Chosen Solution
Sometimes with no apparent correlation with internal hardware temperature my iMac screen shuts off, but the computer itself still running (I can type something, use it…as usual, but the monitor is off. I can see my desktop if I point it to direct sun light). I’ve tried to restore the original configuration but it’s the same. There is not an exact delay time before shuts off; it can happends some minutes after startup, some hours later or some weeks later. To restore it I need to put it in stop (with the power button) then press it again to awake it. It turn on again without any problem. iMac 27 Mid 2010-Intel i3 3.2-2GB DDR3 Ram, OSX10.8.2,ATI Radeon HD 5670 512 MB. Any idea?
Your LCD backlight is on the way out. You could either connect to an external monitor or get/do a LCD replacement. If this answer was helpful please remember to mark it accepted.''
I just read this: The cause is heat. The SMC fan is not cooling down the system which is then shutting down because of the heat. There is a temperature sensor in the drive to supposedly prevent this. However, if you’ve changed your drive like I did, and replaced it with a drive that did not have a temperature sensor, thent he fan controll gets all confused, doesn’t turn on properly, everything heats up and then shuts down. This also happens on factory installed drives when the temperature sensor goes bust. A lot of Apple’s early drives for iMacs and MacBooks were really cheap and the sensor failed. I had to replace my factory isntalled drive because it failed. (Thank you time machine). The solution is a simple FREE software download called SMCfancontroller that turns your fan on at specific RPMs, and turns it off when you aren’t overheating. I installed the software and haven’t had a “go to black” moment since. The software can be downloaded here: http://www.macupdate.com/app/mac/23049/s…
My MacBook never had this problem until I replaced the magsafe adapter with an non-Apple charger (the Ostrich Replacement Macbook Charger 60w Magsafe L-Tip Power Adapter Charger) during the heat of summer. Whenever I was working the CPU heavily while the ambient air temperature was hot (20-25C or so), first the monitor would repeatedly shut off, and soon the entire machine would spontaneously shut down completely. The computer felt quite hot, so at first my solution was to shut it down for 20 or more minutes to let it cool off. But whenever I used the MacBook for long periods of time in hot temperatures, even this proved to be only a temporary solution. Ultimately, I ended resting the machine on top of a medical cold pack just to keep it running for an hour or less at a time. I then tried three of the solutions mentioned in this thread. First, I tried resetting the PRAM, but this had no effect at all, as far as I could tell. Second, I followed @jurgenkoppen’s suggestion and downloaded smcFanControl, but unfortunately it didn’t help nearly enough. I then switched to a different fan controller, MacsFanControl (https://www.crystalidea.com/macs-fan-con...), which worked much better. It’s slightly more complicated to use than smcFanControl, but not by much; I set it to display the temperature of the CPU Diode, which consistently ran very hot very quickly, and I set the exhaust fan to run on auto. These steps immediately began to cool down the whole computer, and maintained a relatively lower temperature. Third, after reading that a faulty power adapter could be responsible, and realizing that I had just recently replaced mine, I tried removing the power adapter and running off the battery. The result was striking: whenever the power adapter was in use, the temperature would climb VERY high fairly quickly, but when the battery was in use, the temp would drop to significantly cooler levels. My MacBook still runs too hot when I work it hard, probably because of faulty hardware, but it doesn’t shut down anymore after I made these changes.
Try a SMC reset. Worked for me on my iMac. Just unplug the computer from the wall for 15 sec. Plug it back in and wait 5 more seconds. Then turn on the computer.
Please reset PRAM 5 times by keep pressing Command, Option, P, R hearing 5 chimes except the first regular one. That will resolve the issue. If not, come back here and let us know. We will try to resolve it.
