Where are the diagnostic LEDs located on a 27" iMac (late 2009, 3.06GHz)?

bittered · Apr 1, 2012

My iMac stopped booting. The fans and hard-drive spin up but the screen stays black and I can hear that the hard drive isn't been read after the initial spin up.

I decided to open it up to take a look at the diagnostic LEDs to see if it might help me diagnose the problem (the most obvious failure is the logic board). I can't find the LEDs anywhere, where are they? Does my iMac even have diagnostic LEDs?

The other possibility is that I can't find the lights because they aren't lighting up, what would this suggest? The fans, the hard drive and the CD drive all spin up fine so at least I know that there is some power running though it.

Intell · Apr 1, 2012

If facing the board when it's mounted in the iMac with the CPU heatsink at the top left, the diagnostic LEDs are on the bottom left corner of the logicboard facing the screen side of the iMac.

bittered · Apr 1, 2012

Intell said:
If facing the board when it's mounted in the iMac with the CPU heatsink at the top left, the diagnostic LEDs are on the bottom left corner of the logicboard facing the screen side of the iMac.

I see them, there's a piece of translucent tape covering it. It seems that none of the LEDs are on when the computer is on or off. What does this suggest?

Intell · Apr 1, 2012

LED 1 is power available
LED 2 is power available and system on
LED 3 is power available, system on, and video card found
LED 4 is power available, system on, video card found, and internal LCD found

If none of them are turning on, then your power supply is likely to be dead.

bittered · Apr 1, 2012

Intell said:
If none of them are turning on, then your power supply is likely to be dead.

Thanks.
If the power supply was dead then would the hard drive (and CD and fans) spin up?

Intell · Apr 1, 2012

The power supply may be partly dead. It could have enough life to supple power to somethings, but not all things. Because the fans also spin, another dead thing could be the logicboard. If you have a multimeter, you can check the power supply to verify it as good or bad.

bittered · Apr 1, 2012

Intell said:
The power supply may be partly dead. It could have enough life to supple power to somethings, but not all things. Because the fans also spin, another dead thing could be the logicboard. If you have a multimeter, you can check the power supply to verify it as good or bad.

Thanks this is very helpful, I can get a multimeter from a friend. Where should I connect the multimeter to (I'm a noob when it comes to electronics).

Intell · Apr 1, 2012

You'll have to search for the power test points for your iMac model. I haven't done that on a 27" so I can't tell you where they are at.

bittered · Apr 5, 2012

Intell said:
You'll have to search for the power test points for your iMac model. I haven't done that on a 27" so I can't tell you where they are at.

Hi Intell, I asked this question on the Apple support forums and a person replied to me saying that the iMacs haven't had diagnostic LEDs in years. Could you have a look at the thread and tell me what you think?

edit: forgot the link https://discussions.apple.com/thread/3845641

Intell · Apr 5, 2012

Whoever said that hasn't opened up an iMac, isn't an Apple tech, or hasn't read the service manuals. iMacs still have diagnostic LEDs, here's a screen shot from the late 2009 27" service manual that details the LEDs (Click to enlarge):

AppleNewton · Apr 5, 2012

the 2009 iMac indeed has Diagnostic LEDs and there are 4 of them in the lower left of the logic board.

There is an area in the upper right area of the logic board below the GPU CPU bracket.

There are 14 pins.
7 6 5 4 3 2 1
14 13 12 11 10 9 8

Logic Board Test Point
Function
Pin 1
Ground

Pin 4
Standby 12V power (permanent power coming from power supply and present as long as AC cable is connected, even if computer is off); corresponds to LED #1

Pin 6
Backlight Control pulse width modulated signal (from logic board to LED backlight board, to adjust the backlight level setting according to user setup)

Pin 11
12V Run-Mode power to logic board (coming from power supply, present as long as system is on or asleep); corresponds to LED #2

Pin12
Power On Request signal (from logic board to power supply when power button is pressed)

Pin 13
Backlight Enable (signal from logic board to Backlight Controller board, to enable backlight

bittered · Apr 6, 2012

AppleNewton said:
the 2009 iMac indeed has Diagnostic LEDs and there are 4 of them in the lower left of the logic board.

There is an area in the upper right area of the logic board below the GPU CPU bracket.

There are 14 pins.
7 6 5 4 3 2 1
14 13 12 11 10 9 8

Logic Board Test Point
Function
Pin 1
Ground

Pin 4
Standby 12V power (permanent power coming from power supply and present as long as AC cable is connected, even if computer is off); corresponds to LED #1

Pin 6
Backlight Control pulse width modulated signal (from logic board to LED backlight board, to adjust the backlight level setting according to user setup)

Pin 11
12V Run-Mode power to logic board (coming from power supply, present as long as system is on or asleep); corresponds to LED #2

Pin12
Power On Request signal (from logic board to power supply when power button is pressed)

Pin 13
Backlight Enable (signal from logic board to Backlight Controller board, to enable backlight

Thanks I found the LEDs. There only seems to be one LED on whether the computer is on or off. It's difficult to see though.

I don't quite understand what you mean by the second part of the post about the pins. Could you go into more detail about what I can do to test it.

edit: Ah, ok, they're obviously for testing with the multimeter. I'll test now and report back.

bittered · Apr 6, 2012

Ok I measured the test points on the logic board and this is what I got:

Standby 12V power: Pin 4 - OK (12v)
Backlight Control pulse width modulated signal: Pin 6 - NOT OK (0V)
12V Run-Mode power to logic board: Pin 11 - OK (12v)
Power On Request signal: Pin 12 - NOT OK (0V)
Backlight Enable: Pin 13 - NOT OK (0V)

AppleNewton, Intell:
What does this suggest? Does this mean my power supply is fine but I need to replace the logic board?

Intell · Apr 6, 2012

I'm traveling right now and unable to check the service manual, but when I get a chance, I'll check it.

Intell · Apr 6, 2012

After consulting the service manual, the next thing you should try is removing the coin battery for two minutes then putting it back in. This will reset the logicboard and hopefully make it work again. The battery is located on the back of the logicboard and may be difficult to get to. If the removal/replacement of the battery doesn't fix it, the logicboard may be bad.

bittered · Apr 7, 2012

Intell said:
After consulting the service manual, the next thing you should try is removing the coin battery for two minutes then putting it back in. This will reset the logicboard and hopefully make it work again. The battery is located on the back of the logicboard and may be difficult to get to. If the removal/replacement of the battery doesn't fix it, the logicboard may be bad.

Just did that and tried booting it up. Same as before :-(.
I also measured the voltage of the battery, 2.8V which I suspect is ok.

I guess the conclusion at this stage is a broken logic board, right?

Troubleman · Jul 24, 2012

Bittered, did are your iMac running? I encountered the same problem as you, with the same voltages and everything... Now I'm worried that my logic board is faulty

(BTW, Is there a way to send private messages on this forum?)

susu · Nov 19, 2012

I have the same problem as you Led 1 and 2 are on, screen stay black, no chime, fan spin, but i don't know if logic board is broken or it is power supply problem. Imac don't eject cd, don't recognize usb keyboard and usb mouse.
I need to know if is a logic board or power supply issue.

ocelittle · Mar 7, 2013

Did anything ever come of this?

peje1 · Jul 14, 2015

I have a similar problem: but it seems I only have 1 LED on.

While the computer seems to be functioning normally except for some very high secondary heatsink temps that go up to 90C and a weird vertical smudge on the LCD the runs the entire vertical length of the far right side of the LCD. Could this be the result of poorly connected cabling?

AHT found no errors.
Imac 11,3 mid-2010, 27'

DEVAS · Dec 3, 2016

27 "Mid 2010. Won't Power ON. I suspected a power supply issue. Replaced CMOS battery. RF filter, and PSU

SMC.
Fans Runs with high speed. But no use.

My PIN and voltage test says.
PIN 4 - 12 V
PIN 12 - 4 V on Power button press goes to 0
PIN 11 - 12 V on Power button press goes to 12 V

LEDs.
Only LED 1 is on.
LED 2 is OFF.

I have removed ODD/HDD and tried to Power on.

Above status remains.. Any further clues to test the logic board with power points...?

Help would be highly appreciated.

Thanks
Devarajan G

Where are the diagnostic LEDs located on a 27" iMac (late 2009, 3.06GHz)?

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