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Repair guides and support for 27" Intel iMacs by Apple, introduced October 2009.

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Late 2015 iMac Black Screen with Chime but no #3 LED

I recently removed my iMac late 2015 i5 27” screen to upgrade the hard drive. (Something I’ve done successfully before). Upon re-assembly, the machine would not boot. Got a chime but then a full black screen.

I removed the screen again, checked all of the connections, and they seem good. I get LED #1+#2 lit up on the diagnostic, but no #3. I’ve read that this means the the video card and logic board are not communicating.

I’ve tried resetting the PRAM and the SMC with no luck.

  • Questions:
    • Are there any further suggestions about what to try to get it running again?
    • Is there any value in pulling the whole thing apart to replace the PRAM battery? I read one place this could be a fix, but it seems like a lot of work if it’s a dead end.

Thanks for any help.

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Great question and self-searching diagnostics. If you haven't considered it already, try disconnecting the reconnecting the video card as you may have damaged (or disconnected) some cables even slightly with a misalignment. (it happens to all of us).

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Thank you for the suggestion. In researching the disassembly of the iMac, I can't quite see where I would do this re-connection of the video card. It seems that all of the GPU components are soldered onto the logicboard. (see step 8 here)

iMac Intel 27" Retina 5K Display Teardown

Do you have any further insights about how I would acomplish this disconnect/reconnect of the Video card?

Thank you!

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1 Answer

@ericsoup So a quick search on this topic revealed this has been answered before. I recommend you do a search in the future to help you solve your issue faster. Hope it helps as it’s from a great source @danj

iMac won`t power on. Power button pressed = LED # 2 lights up briefly

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@dunc Thank you for the link. I had read through that already and had not found an answer to my question about whether replacing the PRAM battery might fix my problem. This section was of much interest to me:

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LED 1 + LED 2 + LED 3 = Power is available, the system is turned on, and the GPU was found.

If the third LED is not visible after the system is turned on:

Verify whether the boot chime is present and fans are running when turned on (reset the SMC and PRAM and verify backup battery voltage for proper startup).

I

f the POST boot chime is not heard, go to the No Startup symptom flow.

If the POST boot chime is heard, go to the No Video symptom flow.

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I have searched around the internet and cannot find the "No Video symptom flow". This looks to me to maybe be from some sort of Apple service manual or something. If anyone knows more about where I can find the "No Video symptom flow", I'd love to take a look. Thanks.

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@ericsoup - FYI it was @dunc who aimed you to the link

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Oops. Sorry about that.

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Eric Bryant will be eternally grateful.
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