iMac Intel 27". EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz), EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz)

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How to run AHT with broken display on iMac?

Hi everyone,

I recently did a bunch of system updates (I was long overdue) and my internal display stopped working. On start up, the grey screen appears and then goes to black before the full operating system is loaded. I've connected an external monitor which seems to be working without issue. Every now and then when I wake up my computer, the internal display works for up to a minute then goes black again.

I've tried a couple basic troubleshooting things that haven't seemed to help:

1. Turning off the computer, unplugging and leaving for a couple hours.

2. Resetting NVRAM

3. I've tried booting up with RecoveryHD, but the screen goes black before I can make a selection.

4. I've tried running AHT but I'm either doing it wrong (holding "D" on startup) or I'm not seeing it as my external display only starts working once the OS has booted up.

I'm wondering if this is a hardware issue or a software issue or even how I can find out. Does anyone know how I can run AHT when the internal display goes black before I can do anything?

A couple years ago I upgraded to 32gb of RAM which isn't officially supported but haven't had any issues with the computer since I made the upgrade.

I'm on a 27" iMac 2009. MC507LL/A

Any and all help would be greatly appreciated on how I can resolve this issue or at the very least know the cause of the issue. Please let me know if there are any additional details that might be useful in resolving the issue.

Thanks very much!

Answered! View the answer I have this problem too

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Your GPU is failing, works till it heats up. Needs a hot air rework station reflow. If I am correct the fourth diagnostic LED will go out when it fails.

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Gotcha-- is this likely the case even though my external display is working without issue?

What is a "hot air rework station reflow?"

Thanks for the super quick response!

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If the external is working, it's not the GPU. Go for the LED driver board: iMac Intel 27" EMC 2309 and 2374 LED Driver Board Replacement

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Awesome thanks. Any idea of how much this sort of replacement would cost me (or where I can find the part # or a supplier of the parts)?

Adding one additional note that I just noticed-- if I wake the computer with my screen brightness turned to the lowest setting, the display seems to function without issue. As soon as I turn it up enough it goes black. Would this confirm it's the LED driver board?

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Here's how to replace it: iMac Intel 27" EMC 2309 and 2374 LED Driver Board Replacement

I'll look for the part in a minute but tells see first if you can repair the board:

But first I'd like you to go over the board with a soldering iron. You're looking for a "cold solder joint". Here's a video on repairing them: https://www.youtube.com/watch?v=5dG0fwE7...

Looks like it is also called the backlight board, Apple part #661-5311 found here: http://www.powerbookmedic.com/Intel-iMac...

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tsekhman will be eternally grateful.
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