I have been trying to restore an old 27" iMac from 2011. I've done a few of these before but this one has an issue I just can't figure out.
Essentially, it restarts by itself, about 50% of the time you use it. Sometimes it works perfectly, but sometimes, mostly halfway through the apple logo boot progress bar, it'll just power off and then restart by itself.
I have tested (from donor working iMacs I have):
New GPU
New PSU
New RAM
Nothing on the logic board looks burnt out or corroded. None of the plugs or cables are unplugged or damaged in any way. When it does power on, everything works fine and the temperatures don't go crazy. I have re-pasted the CPU and GPU. The Mac also boots into recovery mode just fine, it;s only when it tried to go into the full OS that it has issues.
The first slight clue: on the times that the Mac does boot correctly and doesn't shut itself down, the CPU fan whirs up audibly, and stays on full blast until MacFansControl takes over on startup and restores it to the setting I have it on. The strange bit is that I have put an SSD in this Mac, so this behaviour is what I would be expecting from the HDD fan. However the HDD fan runs normally on Auto and the CPU fan is the one that whirs up to full if you set it to Auto. I have checked and both fans are plugged into their correct sockets on the mobo. Again, the CPU fan only does this if the iMac actually completes the boot as oppose to restarting as described above.
The second slight clue: if you start the iMac up with the screen taken off and all four screen cables unplugged, it works every time. As least as far as the CPU fan whirring up to full and the iMac stays on. I have tried unplugging the screen cables one at a time to see which one might be causing the issue, and it seems that the main data cable is the one that causes it to power off. The screen itself seems to work completely fine. There is a 'bit' of damage on the data cable connector, one of the pins seems like it was slightly bent. But I have bent it back into shape and it seems odd that the screen would work completely fine if this cable was the one causing issues. I wonder if this is instead pointing to the GPU - ie only when the iMac detects a screen data cable plugged in does it actually instruct the GPU to start making pixels, and this causes the shutdown...? The iMac does however work fine if I remove the screen and plug it into an Apple Cinema Display via the displaybort/thunderbolt adapter on the back.
Any help? I'm a bit stumped with this one.
Essentially, it restarts by itself, about 50% of the time you use it. Sometimes it works perfectly, but sometimes, mostly halfway through the apple logo boot progress bar, it'll just power off and then restart by itself.
I have tested (from donor working iMacs I have):
New GPU
New PSU
New RAM
Nothing on the logic board looks burnt out or corroded. None of the plugs or cables are unplugged or damaged in any way. When it does power on, everything works fine and the temperatures don't go crazy. I have re-pasted the CPU and GPU. The Mac also boots into recovery mode just fine, it;s only when it tried to go into the full OS that it has issues.
The first slight clue: on the times that the Mac does boot correctly and doesn't shut itself down, the CPU fan whirs up audibly, and stays on full blast until MacFansControl takes over on startup and restores it to the setting I have it on. The strange bit is that I have put an SSD in this Mac, so this behaviour is what I would be expecting from the HDD fan. However the HDD fan runs normally on Auto and the CPU fan is the one that whirs up to full if you set it to Auto. I have checked and both fans are plugged into their correct sockets on the mobo. Again, the CPU fan only does this if the iMac actually completes the boot as oppose to restarting as described above.
The second slight clue: if you start the iMac up with the screen taken off and all four screen cables unplugged, it works every time. As least as far as the CPU fan whirring up to full and the iMac stays on. I have tried unplugging the screen cables one at a time to see which one might be causing the issue, and it seems that the main data cable is the one that causes it to power off. The screen itself seems to work completely fine. There is a 'bit' of damage on the data cable connector, one of the pins seems like it was slightly bent. But I have bent it back into shape and it seems odd that the screen would work completely fine if this cable was the one causing issues. I wonder if this is instead pointing to the GPU - ie only when the iMac detects a screen data cable plugged in does it actually instruct the GPU to start making pixels, and this causes the shutdown...? The iMac does however work fine if I remove the screen and plug it into an Apple Cinema Display via the displaybort/thunderbolt adapter on the back.
Any help? I'm a bit stumped with this one.
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