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iMac shutting off shortly after boot

So I have been working on this iMac as a bit of a hobby at this point.

Long story short...

Bought a 27" iMac on Craigslist, wasn't working, so I bought some parts for it (Power Supply from iFixIt, random screws and brackets, etc. and eventually a replacement logic board). Nothing worked until I had the new (well, older) logic board in and it worked great...for about a day.

Then, randomly, I check on the computer and it's turned off. Odd...I figured it had a sleep issue, so I power up and turn off the sleep timer. That ought to fix it. Nope. Ten minutes later it's down again.

Now it won't boot up unless I give it several minutes and then it will hit the chime and then shut down again. If i give it a few hours, it will boot fully and I can putz around for a minute before it shuts down again.

I'm thinking it's heat related. I ordered new thermal paste as the stuff I used on the i5 I bought was purchased originally 2 years ago (when I replaced the CPU's in my Mac Pro 1.1 to Quad Cores). But I assume heat because:

1. It worked great for the first issues whatsoever. Installed everything, etc.

2. It works longer when it's unplugged longer.

I guess it COULD be the logic board, but it seems unlikely since it did work really well for awhile there.

Anyone have any thoughts on this?

Thanks :D

Answered! View the answer I have this problem too

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Look at the crash log.

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I will see if I can get it that far and report my findings...some googling led me to perhaps (hopefully) a bad CPU? It's a used i5 that I bought on would be great if it was that.

by murphmanstudios

There was no file in the DIAGNOSTIC REPORTS folder. That doesn't help. Thoughts? It was running for about 5-6 minutes before poof...dead.

by murphmanstudios

You could have yet another bad logic board - try target booting it. Does it run any longer in Target mode?

by originalmachad

I just tried Target Mode and it dies at the same time roughly. I think I need to take it apart one more time. I bought some new thermal paste so I'll try that (even though it's a reach), and then test some of the cables in there to see how they work. Sensors, mainly. I wonder if a sensor cable is broken and it freaks out when it gets a reading and dies. I hope it's not a logic board because that's just annoying...but it certainly could be. :(

by murphmanstudios

The only thing making me think that it is NOT the logic board is that it ran great the first day and then started having the issue. And that the longer it's unplugged, the longer it will run. That, to me, says heating...even if the temperatures don't say anything. It's odd.

by murphmanstudios

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Try lowering the displays brightness to the lowest possible setting that you can still see it. Does it stay up longer?

I would also try installing a temperature monitoring software program to see if it reports anything getting hot.

As you didn't tell us what exact model & OS you have it will be tough going any further.

Please give us the last four digits of the systems serial number and the OS you are running on it. Also, can you tell us if the logic board you swapped out is the same or different from what it had in it. If it's different tell us what the CPU & GPU specs it has.

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Thanks for the response, Dan. I will test the display brightness right away. I have iStat Pro, smcfancontrol and a paid hard drive fan controller that I purchased since the hard drive isn't Apple OEM. The model is kind of a tricky situation. The model WAS a 2010 27" 3.2ghz i3, but I bought a 2009 27" 2.66ghz i5 board with the stock i5 chip. The serial number is still underneath the mount, but when you check ABOUT THIS MAC (If you can make it that long), it actually says "SYSTEM SERIAL NUMBER". I'm assuming the switched logic board is confusing it. GPU is ATI Radeon HD 5670 (512MB).

by murphmanstudios

No dice on the lowered brightness.

by murphmanstudios

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murphmanstudios will be eternally grateful.

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