Won't power up and only 1 diagnostic LED

Question

iMac Intel 27" EMC 2429

Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor, ID iMac12,2

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joeadkins @joeadkins

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crwdns2931729:0crwdne2931729:0: Sun 26, 2015

crwdns2933897:0crwdne2933897:0

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.

crwdns2934271:0Sun 26, 2015crwdnd2934271:0 mayer crwdne2934271:0

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

crwdns2934271:0Sun 26, 2015crwdnd2934271:0 joeadkins crwdne2934271:0

@joeadkins: I have the exact same problem. Did you ever find a solution?

crwdns2934271:0Yan 5, 2022crwdnd2934271:0 king50572 crwdne2934271:0

Same problem here in 2022. Tried two different PSU and nothing. I did notice more pins with voltage using on one of the PSUs but nothing between 1 and 4 :(

crwdns2934271:0N'w 4, 2022crwdnd2934271:0 Hen crwdne2934271:0

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oldturkey03 @oldturkey03

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crwdns2931729:0crwdne2931729:0: Sun 26, 2015

crwdns2933897:0crwdne2933897:0

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.

Check some of the test points with a multimeter

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 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

crwdns2934271:0Sun 26, 2015crwdnd2934271:0 mayer crwdne2934271:0

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.

crwdns2934271:0Sun 31, 2015crwdnd2934271:0 joeadkins crwdne2934271:0

Sounds like a bad PSU

crwdns2934271:0Yan 2, 2015crwdnd2934271:0 oldturkey03 crwdne2934271:0

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

crwdns2934271:0Yan 5, 2015crwdnd2934271:0 joeadkins crwdne2934271:0

Same deal as joeadkins. Pin results of 1 & 11 give 12v but 1 & 4 give nothing. Replaced power supply and still the same result

crwdns2934271:0Nhl 13, 2017crwdnd2934271:0 Cory Dorning crwdne2934271:0

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Please clarify, your question states 1 LED on but the body of the question states two. — mayer
Sorry for the confusion: The manual states troubleshooting for 0 or 2 lights, but not 1. I only have 1 light. — joeadkins
@joeadkins: I have the exact same problem. Did you ever find a solution? — king50572
Same problem here in 2022. Tried two different PSU and nothing. I did notice more pins with voltage using on one of the PSUs but nothing between 1 and 4 :( — Hen
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 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 — mayer
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. — joeadkins
No luck. No more PSU hum, but same LED status and pinout test results. — joeadkins
Same deal as joeadkins. Pin results of 1 & 11 give 12v but 1 & 4 give nothing. Replaced power supply and still the same result — Cory Dorning

bandish @bandsih crwdns2934065:0crwdne2934065:0: 25 1 · Answer 1 · 2018-02-06T22:58:33-07:00

bandish @bandsih

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crwdns2931729:0crwdne2931729:0: Yan 6, 2018

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sir my imac problem no power . i have check power supply 450 volt capacitor is burn . so replace capacitor pluse fuse is gon fuse replace but power supply voltage coming 12 volt 5 volt and 3 volt , motherbord led is not start so reply ,plz help me .

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pls help, my imac 27 inch late 2011 when you plug it the 1st led indicator is lights up in a while then off.

crwdns2934271:0N'w 26, 2020crwdnd2934271:0 jerryfonts_123 crwdne2934271:0

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iMac Intel 27" EMC 2429

Won't power up and only 1 diagnostic LED

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UPDATE

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