iMac shutting itself down

[Paulywauly]

Hi Guys,

I've noticed over the past couple of weeks my main iMac has switched itself off overnight on its owna couple of times.

Its odd because it's not scheduled to do this and the console logs just seem to stop without mention of problems.

What else is odd is when press the power button after the random shut downs. The start up chime sounds, then, it goes straight to the lock screen that appears when coming out of screensaver NOT the initial mac login page. Its very unresponsive at first for a minute or two and there seems to be alot of disc activity.

I've ran a full test with Tech Tool Deluxe and it hasn't found anything and am unsure what to do next. Any advice appreciated! Cheers

EDIT: iMac is a 21.5" late 2009 model.

 

[Icy1007]

Wirelessly posted (iPhone 4: Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_3 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8J2 Safari/6533.18.5)

Overheating?

 

[Paulywauly]

Wirelessly posted (iPhone 4: Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_3 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8J2 Safari/6533.18.5)

Overheating?

I'm sure I've ruled that out actually. Room temperatures a pretty constant 19-22C. It was just sitting there idle so the CPU would have only been running around 35C. Even when I've pushed it the system to the max (encoding and folding on a hot day with a zillion apps open etc) the hottest any of it ever gets is in the late 50's (hard disc around 48c)

Also, I'm not sure if that would explain the weird startups?

 

[SantoW]

Sounds like it's gone to sleep rather than shut down

 

[Paulywauly]

You're right it sort of does seem more like sleep than a shutdown. The main differences though are:

1. The keyboard isn't waking it it up as normal, only power button works
2. There is a startup chime
3. Its very unresponsive for the first minute or two, lots of disc activity
4. All my open applications have closed, it loads up all login applications

I'm not sure if its relevant but the iMac has never actually slept properly since i bought it, it seems to wake itself up at random (despite SMC resets, switching off Bluetooth devices and disabling network power ons) It's just never been an issue till now because i need to leave it switched on all the time anyway

 

[DanielSosMac]

same issue

I'm having the same issue with an intel imac. It shuts down without warning or apparent reason. Sometimes we can leave it working overnight and nothing will happen. It seems to shutdown a lot when forced to process a lot of information.

How can I tell if its the alimentation or overheating?

I'm running a couple tests with smc fan control leaving it under 37 degrees. But how can I tell? I changed the alimentation chord but it doesnt seem to be that.

 

[GGJstudios]

Try resetting the SMC to address unexpected shut downs: Intel-based Macs: Resetting the System Management Controller (SMC)

For unexpected wake from sleep: Mac OS X: Why your Mac might not sleep or stay in sleep mode

Macs are designed to automatically shut down to prevent damage if they truly overheat (around 100C/212F - 105C/221F, depending on your processor), so unless your temps are that high, it's not heat-related.