Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor

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Won't power up and only 1 diagnostic LED

My iMac just shut down, and won't power up. I'm guessing either power supply or main board (already reset pram and other troubleshooting steps).

For the diagnostic LEDs, only the first lights up no matter what order I try.

According to the manual 0 LEDs is the power supply, and 1 and 2 is working, but it doesn't say anything about just 1.

Before I spend $200 on a power supply, are there any other ideas?

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Please clarify, your question states 1 LED on but the body of the question states two.

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Sorry for the confusion: The manual states troubleshooting for 0 or 2 lights, but not 1. I only have 1 light.

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mayer, I think the reference here is to the manual. Sounds like the OP only has 1 LED working on the iMac. All that the 1 LED shows is that the computer is plugged in and that the PSU receives power. It does not validate that the PSU is working. I would suggest to measure the output of the PSU.

UPDATE

Check some of the test points with a multimeter

Block Image

Pin 1 Ground

Pin 3 5V power for Hard Drive(s). Note: If present, the 12V power will come directly from the Power Supply to the Hard Drive thru the power harness cable.

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

"Press power button and Verify if system turns ON. You may alternately check for presence of a 12V DC signal between pin 11 and pin 1 of logic board when power button is pressed."

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I think you are correct so I changed this to an answer. Fortunately iFixit prices are better now and the part can be had for $150 now: iMac Intel 27" (EMC No. 2309, 2374, 2390 or 2429) Power Supply If the output is to low he can replace it using this guide: iMac Intel 27" EMC 2429 Power Supply Replacement

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Ok. As soon as I plug in, I get 12V on 11 but none on 4. Exactly the opposite of what I was expecting. LED one is lit, and the PS makes a slight hum.

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Sounds like a bad PSU

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No luck. No more PSU hum, but same LED status and pinout test results.

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joeadkins will be eternally grateful.
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