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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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Power On Issues (delays) After Power Failure

Over the years, my iMac has been taking longer and longer to power up after a complete loss of power (e.g. unplugged, or power failure). When power is returned, the power button does not start the machine (no response). Initially, after a couple minutes it would work. Over the years, this symptom has expanded to longer and longer for the button to work. About a year ago, it would take a couple hours. Recently, this timeline is now up to 24 hours. Once the power-up succeeds the machine boots and works normally.

Obviously, something is decaying in the machine (probably a capacitor, but could be something else). I'd like to replace the whole machine - but just don't have the money. Anyone know definitively what might be the issue? I'd really like to know if someone has actually root caused a similar issue to exactly the component on either the power supply or logic board that would be responsible for the condition.

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Within your system is a PRAM battery which also powers the clock and holds some of the settings.

Give this a try to test it. Disconnect the WiFI and Ethernet connections so the system can’t connect to the internet. Then double check the date and time should be correct, now shut the system down and unplug the power cord, press and hold the power button a good 15 secs and leave the system off for a good eight hours (more if you can). Now after leaving it off, plug in the power cord only and check the data and time is it correct or off? If off the PRAM battery has died and needs replacing. Follow this guide to gain access to it iMac Intel 27" EMC 2429 Display Replacement you’ll see the button cell near the bottom edge.

The reason your system takes so long is the startup setting becomes lost with the power outages. You may have also made it fail prematurely as your system is dependent on vampire power to keep the systems from being lost and really only use the battery when the cord is disconnected or the power feed is off. Even still a fresh battery will sustain the settings a good year at room temperature.

Once the battery is replaced go into your Startup setting making sure your boot drive is set.

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7 Comments:

Thanks Dan. Interesting on the battery. With the most recent power failure I was unable to get the machine to turn on even after the usual few hours of applied power. I have suspected the PRAM battery so I actually replaced it to see if it would help. After plugging the machine in there was no difference. It took over 24 hours for me to get it to power on (and it mysteriously powered itself up after 24 hours).

Question: after the battery replacement is the machine supposed to work straight away? Or would the second power up be the one that would work normally?

I’ll try your trick with the clock to see if time keeps. I am honestly afraid to pull the power cord fearing that one of these times it’s never coming back.

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@curlyq - mmm… Then the logic board needs repairs as a cap or two are blown. Time to pop the logic board and do a deep inspection of the caps.

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Was afraid of that. Not sure inspection will reveal a problem.

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@curlyq - Look for discolored components, post a clear picture of any that look suspect.

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Are you convinced it’s the logic board and not the power supply?

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CurlyQ will be eternally grateful.
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