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Vip887

macrumors newbie
Original poster
May 29, 2016
9
2
London
Hi,

I have a iMac mid 2011 27" a few months ago it stopped booting up first time, I had to repeatedly reset the smc to get it running. Now it is dead and does not power on at all no chime no noise nothing, I have taken it apart and exposed the diagnostics LEDs and there are all unlit, so I figured I need a new logic board or power supply.

So today I thought I would go ahead and try it out again, I decided to plug the mains cable back in, after around an hour plugged in I notice the first LED was lit, so I pressed the power button and I heard the chime and it started to boot the first 3 LEDS all lit up and then all the fans came on full blast, I powered it down holding the power button and the first LED was still lit, after switching it on again it did the same thing but no loud fans this time, so I reassembled the display and turned it on powered up to the apple logo and then after a minute it switched off by itself and now it is back to dead again!

Any ideas what this issue is? Logic board or power supply?

Thanks
 

ElCani

macrumors regular
Jun 15, 2012
112
26
I replaced a completely dead PSU on a 2011 iMac with a used one that turned out to be faulty. The Mac booted but the CPU fan ran at full speed and the CPU throttled to about 1/2 power. Apple Hardware Test showed an error code that related to a PSU temperature sensor, so I replaced the PSU again and all was well.

Not sure how much that helps, but it shows that a dodgy PSU can lead to the CPU fan running at full speed. It a cheap-ish part and an easy job to replace, so worth trying.
 
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Vip887

macrumors newbie
Original poster
May 29, 2016
9
2
London
I replaced a completely dead PSU on a 2011 iMac with a used one that turned out to be faulty. The Mac booted but the CPU fan ran at full speed and the CPU throttled to about 1/2 power. Apple Hardware Test showed an error code that related to a PSU temperature sensor, so I replaced the PSU again and all was well.

Not sure how much that helps, but it shows that a dodgy PSU can lead to the CPU fan running at full speed. It a cheap-ish part and an easy job to replace, so worth trying.

Thanks will give that a try also, I am thinking it is either a power supply or logic board issue, when the Mac does not switch on all of the LEDs are unlit, which suggests it is not receiving any power. LED 1 shows it is receiving a trickle charge, when it is lit then it boots up, although it seems to be random it is lit or not.
 

Vip887

macrumors newbie
Original poster
May 29, 2016
9
2
London
Update, so I redid thermal paste but had no effect on the power, so took the plunge and picked up a new power supply, installed and is now running perfectly, LED lit up straight away and powered up first time, been running for the last 24 hours, tried a few restarts and cold boots and is running perfectly, thanks for your help!
 

ElCani

macrumors regular
Jun 15, 2012
112
26
Good news! They're good machines the 2011 27s, I installed a 256GB SSD in mine around three years ago (configured as a fusion drive with the 2TB HDD) and it still serves me very well.
 

Vip887

macrumors newbie
Original poster
May 29, 2016
9
2
London
Good news! They're good machines the 2011 27s, I installed a 256GB SSD in mine around three years ago (configured as a fusion drive with the 2TB HDD) and it still serves me very well.

Yeah it is a great machine, I have also put a ssd in mine, makes a big difference, from the speed you wouldn't know it's a 4+ year old machine.
 
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