IMac won’t turn on

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IMac won’t turn on

This is a 27” iMac model A1312 and obsolete from Apple perspective but for the simple stuff I do it is quite adequate.

The unit was given to my daughter, an artist, but hasn’t been used for at least 3 years and hasn’t even been plugged in during that time. Anyway, I plugged it in an pressed the power button - nothing happened. No light, no sound.

Did the drops of WD-40 around the power button on the theory that dust was preventing the button making contact. No change.

Used a different power cord, no change.

Had a support call with Apple and they had me try a number of things but to no avail. They made an appointment for me at the nearest Genius Bar, over an hour away. I lugged this very heavy unit to the store only to be told that because it is obsolete, they can’t touch it!!

Back home with nothing to lose, I decided to open the unit up. Removed the glass and the LCD and found where the power button plugs into the logic board. Unplugged it and used a paper clip to short across the contacts, simulate pressing the power button, no action.

Measured the voltage at the point where 240 volts comes into the power module and it is indeed 240v.

Lacking any other advice, I decided to replace the power module which I did to no effect.

I measured the voltage at the power switch connection on the logic board and it was 2.8v instead of the expected 3.3v. This is when I realised there must be a battery somewhere. After some research, determined that it is on the topside of the logic board. Found a uTube video which showed me how to remove the logic board. I didn’t completely remove it, just to the point where it allowed me to access the battery. Measured the battery voltage across the battery and it was indeed 2.8v. The unit had been plugged in for several weeks but hadn’t fully charged the battery so, replace it. The BR2032 battery is not common where I am, had to order it from interstate and am still awaiting delivery. I hope this is the source of my problem and that I can successfully reassemble the unit.

iMac 27″

Posted on Sep 13, 2023 1:03 AM

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Answer 1

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BDAqua

Level 10

250,673 points

Sep 20, 2023 7:04 AM in response to AlanLH

This is from the Apple service manual, not sure which one:

Diagnostic LEDs

LED 1 - Indicates that the trickle voltage from the power supply has been detected by the 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 the computer has been shut down correctly, LED 1 behavior may differ:

If a startup event is scheduled in System Preferences/Energy Saver, then LED 1 will stay on after a correct shutdown.

If no startup event is scheduled in System Preferences/Energy Saver, then LED 1 will turn off and will stay off as long as the power cord is kept connected and an AC power source is present. Disconnecting the power cord and plugging it back in will turn this LED back on, even if the 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 the AC receptacle.

If the power supply board is faulty.

LED 2 - Indicates that the computer is turned on. This LED will be on as long as the computer is turned on (but is not asleep) and the power supply and voltage regulators are working correctly.

LED 3 - Indicates that the logic board and GPU are communicating. This LED will be on when the CPU is communicating properly with the GPU. If LEDs 1 and 2 are on and LED 3 is off, then the backup battery (on the back of the logic board) may need to be reseated or the

logic board may need replacement.

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

LED 5 - Indicates that the logic board and LCD panel are communicating. This LED is on when the computer is turned on, a video signal is being generated, and the LCD Panel is signaling to turn on the backlight. If LED 5 is ON and there is no image on the display, then the LCD backlight or circuitry on the logic board may be malfunctioning. The LCD panel or the cables or the logic board may need replacement.

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Answer 2

BDAqua

Level 10

250,673 points

Sep 20, 2023 7:04 AM in response to AlanLH

—————-

LED Startup Sequence

LED 1 = Power is available. If no LED is visible:

Disconnect the power cord from the computer and wait 15 seconds to reset the power supply and LED status.

Reconnect the power cord and check the LED status again.

Verify the AC source.

Verify that a known-good power cord is connected. Verify the cable connection between the AC inlet and the power supply.

Verify the cable connection between the power supply and the logic board.

Verify the power supply.

LED 1 + LED 2 = Power is available and the system is turned on.

If the second LED is not visible when the power button is pressed:

Verify that the power button is connected to the power supply.

Verify power button functionality.

Verify the cable connection between the power supply and the logic board.

Verify the power supply. Verify the logic board.

LED 1 + LED 2 + LED 3 = Power is available, the system is turned on, and the GPU was found.

If the third LED is not visible after the system is turned on:

Verify whether the fan is running when turned on (reset SMC

and NVRAM, verify backup battery voltage for proper startup). If the fan is not heard, go to the "No Startup" troubleshooting flow.

LED 1 + LED 2 + LED 3 + LED 4 = Power is available, the system is turned on, the logic board is communicating with the GPU, and the internal LCD was found.

If the fourth LED is not visible after the system is turned on:

Verify the embedded DisplayPort cable (eDP) connections between the LCD panel and the logic board. Inspect the LCD display cables for cable damage.

Verify external video functionality and proceed according to the result:

If an external display works, verify/replace the LCD panel. If an external display does not work, verify/replace the logic board.

LED 1 + LED 2 + LED 3 + LED 4 + LED 5 = Power is available, system is turned on, GPU is working, logic board is communicating with the LCD panel, and the LCD panel has turned on the LCD backlight.

If the fifth LED is not visible after the computer is turned on:

Verify the eDP cable and backlight cable connections between the LCD panel and the logic board.

Inspect the LCD display cables for cable damage.

Using a flashlight, check the following areas on the panel:

Look in the top left corner of the panel to see if there is a dim Apple logo (the Apple menu logo).
Look in the center of the panel to see if there is a login screen.
Look at the bottom of the panel to see if there is a Dock visible.
If the following items are visible, there maybe something wrong with the display cables or the logic board. Proceed to the next step.
Replace the LCD panel, check that all the LED’s are visible, and that the backlight functions. If the computer does not function after replacing the LCD panel, replace the logic board.

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Answer 3

AlanLH Author

Level 1

5 points

Oct 2, 2023 1:52 AM in response to AlanLH

Receiving no further clues and unable to find diagnostic LEDs, I punted and replaced the logic board and it worked!! Computer up and running. Thank you all for responding to my cry for help.

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Answer 4

AlanLH Author

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5 points

Sep 18, 2023 4:12 PM in response to BDAqua

Complication: when I removed the original battery, must have broken off a piece of the “container” because the new battery won’t stay in. Had form a ridge with a glued piece of plastic. Reassembled and pressed the power button - nothing happened! Need to re-check my assembly.

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Answer 5

AlanLH Author

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Sep 20, 2023 8:36 PM in response to BDAqua

Thanks for all that info. I had seen it before but as I couldn’t see any LEDs on my board, It didn’t apply to me. An article on the web said that all iMacs have diagnostic LEDs but I can’t see them on my EMC 2374 system. I can find images for 2010 & 2011 models but the board layouts are different. If my unit does have LEDs then as no light is showing when power is connected, it means either I bought a dud power supply or the logic board has a problem or both.

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Answer 6

AlanLH Author

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5 points

Sep 20, 2023 1:06 AM in response to BDAqua

Retraced my steps. My fix for the battery “container” needed further work. Re-assembled and still nothing happens when the power button is pressed or the power button socket on the logic board is shorted. Measured 3.7v at the power button socket. Now I’m fresh out of ideas. Given that the unit was ok when last used, was powered down and moved around but never treated roughly or dropped or plugged in so it’s hard to imagine that anything has been “burnt out”.

Any ideas?

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Answer 7

BDAqua

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250,673 points

Sep 20, 2023 2:35 AM in response to AlanLH

I vaguely remember something about the power switch on some older macs, one was that the button had to be pressed in the top right corner, ( I think), and the other drawing mostly blanks on it.:(

I assume polarity is right?

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Answer 8

AlanLH Author

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Sep 20, 2023 2:53 AM in response to BDAqua

Thanks for the response. Yes, there is one spot on the power button which feels like it is doing something. I’d like to unplug the power button connector from the logic board and test whether pressing the power button connects the two wires but my simple multimeter doesn’t have thin enough probes to fit into the end of the plug. As for polarity, there is no choice as the plug only fits one way into the logic board socket.

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Answer 9

AlanLH Author

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Sep 20, 2023 4:59 AM in response to BDAqua

Apologies for my misunderstanding, the battery can only go one way.

Whilst I don’t understand how it could be, I’m beginning to wonder whether the logic/motherboard has a problem and whether I should replace it just call it quits.

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Answer 10

BDAqua

Level 10

250,673 points

Sep 20, 2023 7:01 AM in response to AlanLH

I think at one time t was possible to insert the Battery upside down, & they changed that... just didn't want to leave the obvious hanging...I didn't think you would insert it wrong in the fist place! :)

Any diagnostic LEDs on the Logic Board?

https://logi.wiki/index.php/iMac_Logic_Board_LED_Indicators

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Answer 11

BDAqua

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250,673 points

Sep 20, 2023 8:48 PM in response to AlanLH

I think you're right, I can't post the kink because Apple will remove it, google

IMAC INTEL 27" EMC 2309 & 2374

You'll see it should have LEDs, they even sell LED Driver boards, not that I think that is it.

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Answer 12

AlanLH Author

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5 points

Sep 20, 2023 9:28 PM in response to BDAqua

Thank you, I had a look at the site and I even posted a question asking where the LEDs are. All the photos I’ve seen so far seem to have different logic boards. I have scanned my logic board with my iPhone magnifier app and have been unable to see the word LED.

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