Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor, ID iMac12,2

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iMac has white screen, then reboot again and again.

My iMac 27” 2011 recently has an issue:

  1. After power it on, Apple logo shows up with progress bar loading.
  2. It then becomes white screen for a while.
  3. Then it turns black and reboot.
  4. Repeat step 1, 2, 3.
  5. Try to run Apple hardware diagnostic program, it seems fine.

Any clue what problem this could be and how to resolve it?

Thanks!

Tom

Update (10/10/2018)

Hi Dan,

Here are some pictures I just took.

Thanks,

Tom

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What happens when you try starting your system up using an external boot drive? Or, have you tried using the recovery partition?

Reference: Mac startup key combinations

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I did try that, the white or grey screen just stay there forever, won’t proceed to the next step point. Is this because of bad GPU?

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Can you get your system into Target Disk Mode. We just need to see if the FireWire/Lighting Icon is visible on the screen.

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Hi Dan, can you advise how to do it?

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Just press Option+T during system boot up?

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OK the good news first your logic board is AOK!

Now the bad…

Your GPU board is gone ;-{

The test forces the onboard graphics services within the CPU to run bypassing the dedicated GPU.

Here’s the IFIXIT guide you’ll need to follow Installing iMac Intel 27" EMC 2429 Dual Drive Kit (HDD or SSD) it will get you mostly there. Once you get to Step32 you’ll need to finish pulling the logic board out. Carefully remove the GPU bracket and then the GPU board. Here’s the replacement GPU’s

You’ll need thermal paste as well as thermal pads to replace what you have.

iMac 27" (Mid 2011) Graphics Card Image

Product

iMac 27" (Mid 2011) Graphics Card

$99.99

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Thanks for your advise, Dan, really appreciate!

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Hi Dan, you are right, problem is the GPU, so took it out, got it "cooked" in the oven to reseat pins, now it works again. I also replaced the original hard drive with the 2.5" SSD drive. The problem is that the fan is quite loud after turn it on, I have to use software to control fan speed. It seemed that the loud fan not only from hard drive sensor, but also from CPU (I have to manually control the fan speed for cpu temperature). So, question is, are cpu and hard drive using the same temp sensor in this 2011 model? If the sensor won't detect the replacement hard drive temp, will it affect CPU temp as well?

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Keep in mind cooking the GPU is short lived :-{

You may have damaged the logic board there some small inductors near the sensor cable which you can snap off if you’re not careful. Look at Step27 & Step28 in the guide.

The SSD is missing the needed thermal sensor which your system needs. You’ll need this sensor OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade

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Thanks, I will check it.

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Hi Dan, by checking what I did, the only thing I can think of is the power button connector, step 29, two wires came out from that little socket, I then insert it back, but not sure if I misplaced it (which wire goes to which hole). If I misplaced it, will it affect the sensor system?

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@lovecd - For your fan issue heres the details!

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Now with a better view you can see the small inductor has been pulled off!

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Here you can see where it is just hanging on! Be very careful as you really don’t want to loose it

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Thanks Dan, not sure if I can get it welded back as it is too small to see. Any suggestions?

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You'll need a micro soldering iron and a good magnifier to do this. You may need to just find someone local with the tools and skills as it's not easy!

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Tom will be eternally grateful.
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