I have an iMac that is stuck in a boot cycle

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iMac Intel 27" EMC 2429

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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chrisallanlittle @chrisal

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Gepost: 4 okt. 2017

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I have an iMac that is stuck in a boot cycle

I recently replaced the video card in my 27" iMac after it wouldn't boot to the Apple logo and all you see is a grey screen.

The iMac is now stuck in a boot cycle with just a grey screen and no Apple logo. You just hear a chime and then the Mac keeps rebooting.

I also installed an SSD using iFixit's Dual Hard Drive Kit at the same time that I replaced the the video card. I have gone back to the eBay member that I purchased the replacement video card from and they said that their video cards go through a rigorous testing process and are 100% working.

Is it possible that my logic board faulty?

What is the best way to test the logic board to see if it is faulty. I have removed the SATA cables from the original HDD to see if this was the cause for the boot cycle. I'm used to working on PC's where you can just use a POST test card to check for faults. Is there anything similar for Mac or is this an Apple Genius only thing?

Please note: I have inherited this iMac which appeared to have chocolate milk stains or something on the inside of the case which I have cleaned using electronic cleaning solution.

I have removed the RAM and removed the LCD and powered up the iMac and I can see that I only have two out of four LED's lit on the motherboard. LED501 is on, LED502 is on, LED503 is off and LED504 is off. LED504 is fine as I understand that this indicates the connection with the LCD which is removed at the moment. However, my understanding is that LED503 indicates the connection between the logic board and the video card?

I have used thermal paste etc between the heat sink and the video card - standard stuff right.

What could be wrong? Faulty video card or logic board?

Again, this is meant to be a replacement video card or have I been sold an expensive dud!

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Dan @danj

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Gepost: 5 okt. 2017

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Here is the exact wording from the service manual:

LED #3 Indicates that computer and video card are communicating. This LED will be ON when computer is communicating properly with video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the backup battery (on back of logic board) may need to be reseated, or the video card might be installed incorrectly or needs replacement.
LED #4 Indicates that computer and LCD panel are communicating. This LED will be ON when computer is turned on and video signal is being generated. If LED#4 is ON and there is no image on display, then the LCD panel, the LED backlight board, or the cables between LCD and logic board or backlight board might be installed incorrectly, or need replacement.

So it does look like the GPU card still has a problem. Double check that its seated correctly. If it is still failing you'll need a new one.

I would double check to make sure its a real Apple board as some people try to sell other boards which won't work as they don't have the needed firmware onboard. Here is the specs and Apple's P/N for the different board that fit this system: