iMac Late 2014 27" shutdown

[PsychoSync]

Hi

I have not-so-random shutdowns problem with my 2014 Retina 27" iMac. When i try to install a fresh install of Catalina from USB onto the internal Apple SSD, first stage of the install works fine then at the installation after the first reboot when the ETA bar is shown it will reach the end but i get kernel panic when it's done and it reboots by itself. Then i am stuck in this loop forever, ETA-->reboots-->ETA-->reboots......

Trying to install Windows 10 is pretty much the same, first sage of the install works fine then after it reboots and installs hardware stuff it reboots and then installation gets corrupted.

I searched for problems on these iMacs on the internet and random shutdowns problems seem frequent with these models. Most of the time the solution is to replace the power supply. There are 2 manufacturers for the PSU: Delta and LiteOn. If one is better than the other is not clear, the problem seems to happen with both anyway. So i purchached a LiteOn one from ebay thinking that i might have a Delta brand inside but it was a LiteOn inside my iMac. Anyway the new power supply did not change anything, still getting these annoying shutdowns. Either the new psu has the same defect or the problem is not the psu. Also, most of the posts i saw had to unplug the iMac from AC to do a SMC reset to be able to start the mac again, not me. The iMac just reboots out of nowhere and almost never gets to the desktop.

List of things i tried:
- Resetting the PRAM a million times.
- Resetting the SMC less than a million times but not far from it.
- Running the diagnostics: it found nothing wrong (can it find anything at all anyway...)
- Running EtreCheck. I see people using that often to troubleshoot. Did not find anything anyway.
- Re-installing OS from Internet Recovery. It installs but still gets stuck in a shutdown reboot loop with ETA progress bar...
- Removing the Apple SSD in case it is causing the issue. Did not help...
- Booting already installed OS (Big Sur beta, Catalina) from USB hard drive: gets to the desktop, no graphical glitches or anything weird, freezes after 1 min max and reboots.
- Booting in Safe Mode works and seems to last for a long time without locking up.

Now after replacing the psu and seeing that the iMac was still shutting down i thought that it must be a GPU failure or a bad capacitor feeding the gpu like the 2011 15" MacBook Pros. A lot of 2009-2011 iMacs have faulty gpus that freeze when initialized during boot sequence but these iMacs can still get to the desktop when booting in safe mode albeit really slow.

So one last thing i tried yesterday:

I booted in Safe Mode and quickly changed the resolution to 2560x1440 (low resolution not scaled) instead of the native 5120x2880 and rebooted normally. The iMac has been running fine for more than 2 hours yesterday and still works fine this morning without any shutdown issues. I ran Cinebench, Geekbench, Unigine Heaven (in 1080p) and it is stable. I pushed the iMac a little more by setting the resolution to 3200x1800 and ran Unigine Heaven windowed in 720p while watching temps and other sensors in iStats and it froze 1 min or so after starting the benchmark. So my best guess is that it really is a Power Supply problem and i just bought a used one that has the exact same problem.

Here are the stats just when it froze in case you can find something strange. PSU is rated for 300W, 174W should not be a problem...

 

[PsychoSync]

Attached kernel panic log.

 

[PsychoSync]

So i have determined that the problem in my case is not the power supply after all. I read a lot of posts from different sources of people replacing the PS and it solved the problem but a lot of them reported that it came back after a while. So before spending any more cash on another power supply i wanted to try an bypass it with a known good power source.

The iMac's power supply uses a 12 pins mini molex connector, the 6 pins one one side are ground, the other side all 12V. I had 2 mini molex connectors just like that that i used in a Mac Pro to feed a GPU. I made a custom cable that connects directly to the iMac's motherboard and fed 12V to it with a 700W PC power supply. I left the original power supply inside the iMac to be able to power it on, all of it's cables connected but not it's 12V output, the PC power supply is connected in it's place.

I fired up the iMac and changed the resolution the the max, did some tests but after 5 minutes it shut down again and again and again....

So looks like there is a problem with the Logic Board. It looks good, the 4 green leds light up, i can use it in a "lightweight" mode, must not push it too much. Even with the lowered resolution, if i leave it running idle, i leave for 30 minutes and when i come back i can see it has rebooted.

I am not going to bring that mac at Apple, it's gonna be too expensive. The problem must be related to some capacitor(s) failing? Maybe it's a simple component that needs to be replaced i'm just not savvy enough in electronics to figure it out by my self, i need Louis Rossman's brain, and tools, and time.

A good start would be to have the schematics though, and the pcb traces etc... Anyone knows where to find them? Searching on Google, i always end up on forums who want money.

 

[PsychoSync]

I replaced thermal paste on CPU and GPU + thermal pads today just to eliminate the possibility that it is overheating. Looking at temps with iStats nothing is overheating but just in case... It still freezes and reboots when wattage is higher than 130ish Watts but i am writing this at full 5k res right now. I'm gonna hold on to this iMac for a while until something comes up or i find some more info about what to check. I know a Logic Board swap would fix it but $$$.

 

[PsychoSync]

This thread is my private blog...?

 

[avz]

I have a similar issue with my 27" Late 2012 iMac. I bought it used 1 week ago and since that updated the firmware(by updating the macOS). After the firmware update the shutdowns became permanent. So it is possible that the firmware update might be the real issue and not the power supply. My firmware version is 292.0.0.0.0. I can't say for sure as I haven't used it long enough with the older Mountain Lion firmware but it definitely used to be rock solid.

 

[PsychoSync]

Thanks for your reply! I should look into that. The firmware is pretty much the only thing i haven't looked into yet. Yesterday i did a complete MemTest86 that took 2h15 minutes to complete! No Ram errors at all so the ram is good.

 

[avz]

Alright, I am typing this on my 27" Late 2012 iMac. I had to reinstall Mountain Lion(10.8.5) to make it usable again, as previously clean installed Mavericks(10.9.5) sent it to continuous restart loop without allowing me to do as little as set mouse preferences in System Preferences. This seems to be a very complex issue indeed as it matters what OS you are using. I actually like Mavericks and it is rock solid on my Late 2008 MacBook, but I can definitely see why some people had issues with it on some other Apple hardware. I will see how it goes with Mountain Lion and it is quite possible that I will have to leave it on Mountain Lion for good. In this case I hope that an updated firmware will provide me with better security, making updating firmware not an entirely futile exercise.

 

[robotica]

Have you tried to just put one stick of ram in at a time? Strange issues like this are often caused by faulty ram. Just put a single stick in and boot it to see what happens. If the same thing happens try the other stick on its own.

good creative problem solving with that psu hack!

 

[avz]

Have you tried to just put one stick of ram in at a time? Strange issues like this are often caused by faulty ram. Just put a single stick in and boot it to see what happens. If the same thing happens try the other stick on its own.

good creative problem solving with that psu hack!

You might be onto something here. Not necessary faulty RAM, but I noticed that some RAM modules are somewhat thinner than others. I took out 2X8GB DDR3 modules out of my Mid 2012 MacBook Pro and used them in an iMac. They worked absolutely fine in a MacBook Pro so I assumed they should be OK in an iMac. Now I removed them and using only 2X2GB DDR3 modules that originally came with my Mid 2012 MacBook Pro. They seem to sit nice and tight so fingers crossed.

 

[PsychoSync]

I will try just one stick of ram, sure why not but i'd be surprised if it helps at all. I already tested the ram for 2h with MemTest86 last night and no errors. Also, the ram that is installed right now is 2 sticks of 4GB that was installed in my MacBook Pro Mid-2012 before and never had any problems. I had the same problem with the original 2x4GB of ram that was installed when i bought the iMac.

I tried installing MacOS Big Sur Beta to get a new firmware update. The firmware has been updated from 235.0.0 to 426.0.0. The OS failed the installation though like any OS i tried installing. MacOS will only boot from pre-installed systems on this Mac. If i start a youtube 4K video, the iMac will freeze after 4-5 minutes. If i start a 3D intensive game, it will freeze after 4-5 minutes. If i just browse the web, listen to music on Spotify etc.. It will last for hours without failing.

I'll report back on the firmware update as soon as i see and improvement or not.

 

[USB3foriMac]

To me the problem sounds as if the GPU has some damage.
If you want to check for this, I suggest the following: Boot a live Linux Mint 20, and see whether that is working fine.
If not, I'd say you can confirm that there is a hardware issue. In fact after all the tests you have already done, you won't be able to fix it unless you change the logic board.
If it is working fine, Linux might have got around your issue as the GPU drivers are totally different. If you subsequently assume that the issue is software related, you could experiment with a fresh install, but if that results in the same problem it means that your hardware fault can't be tolerated by MacOS. I had such case before (Linux working, MacOS didn't), however with a different GPU and on a Mid 2011 where GPU could be replaced.
If Linux is working, it could be an alternative.

 

[PsychoSync]

Yeah, i am pretty sure there is some damage somewhere on the logic board like you said. But i am holding on to it just in case it is not the gpu chip but maybe a component feeding it that is damaged. Like the MacBook Pro 2010 that some models are failing due to a bad capacitor:

.

My guess is that if the gpu was failing i would probably see artifacts on the screen or the iMac would only boot in safe mode like the 2009-2011 gpu failures.

 

[theadder]

I had an issue with my late 2014 Retina iMac. It was shutting off and it seemed to be the GPU. This may not be helpful to you since you’ve already opened it, but you never know:

Late 2014 27” iMac Sleep/Power Off Issue

I have a Late 2014 27” Retina iMac (iMac15,1) with i5-4690 CPU, M295X GPU, and 32GB Crucial RAM. I have an issue where the iMac will go into a sleep state during use; rarely, the system powers off entirely and I have to turn it on again. The sleep/power off issue occurs during certain types...

forums.macrumors.com

 

[avz]

In my case it seems to be the RAM issue as my iMac is going strong without shutdowns for almost 3 hours. I can't say the RAM is faulty as it is working great in a MacBook Pro, but my iMac called BS and demanded respect(I believe that the origin of the "faulty RAM" is from the Windows laptop).