iMac Intel 27" EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz) ID iMac10,1, EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz) ID iMac11,1

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Power supply replaced, still not turning on

I just purchased the replacement power supply for my Late 2009 iMac - installation steps went fine.

However, still not powering up, I have reinstalled 3 times thinking I may have missed a step.

All connections are made up correctly. Is there a chance I got a bad power supply? If not, what is my next step in troubleshooting.

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Why did you think it was a bad power supply?


What are the diagnostic LEDs ( to the right of the RAM slots) on the logic board telling


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2 Answers

Most Helpful Answer

ralph scott like @mayer pointed out already, let us know what your diagnsotics LED’s are telling you.

Diagnostic LEDs

The iMac (27-inch, Late 2009) computer has four built-in diagnostic LEDs on the main logic board that can help you to troubleshoot the computer. The LEDs are located on the bottom left edge of the logic board (under a piece of black mylar tape) and can only be seen when looking through the lower vents of the enclosure.

Block Image

LED #1

• Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will remain ON whenever the iMac is connected to a working AC power source. 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 source 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 and that logic board voltage regulators are functioning properly. 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 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 display, then the LCD panel, the LED backlight board, or the cables between might be installed incorrectly or need replacement.

Some test points are accessible on the logic board when the LCD panel is removed. The test points are defined below.

Block Image

Logic Board Test Point Function

Pin 1


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


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

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While rare, I have seen the actual power button fail. With the unit open (and unplugged...), follow the power button and test to verify that it is actually functioning.

Post the results here. Also post the "A" number for your unit so I can look up the prints and provide a few test points.


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I think checking the onboard diagnostic LED's would be the first step here. I'm not certain the power supply is the issue. It's also possible the power socket or cord is bad.


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ralph scott will be eternally grateful.
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