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Sudden restarts and shutdowns on iMac

I recently bought a refurbished iMac from Best Buy. When I inspected the unit after I unpackaged it, nothing appeared wrong externally. Also, the setup and installation of the OS went smoothly. However, after turning it on and browsing the web on Safari, the iMac suddenly shuts down. I thought that the power cord might've had a short in it at first, but have since moved away from that conclusion. Now, whenever I turn on the computer it will boot up fine and then suddenly shutdown or restart.
And as I write this, there is a new symptom: as soon as the iMac loads the desktop, the fans start up really loudly and then the computer shuts down.

Help is much appreciated.

iMac late '08, Mac OS X (10.5.6)

Posted on Jan 14, 2009 9:51 AM

Reply
36 replies

Jan 14, 2009 3:42 PM in response to randmc323

Before taking it back try this, if these don't work call Apple for support. If the machine is covered by AppleCare or is less than 90 days old you are entitled to free phone support. The phone number is in you manual

Try doing a SMC and PRAM reset by:

1. Turn off your iMac and unplug everything from it for 30 seconds.

2. Plug in the power cord, keyboard and mouse only.

3. Press the power button and immediately hold down the Option, Command, P & R keys until you hear the second gong then release the keys. Your machine will start up normally, then check for the issues you are having.

Jan 14, 2009 8:26 PM in response to randmc323

well, i have the same/similar problem - random restarts. and mine is a new 24"/2.8 GHz/10.5.6/iMac bought about 5 months back. problem started about a month back and incidentally a few days after i upgraded to 10.5.6. so, i assumed the issue was with the upgrade. please see the thread http://discussions.apple.com/thread.jspa?threadID=1835711&tstart=0 for my initial experiments. i have since concluded the issue is not 10.5.6.

i then called apple and they tell me that they have had a couple of similar cases and the issue was the main DC power cable which had to be replaced. they have ordered one for me and am waiting for the part to arrive and for the repair. hopefully, the cable indeed is the issue and the problem will be solved once and for all. will keep you posted.

you may want to investigate along similar lines...

Feb 12, 2009 8:37 AM in response to Scott Fields

We have a long history with this problem. So far 4 out of 10 iMac purchased at the same time had this problem. We have replaced the logic boards - then the machine works for a day or two then fails. We next replace the power supply, works for a day or two then fails. We suspect thermal issues, no help from Apple - then read about the cable harness and that fixed the problem. The next machine that fails we plan to swap the cable first. I just want to note that some of the machines are up and running now for months with just the replacement of the logic board or power supply. Nothing seems to remail consistent but the problem.

Feb 12, 2009 12:17 PM in response to randmc323

I have a 20' iMac purchased less than a year ago. Started having the same problems a few months ago. Would restart at random times, even when in sleep mode. An Apple cert. tech replaced the logic board and power supply. Everything worked for a few months, but now the same issue is back. In addition the fan speed increases dramatically even when the temps are normal. Surprisingly, the tech indicated the the logic board he put in was used and thus may have gone bad again. So it will be back again for another logic board.
Would be nice if folks posted any successful resolutions to this issue.

Feb 13, 2009 9:48 AM in response to randmc323

I'm having the same problem. iMac was new in August '08. A few weeks ago (Jan '09) it restarted for no reason. Became more and more frequent over a week. First it was a few times a night, then every hour and then every 2-3 mins. Took to apple store - they replaced the power supply. Issue re-appeared in less than 24 hours after repair. Took back to apple store, they checked the internal power supply cable because there was apparently an issue with a previous version of the cable, but I have the updated one. Also said the logs aren't showing anything unusual and the apple store hasn't been able to re-create the issue.

Feb 14, 2009 12:41 AM in response to randmc323

same problem here on 3 yrs old iMac 20" model iMac5,1. Appeared recently, increasing in frequency. Do you think Apple store people will be willing to check it for me?

Not sure if this related but I see graphic problems occasionally, dogear shaped areas either distorted or strangly tinted (pink, blue sometimes). Not sure if this is connected with the above.

Also need pointers to RELIABLE instructions on how to open this iMac.

Sudden restarts and shutdowns on iMac

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