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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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iMac EMC 2429 wont turn on

Hi Everyone,

Today my 27" iMac shutdown suddenly and now will not turn on. After looking through forums I have found that the most likely cause is either the PSU or the motherboard but am having trouble finding out which one it is.

The only LED light indicator that comes on, even when power button is pushed, is the first one from left to right when looking at the iMac.

When I measure the testing points on the board as per this post..

How can I test the Power Supply (voltage values)?)

the only ones that have any power are pins 8,9, and 10. If I am doing it right.

Any suggestions for next steps or solutions is greatly appreciated.


Answered! View the answer I have this problem too

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Chosen Solution

Start by replacing the PSU

Here's how:

iMac Intel 27" EMC 2429 Power Supply Replacement

LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.

LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.

LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.

LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

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Hi Mayer,

I installed the new power supply from ifix it today but unfortunetly that did not change anything. Still only showing the one led light and i get no response at all when pressing the power button.

Any suggestions where to go next?


Did your read outs on the output of the board change with the power supplies?


I just did that, pins 8,9,10,and 11 are reading 12 volts. Pin 14 and 6 i get 4 volts, nothing out of any of the other ports. Those readings are coming without me pressing the power button at all.


We may need need @oldturkey03 and/or @danj to chime in on this one. When you power on, does the second diagnostic LED flash or any of the others?


Nope. The only Led that does anything is the first one. Nothing changes when I push the power button.


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Ia have similar imac (same year but 21 size) . The Led 1 lights on when connected. But when I push the power button ligh goes off. Any way when I measure with the multimeter pin 4 reads 12V. Any ideas?

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