iMac Random Shut Down

[cxviking]

My 27” iMac, OS (late 2012), OS10.13.6 (High Sierra) has a troubling problem. The computer shuts down randomly for no apparent reason. Not related to doing something specific or memory intensive, sometimes just after an hour being on, sometimes after several hours or even days. Powers itself off with no sound, not even a click, just as if the plug was pulled.

Tried redoing hard drive from backups, changing plug-ins, booting from external drive, all to no avail and the problem persists. The only thing that actually allows power on again after a shutdown is to unplug the computer for one minute, then plug it in again and restart. Ran a utility, smcFanControl 2.6, and it shows the fan rpm as 1254, temp hovering around 102 -104 degrees F. (Is that normal?) But it will shut down even at 5 am when the house is cool, so don’t think it is overheating. Checked all previous posts here about random shut downs - the only similar entry was in 2015 in which the guy stated it was fixed but didn’t respond on how it was fixed. Any ideas will be gratefully accepted and tested!

 

[MacInWin]

Sounds like the power supply is going south. Could be overheating, or just about anything. If one component in the PS is overheating, that's all it takes.

 

[cxviking]

Thanks for the quick reply, MacInWin. I will check with our local Mac Repair shops to get an estimate on installing a new power supply. It is a frustrating problem even if I decide on a new iMac, since my spouse always inherits my older one, so need it fixed. She's using a Late 2009 model that is still going strong, but has reached its limit on OS upgrades. I'll continue to monitor this thread in case other ideas surface. Cheers, stay safe.

 

[MacInWin]

I wouldn't assume 100% that the PS is the issue, but the local shop can check it out for you. Let us know what they find.

 

[cxviking]

It's the best guesstimate so far - today was a 4 x shutdown day ... so far!

 

[chscag]

It's the best guesstimate so far - today was a 4 x shutdown day ... so far!

Can you hear the fan running in your iMac? Do you have an app that shows fan speed and temperatures? If you don't, my advice is to download and install one. Here's one that's free and will show the fan speed and temperature:

Macs Fan Control - control fans on Apple computers, also on Windows via Boot Camp

Control fans on Apple computers, also on Windows via Boot Camp. Monitoring of fan speed and temperature sensors. Solve fan noise and overheating problems, iMac HDD replacement

crystalidea.com

This will give you some idea if your iMac is overheating and shutting down or if it is in fact something wrong with the power supply. If the fan is not running in your machine, it will shut down.

 

[Randy B. Singer]

The computer shuts down randomly for no apparent reason.

Things that can cause this:
- Bad power supply
- Cold solder joint (this often mimics a bad power supply, but can be fixed with just the touch of a soldering pen)
- Overheating (often due to Mac being filled with fluffy dirt and needing to be vacuumed out, sometimes due to failing or failed fans)
- Large current draw on circuit (often due to a huge load cycling on, like a laser printer, air conditioner, or a refrigerator)

I've seen all of these. The most common being a cold solder joint. You can usually see one solder point on the motherboard that is black, and if you run your fingernail over it you can feel a crack. It's usually a solder joint holding in the power supply, so shops usually charge you to replace an entire power supply when it isn't needed, but that does solve the problem. DO NOT check this yourself. The power supply, even when your Mac is unplugged, holds enough of a charge to kill you!

It's best to go to a good repair shop that boasts that it will do "component level repairs." i.e. they don't just replace entire boards, they actually replace just the (inexpensive) component that is bad.

 

[cxviking]

Can you hear the fan running in your iMac? Do you have an app that shows fan speed and temperatures? If you don't, my advice is to download and install one. Here's one that's free and will show the fan speed and temperature:

Macs Fan Control - control fans on Apple computers, also on Windows via Boot Camp

Control fans on Apple computers, also on Windows via Boot Camp. Monitoring of fan speed and temperature sensors. Solve fan noise and overheating problems, iMac HDD replacement

crystalidea.com

This will give you some idea if your iMac is overheating and shutting down or if it is in fact something wrong with the power supply. If the fan is not running in your machine, it will shut down.

Thank you for the advice, chscag. I did have a fan control utility, but it wouldn't let me up the RPM. I downloaded the one at the link you supplied and it provides much more detail and does permit me to set the fan to a higher speed. The temps do drop from the lower 100s to the mid 90s. I'll monitor that for a while and see if it helps.

 

[cxviking]

Things that can cause this:
- Bad power supply
- Cold solder joint (this often mimics a bad power supply, but can be fixed with just the touch of a soldering pen)
- Overheating (often due to Mac being filled with fluffy dirt and needing to be vacuumed out, sometimes due to failing or failed fans)
- Large current draw on circuit (often due to a huge load cycling on, like a laser printer, air conditioner, or a refrigerator)

I've seen all of these. The most common being a cold solder joint. You can usually see one solder point on the motherboard that is black, and if you run your fingernail over it you can feel a crack. It's usually a solder joint holding in the power supply, so shops usually charge you to replace an entire power supply when it isn't needed, but that does solve the problem. DO NOT check this yourself. The power supply, even when your Mac is unplugged, holds enough of a charge to kill you!

It's best to go to a good repair shop that boasts that it will do "component level repairs." i.e. they don't just replace entire boards, they actually replace just the (inexpensive) component that is bad.

Thanks for the advice, Randy. Sounds technical to me but I will inquire at my repair shop to see what he is willing to do. No worries about me doing it myself, as I can't access the internal workings without the proper tools. Thanks to this great forum for giving me some direction to look at.

 

[Randy B. Singer]

My 27” iMac, OS (late 2012), OS10.13.6 (High Sierra) has a troubling problem. The computer shuts down randomly for no apparent reason.

One last suggestion!

Wet your fingers and run them along the bottom of the iMac's screen's bezel. There are air inlet ports there. Often doing this will clean away a huge dust bunny. Once that dust bunny is gone, your Mac might run quite a bit cooler.

 

[cxviking]

That may be the case, Randy. Today I brought it in to my tech guy and he is going to open it and give it a thorough cleaning, and try to see if it’s an overheat problem. He’s not one to do cold soldering - says all the soldering now is done by machine. Also hope it’s not the power supply because he says probably not one available for a Mac as old as this one. Before I brought it in, I ran the fan at 1600 rpm and it never powered down in 6 hours, so that sounds hopeful.

 

[lbsimon50]

If, as the OP says, the only way to restart a computer is to unplug the power cord, most likely the culprit is the power supply. Power supplies fail. They are fairly reliable, but not immortal.

 

[cxviking]

Status Update: First, thanks a million for all the suggestions and advice that was forthcoming when I stated my problem. Also, I have found the fan control utility to be a real boon and gives me a level of comfort when I can up the fan speed a bit as required.

My tech opened and thoroughly cleaned out the internal compartment of my iMac. Since back in my house, it has been running for the equivalent of two days now with nary a shutdown, temps now showing in the low 80s and low 90s, so I am hopefully optimistic that it was just dirt and overheating that was causing the problem. Only more time will tell for sure! Again, thanks all - this forum rocks!