this thread:

forums.macrumors.com

Looking for schematic diagram of 27" 2009-2011 power supply, PA-2311-02A (liteOn)

Hi all, I'm looking for a schematic diagram of the 27" 2009-2011 models power supply, my model number is PA-2311-02A (LiteOn branded). If you happen to have the one of model ADP-310AF, I appreciate this, too. I have repaired several standard switch mode supplies, but Apples is more complex, with...
forums.macrumors.com forums.macrumors.com

there is information that a faulty PSU may cause LED1 to glow. Therefore, before I spend $ on a new power supply or MB, I would like to make sure that it is the cause.

Any help and suggestions are welcome.
 
I had a 2010 iMac with exactly the same symptoms.
The #1 LED lit means that the power supply is supplying "trickle voltage", which the logic board needs before it will try to turn on.
Then, press and release the power button, and the #2 LED lights briefly (maybe even a momentary fan movement), then the #2 LED drops off again. Nothing else happens.
What's the fix? First choice (and the most likely fix at that point) is the power supply.
In this kind of symptom, the power supply is partly failed, and won't support the power needed to POST, and certainly won't turn on any video.
Swap out the power supply.
Yes, there is the possibility of a bad logic board -- but the power supply is more likely, and you should try that first.
 
The first think you should try, is reassemble the original parts, to see whether there was something wrong during your process.
If the iMac was running, proceed the upgrade one by one. NVRAM reset after each change.
First the CPU.
Test run
Then the GPU
 
Nguyen Duc Hieu said:
The first think you should try, is reassemble the original parts, to see whether there was something wrong during your process.
If the iMac was running, proceed the upgrade one by one. NVRAM reset after each change.
First the CPU.
Test run
Then the GPU
Click to expand...
I already started this proces. CPU is original again but no progress. (resetting NVRAM - you mean removing battery from MB?) For GPU I had no more time to do this - disassembled iMac occupies to much space :) But this inset step on my list.

BTW - is it possible to damage PSU by just only removing it?
 
Iwan Reddevil said:
I already started this proces. CPU is original again but no progress. (resetting NVRAM - you mean removing battery from MB?) For GPU I had no more time to do this - disassembled iMac occupies to much space :) But this inset step on my list.

BTW - is it possible to damage PSU by just only removing it?
Click to expand...

I myself experienced this issue (unable to power on) on my system.
The cause is assumed to be a short circuit on the GPU, due to bad re-assembling process and insulation of the components on the CPU vs heatsink.

Reassemble the original parts (CPU/GPU) to verify that your machine is still working.
Then change the CPU.
Confirm that it still working. (with original GPU)
Then change the GPU.
Carefully examine the GPU (MXM card), use Kapton tape wherever you can (without obstructing the heat transmission/dispassion)
Then power it up again (don't need to re-assemble the LCD panel) to see if the fans are spinning, video signal is sent to the expternal monitor.
Then (power it off, unplug the power cord) reassemble the LCD panel.

Can't say anything about the PSU, it's 12 years old already. But if it works with the original parts, then it's not the issue.
 
Silly question: have you reinstalled the RAM? I have forgotten to do this in the past (2011 27”) and it resulted in that exact behaviour.
 
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Small update: returning to 'old' CPU and GPU caused that iMac started normally.

But now I have almost 2nd iMac for parts. Only LED panel, front glass and wifi module and I have 2nd iMac :)
 
Last edited:
Another update: I seems that problem is caused by GPU - GTX880M. I put this card to the two different iMacs (mid 2011) and in both cases situation is the same - only one LED is on and iMac won't start. When I change card to old one - iMac starts normally. When GPU removed - iMac start (two LEDs are on, can hear starting chime, fans spins).

Seller claims that GTX is working (checked it before shipping on probably iMac 2010). Is there any way to check if the GPU is not damaged?
 
Just revived a 27 inch 2011 iMac displaying the same symptoms: 1 led lit, no fans spinning when powering on. It was the faulty original graphics card preventing the POST. Replacing it with a known working MMX-B card solved the problem and the machine starts up.
 
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Boot up your iMac.
Go into your System Information app (it's in your Applications/Utilities folder)
Open that -- the first screen will be Hardware Overview.
and, there you should see -- Boot ROM version.
 
DeltaMac said:
Boot up your iMac.
Go into your System Information app (it's in your Applications/Utilities folder)
Open that -- the first screen will be Hardware Overview.
and, there you should see -- Boot ROM version.
Click to expand...
Thank you for this info - it is obvious for me. I was not precise in my previous question. How can I check bootROM version if GPU card fail to start iMac. What tools should I have to be able check this GPU?
 
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