iMac 27 A1312 Mid 2011 not powering up

I am attempting to fix a lifeless A1312 27" iMac (mid 2011) and would appreciate some help if anyone can provide it.

The iMac does nothing when I attempt to power on, so at first I suspected a power supply. I opened it up and tested the power supply and found it to be supplying the right power.

I looked at the diagnostic LEDs and LED501 was not lit. I unplugged and discharged the power by holding the power button and then plugged back in. LED501 blinks twice, the first time longer than the second, and then goes out and stays out.

Not prepared to believe my tests proved it wasn't the power supply (as all my research suggests that LED501 not staying lit implies the power supply is faulty) I replaced the power supply with a known good, brand new one. Symptoms are the same.

I then unplugged everything from the board except the power supply and tested again, with the same result.

I suspect the logic board is faulty, but I'm confused by the symptoms which don't seem to match anything I can find in the service manuals or online. Does anyone know what's wrong, and whether it's worth trying to replace the logic board?

I suspect the logic board is faulty, but I'm confused by the symptoms which don't seem to match anything I can find in the service manuals or online. Does anyone know what's wrong, and whether it's worth trying to replace the logic board?

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Welcome to the Mac-Forums.

It does appear that the logic board may be at fault. Whether it's worth the time and expense to replace it depends on how much it will cost to replace the board. There are no new boards to be had, only refurbished or pulled from another iMac. You might check eBay to see if one is available.

chscag said:

Welcome to the Mac-Forums.

It does appear that the logic board may be at fault. Whether it's worth the time and expense to replace it depends on how much it will cost to replace the board. There are no new boards to be had, only refurbished or pulled from another iMac. You might check eBay to see if one is available.

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Does it matter if I replace with 661-5948 or 661-5950? Would they both be compatible? I haven't taken mine back out yet to see which it is yet, hoping to avoid having to do that too many times!

HolmPC said:

Does it matter if I replace with 661-5948 or 661-5950? Would they both be compatible? I haven't taken mine back out yet to see which it is yet, hoping to avoid having to do that too many times!

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If those part numbers are from the same generation 27" mid-2011 iMac…then they might be compatible. Apple logic boards have very specific shapes, port locations, and wiring setups. Apple many times likes to change (or improve) things from model release to model release. So a logic board from a 2010 or 2012 may not fit a mid-2011 27" iMac. Especially if there were major design changes.

Most of the time with Apple computers (if doing repair work). Bad parts are replaced with exactly the same good parts. There's not much part interchangeability between different Mac models (especially complex parts like logic boards). With Apple laptops…sometimes keyboards, trackpads, batteries, etc can be swapped between multiple years (but no guarantee). With 27" iMac's…maybe a power supply, maybe a display panel can be swapped between different model releases (if not too far apart in age).

But regarding the two part numbers above that you mentioned. Go to powerbookmedic.com…and check their database. They sell lots of parts for Mac's…include part numbers…and many times you can figure out if a specific part number will fit a specific computer model. There are pictures as well. And this can help to see if the shapes of the parts are even the same.

- Nick

pigoo3 said:

But regarding the two part numbers above that you mentioned. Go to powerbookmedic.com…and check their database. They sell lots of parts for Mac's…include part numbers…and many times you can figure out if a specific part number will fit a specific computer model. There are pictures as well. And this can help to see if the shapes of the parts are even the same.

- Nick

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Thanks Nick. It appears the one ending 48 is the board for the i5 2.7ghz model and the one ending 50 is for the 3.4ghz i7 model. As I don't know whether the processor makes a difference to the board itself, I'd better stick to the one ending 48.

Looks like I can pick one up on EBay for about £160, and there are repairers quoting me about £340 for the repair.

Thanks.

HolmPC said:

Thanks Nick. It appears the one ending 48 is the board for the i5 2.7ghz model and the one ending 50 is for the 3.4ghz i7 model. As I don't know whether the processor makes a difference to the board itself, I'd better stick to the one ending 48.

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If these two logic boards are from the exact same model release (mid-2011 27" iMac in this case)…both should fit physically…and should have the same wiring connections. If you can get either board for the same price…the 3.4ghz one is significantly faster. But since this sort of thing is not done that often…going with an exact replacement is always best. If you do go for the 3.4ghz logic board. Be sure you can return it if it doesn't fit.

HolmPC said:

Looks like I can pick one up on EBay for about £160, and there are repairers quoting me about £340 for the repair.

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Usually replacing logic boards in Apple computers is not always cost-effective. But if the cost numbers work in your favor…go for it!

- Nick