imac doesn't turn on after reassembling

I don't know if this topic is still relevant, but maybe my answer will help others...

I faced the same problem with iMac 21" (Mid 2011). Installed SSD, cleaned dust, assembled - not working.

Double and triple checked everything and then found a little catch:

On the opposite side of the main logic board near to all HDD, ODD, etc. connectors

there are TWO EXACTLY SAME power connectors (one is closer to the center of the board and another one is HDD power connector). The thing is that if you mistakenly connect HDD power cable to the central power connector - then you will place another one in the wrong place too, and iMac will not start...

So if you're facing this problem - switch places of these 2 power cables on the back side of the logic board. Then you may connect AC cable and if you see that LED501 is lit - then you got things right! (Acm001 wrote about those LEDs)

!!Disconnect AC cable and wait a couple of minutes for condensators on Power Supply Unit to drain!!

Then continue reassembling your iMac. Hope that helps!

First check the obvious such as the power cord to ensure it's plugged into both the iMac and your power source. Once you have done that then methodically take it apart again using a guide such as a iFixit or OWC (http://eshop.macsales.com/installvideos/imac_mid_2011/) videos. Almost for sure you will find something you either did not connect or that you damaged. If you still can't find anything then you likely damaged the machine, if you still don't want to take to an Apple Authorized Service Provider then, well... you have bricked your iMac.

Diagnostic LEDs

There are four diagnostic LEDs on the front center of the logic board that can help troubleshoot

the computer. Note: The audio cable covers the diagnostic LEDs, so you will need to disconnect

or resposition the audio cable in order to view the LEDs.

LED Functions

LED #1

Indicates that the trickle voltage from power supply has been detected by main logic board.

This LED will turn ON when you connect the iMac to a working AC power source. The LED will

remain ON as long as the computer is ON or asleep.

When computer has been correctly shut down, the LED#1 behavior may differ:

• - If a startup event is scheduled in System Preferences > Energy Saver, LED#1 will stay ON

after a correct shutdown,

• - If no startup event is scheduled in System Preferences > Energy Saver, LED#1 will turn

OFF and will stay OFF as long as power cord is connected and AC power source is present.

Disconnecting the power cord and plugging it back will turn back this LED ON , even if

computer is still off.

• After disconnecting and reconnecting the AC power source, this LED could remain OFF if

the AC power source is missing or disconnected, if the logic board is disconnected from the

power supply or from the AC outlet, or if the power supply board is faulty.

LED #2

Indicates that computer is turned on. This LED will be ON as long as computer is turned on (but

is not asleep) and power supply and voltage regulators are working correctly.

LED #3

Indicates that computer and video card are communicating. This LED will be ON when

computer is communicating properly with video card. If LEDs 1 and 2 are ON and you heard the

startup sound, but LED 3 is OFF, then the backup battery (on front of logic board) may need to

be reseated, or the video card might be installed incorrectly or needs replacement.

LED #4

Indicates that computer and LCD panel are communicating. This LED will be ON when

computer is turned on and video signal is being generated. If LED #4 is ON and there is no

image on display, then LCD panel, LED backlight board, or cables between LCD and logic board

or backlight board might be installed incorrectly, or need replacement.

LED Startup Sequence

LED #1 = Power available.

If first LED is not lit:

• Disconnect the power cord from computer, then reconnect it to reset the LED status.

• Verify AC power source.

• Verify known-good power cord is connected.

• Verify cable connection between AC inlet and power supply.

• Verify cable connection between power supply and logic board.

• Verify power supply.

LED #1 + #2 = Power available, and computer is powered on.

If second LED is not lit when power button is pressed:

• Verify power button cable connection to logic board.

• Verify power button functionality.

• Verify cable connection between power supply and logic board.

• Verify power supply.

• Verify logic board.

LED #1 + #2 + #3 = Power available, computer is powered on, and video card found.

If third LED is not lit after power on:

• Verify that video card is seated properly.

• Verify if startup sound is present and fans are running when powered ON (reset SMC

and PRAM, reseat or check backup battery if necessary for proper startup):

If POST startup sound is not heard, go to No Startupsymptom flow,

If POST startup sound is heard, go to No Videosymptom flow.

LED #1 + #2 + #3 + #4 = Power available, computer is powered on, video card found, and

internal LCD found.

If fourth LED is not lit after power on:

• Verify internal DisplayPort cable connections between LCD panel and logic board.

• Inspect all LCD cables for damage.

• Verify external video functionality, and according to result check the following items:

If external display works then verify/replace the LED backlight board.

If external display works then verify/replace the LCD panel.

If external display does not work verify/replace the logic board.

pejman1998 wrote:

im sure no power gets in logic board btw im sure power is avalaible in power supply because it shocked me once i checked cables 5 times but im still confused why it doesnt turn on i am sure that the look of logic board is just like what it was earlier before breaking and i've done nothing else than just taking it out and using fan to clean dust beneath it i even tried to take out the battery of bios to check whether it turn on or not but it wasnt successful neither im not rich man and i cant afford a new machine so i believe i will blame myself for years. tanx btw for helping me

That would indicate you may have blown the power supply. If you gent sound or video and all cables are connected then that is likely where it is. You will need to locate one somewhere and then install it, going forward ALWAYs avoid the power supply.

It's your machine, you can try what you like. I can't tell you if it will work or not, perhaps someone else may know.

Do any of the leds turn on? The #1 should be on if the mlb is getting power. No leds on means no power - either bad power supply or blown mlb. Power supply not a lot of $.

You just saved me hours of troubleshooting and potentially hundreds in repair costs! Thank you very much 🙂