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Model A1312 / Mid 2010 / 3.2 GHz Core i3 or 2.8 & 3.6 GHz Core i5 or 2.93 GHz Core i7, ID iMac11,3

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Won't boot with Graphics Card inserted

I treated myself an iMac 27” i7 for Christmas.

I followed a tutorial from OWC on how to install a SSD behind the optical drive, but when I reassembled, the iMac didn’t turn on at all.

However, without the screen and having removed the graphics card, all the fans turn on at max speed (it’s expected, the thermal sensor is on the screen) and I get the chime. When I plug the graphics card back in place, nothing happens any longer, it’s dead.

I tried different key combinations (cmd-alt-Q-R, cmd-alt-P-R) while pressing the power button without luck.

There are no LEDs nor any visual or sound indicator of what the problem can be.

As I unplugged the thermal sensor off of the logic board at some stage, I was wondering if it could be it, but I read that unplugging that sensor would just bring the fans to max speed without preventing it to start, thus it’s unlikely that although I cannot find anywhere if there is a way I could have plugged it in reverse (I can’t find any picture of that particular piece anywhere).

The graphics card was running perfectly before I disassembled it and sustained no shock during the operation. I had replaced the thermal paste on the GPU about a week ago and it was working fine after that, so it’s not the first time I remove it and put it back in place. The iMac was on 24/7 since then and running fine with no glitch at all — pretty much like new. It’s a freshly installed High Sierra (10.13.6) and I have already prepared the SSD with a brand new High Sierra as well (I booted on it when it was on a USB adapter).

Would someone have an idea why the iMac would refuse to start as soon as the graphics card is inserted? Could it be a nother component damaged (such as the socket for the screen ribbon on the logic board)?

At the price of a replacement GPU on a mid-2010 iMac, it’s not economically viable to fbuy a new one (mid-2010 iMacs trade on ebay for less than their graphics card!), so I am trying to explore all possible ways to diagnose the issue and fix it (e.g. where to look for a short, etc.).

Thank you for sharing your ideas, happy new year to all!

Update (01/06/2021)

Weird, I posted an update yesterday but it’s gone…

It’s the original AMD card, I can’t see what it is, something like 5670, but it was working before, even after I reapplied carbon-based non-conductive thermal paste.

It started not starting only after I unplugged and replugged the themperature sensor. From there, the fan didn’t move.

My big question now is: how can I test that sensor? I read it was a simple diode, but I don’t know how to bypass/replace it. I also read that the iMac would simply refuse to start or would shutdown if the GPU was too hot or the sensor was removed. Thus, my next idea is to put a diode between the wires of the sensor and see if it changes something to rule it out, as I’m convinced there’s nothing wrong with the card itself.

I tried to plug the card without its heatsink, to no change. I also tried removing the SATA cable to the new SSD, no change.

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Can you verify your exact model of your iMac. Plug in the systems S/N here everyMac - Lookup then tell us what you have.

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It's an A1312, EMC No. 2390, CK0520PJDNR, assembled in Ireland.

Thank you for your help!

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I have the same problem. Replaced the thermal paste and ended up with not starting iMac. Noticed with GPU installed power supply 12V comes and immediately shots down. Without GPU two LED stays on and power supply provides the power. Looks like shorted GPU but how it is happened, worked perfect with no issues. Any idea ?

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Repasting GPU boards are a bit tricky! You need two different solutions! A thin TIM for the main silicon chip and you need either a gapping paste or thermal pads (the preferred) for the 4 or 8 VRAM chips.

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TIM chemistry hasn’t really improved as most as the hockers will lead you to believe! What someone tries to prove in a lab is not the same in the real world. As there is no standard people can tweak things enough to show their TIM solution is better! Also the amount of improvement of a degree is not that much in the scale of things.

So what is most important is filling the micro gaps in both chip and heatsink surfaces and in its self is effective in heat absorption as well as transfer! There is only a few materials that do both. As an example Copper is a great absorber of heat (when you touch it its cold) But its not the best shedder of heat! Diamond is! (a form of Carbon). Carbon nano tubes in a paste is a new approach but it is highly conductive! So you need to be able to manage its placement very carefully! As an example We dropped a few bombs of carbon nano tubes in the Iraq war on power stations to short them out! It worked so well they are not useable even today after cleaning up the mess. It was also a costly clean up!

Traditional thermal paste is just fine! [gekoppeld product ontbreekt of is uitgeschakeld: IF179-010] and Arctic Silver ArctiClean I’ve been using Kryonaut lately as it has a bit more T-Conductivity than Arctic-Silver on newer CPU chips. Your GPU does not need anything more than Arctic-Silver. The bigger issue is the shedding the heat at the other end!

As I explained you also need a second type of thermal material here for the VRAM chips. Here the better material is thermal pads MINUS PAD 8. You’ll want to get a set of different thicknesses so you can fit the best one for your given board. I use a gap gage dry fitting the heat sink on the board to see what’s needed. You can use some strips of card stock stacked up as well to figure out what you’ll need. When you cut the pad you want to make sure you cover the top of the chip fully or slightly more.

Collecting the heat is not the biggest problem! Its shedding it at the other end!

You need a good thermal monitoring app and ramp up the fans. I use this app TG Pro it offers both the needed view of the sensors and fans and allows you to ramp up the fans when you are running the system hard.

Arctic Silver ArctiClean Afbeelding

Product

Arctic Silver ArctiClean

$9.99

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What kind of graphics card are you installing into the Mac? While mac devices are still outside of my comfortability in experience, that almost sounds to me like the card is drawing more power than whatever that device has for a PSU is able to provide accounting for all other components.

I’m curious to see what others more experienced with that device say.

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For some unexplicable reason, it started fine last week-end and I could install my boot SSD without any problem: it fixed itself.

I still think there's something wrong with the temperature probe somewhere as all the fans are screaming full speedbut it now works beautifully -- and fast! However, I cannot find a proper way to get the readings of all the sensors that would tell me which one is faulty.

I have a replacement temperature probe ordered, I received a replacement screen ribbon cable this morning and I'm going to repaste the CPU soon with carbon-based non-conductive thermal paste so to get this big baby run for the next 10 years.

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@marabiloso - Don't use the carbon thermal paste! Its conductive and if you get it onto the small SMD devices you can sort out the GPU Chip.

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Regis Michel LeClerc zal eeuwig dankbaar zijn.
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