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I inherited an iMac and it has an issue. It boots up fine but within the first five minutes the fans start to slowly ramp up to full speed. After about five to ten minutes of the fans running at full speed the screen goes black and I lose control of the computer. I cannot see anything on the screen, not even the cursor moving. I am also not able to shut down the computer using keyboard shortcuts. I have to hold down the power button.

The computer actually is still working to some degree after the screen goes blank because I have had a video playing from a USB stick and I could still hear the audio after the screen blacked out. I once had a Youtube video playing via the WiFi and when the screen went black the sound stopped.

I took the computer apart thinking that maybe it was full of dust. There was very little dust and blowing it out did not make a difference.

I have done a SMC reset and and a NVRAM reset and neither made a difference.

I installed a temperature monitor and the following is the output in degrees F a little before the computer goes to a black screen:

AC/DC Supply: 96
Airport Card: 133
Ambient Air: 75
CPU Heatsink: 88
GPU Die: 262 <------I think that this is as high as the sensor reads.
GPU Heatsink: 262
GPU Proximity: 241
Hard Drive Body: 100
LCD Proximity 91
Misc: 100
Optical Drive: 99

When the computer is running properly all four diagnostic LEDs on the logic board are lit. When the display goes black LED number 4 turns off leaving the other 3 on. Based on the Mac diagnostic instructions, if the fourth LED is off the issue is with the LCD panel or there is no video signal being generated.

I suspect that the video card is not working properly due to the high temperatures. It looks like the heat from the die is making it to the heatsink so I think that the thermal paste on the GPU is okay.

The computer is an iMac model EMC2134
OS X 10.8.5
24" Core 2 Duo with ATI Radeon HD2600 256 MB video card

I also ran EtreCheck and it said that the self test passed. I can post the results if needed.

I would like to repair the computer even though it is old. I however do not want to bring it in to a repair shop. What do you think is the issue and what leads you to that conclusion?

Thank you, jbr

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  • Since you already know it is the GPU card, what is the question than.
    – Ruskes
    Commented Apr 5, 2015 at 4:53
  • Cheapest start would be change the thermal paste anyway - getting inside that far would also mean you can check all the heat-sink fins for hidden bunnies; then make sure the GPU fans are actually doing their job & spinning up correctly. I have no clue what the graphics card on that machine looks like but dust bunnies do like to accumulate in the most difficult to reach places & if it's anything like my HD 5770 then the only way to really get at them is to strip the card, which necessitates disassembly of GPU & heat-sink & therefore new paste.
    – Tetsujin
    Commented Apr 5, 2015 at 10:29
  • I have already reapplied the thermal paste and it made no difference. The dust is also completely blown out.
    – Jbr
    Commented Apr 9, 2015 at 13:33
  • @Buscar웃 I am not sure it is the GPU card. Based on the fourth LED turning off it could also be the LCD panel.
    – Jbr
    Commented Apr 9, 2015 at 13:36
  • If it is LCD than the fans would not be working like crazy. 262F is way to high, wonder if it is the TC sensor ? Does it actually blow hot air out ?
    – Ruskes
    Commented Apr 9, 2015 at 16:15

2 Answers 2

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https://www.youtube.com/watch?v=qXso8KFG78M

see post from tommyjames 337 half way down the page

https://www.ifixit.com/Answers/View/126496/Graphic+card+overheating,+CPU+fan+in+full+speed,+then+black+screen

"Okay you guys! After months and months of trying, I have found the true solution to the overheating graphics card. I was right with my initial hypothesis. It was indeed the heat sink.

I was mislead for months because the heat sink I purchased initially to test my theory was also malfunctioning in the exact same way. But after isolating literally EVERY part in the iMac, buying a new logic board, and purchasing 5 new graphics cards (without heat sinks) to no avail, I finally decided it must actually be the heat sink, and it absolutely is."

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    I replaced the old video card heat sink with different used one. It solved the problem. The computer works perfectly.
    – Jbr
    Commented Apr 25, 2017 at 1:29
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I have iMac 5,1 core 2 duo, 64bit processor, 32bit efi. came with snow leopard. never had issues on leopard because it's 32bit/64bit compat. lower memory allocation and usage. actually takes a very long time to hear the fan running. upgraded to lion (top my machine can allow 10.7.5) and the fan goes on after 5 minutes. if i'm watching netflix it'll blank out. what i've done to get around this is install a program called hdd fan control where you can adjust when the fan starts to ramp up at a certain degree. if you set it too low (for the fan to start high), it'll get hot too and pull too much power and shut off that well. double edged sword. there is a sweet spot though, just need to find the medium. You can also run the computer with brightness all the way down. Hope this helps. It's the cpu, maybe accompanied by the gpu

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