Late 2013 imac 27 no power

[jsg68]

Hoping somebody here can help with ideas on what might be wrong....

One line summary: imac spontaneously shut off while in use, won't power back up at all, doesn't seem to be the power supply, so what is it likely to be?

More details:

I have a Late 2013 iMac 27, which has been trouble-free since I bought it at the very end of 2014, until two weeks ago, when it spontaneously shut off while I was using it, and then could not be restarted. No noise or anything when it shut down, just the main display, and an external display connected to it, suddenly went black. For a moment I wondered if I could have moved the cursor into a hot corner that just shut down the display. (But I hadn't, of course.)

Pressing the power button on the back, either momentarily produced nothing -- no chime, no display, no fan -- no indication of anything at all, except a USB-connected superdrive would make a clicking noise after the power button was held for 10 seconds or so. So some indication of 5V USB power. (I subsequently discovered that the same click occurs if the drive is connected, and the iMac is first plugged into a power outlet, so apparently the drive is reacting to power that is present on USB whenever trickle power is present, even with the system off, and the power button depressed for 10 seconds must result in a momentary interruption of that power.)

Haven't worked on one of these before, and being a critical machine, preferred not to start experimenting. No appts available for a week at the local Apple Store, so I took it to Microcenter, listed as an Apple-authorized repair center. Long story short, after being strung along and lied to for 8 days, I finally gave up and went back to pick the machine up. (They hadn't even cracked the case.)

Looked online, found info on the four LEDs indicating power supply and motherboard functionality. Ordered a kit and opened it myself, found that just LED 1 was lit, indicating trickle power in an off state. Pressing the power button, long or short, did not produce any change. In particular, LED 2, indicating proper operating power to the motherboard never lit, flashed or anything. Figured that confirmed the power supply was the problem. (This testing done with the display removed and disconnected.)

Ordered a replacement power supply from iFixit (used -- only thing I could order and have shipped on an expedited basis). Got it today. Looks fine except that the plastic is broken/missing on one side of the bracket that receives the data ribbon cable. Don't think that's likely to be a problem, the cable still seems to seat ok. Anyway, I swapped the power supply, then before reinstalling the left speaker, reconnected the power button, plugged the unit in, and tried booting it. No change. Exactly the same symptom I saw previously: LED 1 indicating trickle power lights shortly after plugging the unit in. Pressing the power button, short or long, produces nothing. Plugging in the superdrive I get the same clicking behavior. (This is how I discovered the click occurs just from plugging the iMac into 120V power.)

I have not attempted to test for a bad power button, but the symptoms here suggest against that being a problem. The unit spontaneously lost power while in use--don't see how that could be a power button issue. There's something still happening when the button is depressed for ~10 seconds. And if I unplug the system from 120V, LED 1 stays lit until I press the power button, which indicates it's triggering discharge of a capacitor (if I understand correctly, that's expected behavior). So seems like the power button isn't the issue.

Seems unlikely that the power supply I received is bad and produces exactly the same failure as the original power supply.

So is there something beside the power supply that would seem likely to explain this failure, whereby the unit doesn't even get to the point of even trying to boot?

Very much appreciate any insights the experts here might have.

Jonathan

 

[MacInWin]

Jonathan, Welcome to the forum. Your diagnostics have been very careful. I would have suggested you reset the SMC, except that being off power is the reset process for an iMac. https://osxdaily.com/2010/03/24/when-and-how-to-reset-your-mac-system-management-controller-smc/

You might try a deliberate process to reset the SMC, following the directions in that article, but basically I think you've done that already. Can't hurt to give it another go.

I suggest this because I recent had the same symptom on a MBP where the fans started but it would not boot and an SMC reset rescued it.

 

[chscag]

Very detailed bit of troubleshooting. I would have thought also that the power supply was the source of the problem. That leaves the GPU and logic board itself as the possible culprit. That particular model iMac has had some history of having a failing or failed GPU which shuts it down. I'm not sure how you can test for that without swapping out the GPU daughter board for another. By the way, that board is on the backside of the logic board and requires removal of the logic board.

There are plenty of YouTube videos which show how to remove and repair the GPU. I found several with a quick Google Search.

However, if this is a production machine that you need for work, you might want to forego all that and replace the iMac with a later model.

I used to own that same machine and gave it to my son when I purchased a later model. He's still using it without any problems. (So far.)

 

[jsg68]

Hi Jake, thanks for your reply.

It's been two weeks, so I forgot to mention it, but I did google and find SMC reset as a possible solution, when the problem first occurred. Alas, it didn't help.

Just for good measure, I did just retry the "older imac" instructions in the article you referenced (press and hold the power button for 5 seconds, while the unit is unplugged from wall power), as I don't recall if the article I had previously found gave instructions beyond just unplugging for a while and then replugging, but not surprisingly, still to no avail. (The newer imac instructions I've definitely already done, repeatedly, and to the tune of having left the thing unplugged for more as much as 10 days! Lol)

Any other ideas for things that I might try?

 

[jsg68]

Very detailed bit of troubleshooting. I would have thought also that the power supply was the source of the problem. That leaves the GPU and logic board itself as the possible culprit. That particular model iMac has had some history of having a failing or failed GPU which shuts it down. I'm not sure how you can test for that without swapping out the GPU daughter board for another. By the way, that board is on the backside of the logic board and requires removal of the logic board.

I've seen a video for removing the logic board, so that seems manageable if it's required.

Does it make sense that the GPU could cause the logic board to misbehave to the point where LED 2 indicating proper operating power from the PS wouldn't illuminate?

If it were somehow the GPU, and that's on a daughterboard, then I suppose that if I pulled the daughterboard, I ought to be able to then see LED 2 light up when I try to power it on. I'm just having difficulty getting my head wrapped around how a CPU failure would cause the whole system to behave as if it had actually no power at all....

I probably should have mentioned up front, this unit has the 3.5GHz Quad-core Intel Core i7, and the NVIDIA GeForce GTX 780M 4GB GDDR5 GPU. Originally 16 GB RAM, I added another 16 GB about a year ago. (And another very early step I took...since it was trivially easy to do...but neglected to mention, was pulling RAM, just in case a RAM failure was somehow the issue.)

 

[chscag]

You might want to do some googling as to how others have dealt with the exact problem you're experiencing. A GPU failure can cause symptoms which sometimes make troubleshooting more difficult. Then again, it may not be the GPU which is the source. It might be the logic board itself as I pointed out above. You're dealing with a difficult repair regardless of what it turns out to be, so do as much research as you can.

Go to www.ifixit.com and see what they say.

 

[jsg68]

You might want to do some googling as to how others have dealt with the exact problem you're experiencing. A GPU failure can cause symptoms which sometimes make troubleshooting more difficult. Then again, it may not be the GPU which is the source. It might be the logic board itself as I pointed out above. You're dealing with a difficult repair regardless of what it turns out to be, so do as much research as you can.

Go to www.ifixit.com and see what they say.

Yeah, actually went there just a little while ago to post and, found a thread I had seen, and one I hadn't with screenshots from the Apple manual. Stepping through the steps there. Formally verified that the power button is working correctly. Next step is to pull the logic board and check the PRAM battery. (How nice--and idiotic!--it would be if it turned out to be that simple.)

 

[chscag]

Off hand, I don't believe a bad PRAM battery would cause a complete shutdown. You would likely get a bad date/time indication rather than a dead iMac. But some good experience in store for you pulling the logic board.

 

[jsg68]

Yeah, it would seem an unlikely cause to me, also, but it is step 18 in the Apple troubleshooter, and since it's a potentially easy fix, worth trying. (I also found one video where a professional repairer found it to be the source of the problem for a 2014 imac 21.5, albeit with slightly different symptoms (intermittent power off/inability to boot, with LED 1 and 2 lit).

Anyway, troubleshooter says to look for 2.7V or less. I measured 2.86V. Though labeled 3.0V, nominal on the BR2032 is allegedly 2.8V. So it does not seem that this is the problem. (I can't help but say, even if these are supposed to be 10+ year batteries, it seems insane to bury a part that will *eventually* need replacement so deep that literally the entire system needs to be disassembled to do so! I mean, why not place it on the other side of the board, where it would be easily accessible. Same for the flash drive, at that!)

I continued on to try resetting the real time clock via the reset pads, per the troubleshooter. None of it has made any difference.

Ftr, there's no visible indication of any kind of damage on the motherboard.

I've sent email to the PS supplier (iFixit) asking how long ago the unit would have been tested to confirm proper operation, and whether that testing would have included testing the damaged connector. (That ribbon cable is listed in the service manual as carrying the power on signal and PS temp sensor, and it indicates it can cause a no power symptom.)

Unless the replacement PS I got is dead also, I dunno what to think at this point.

Does anybody know where I can find pinout indicators for the DC power connector, in order to test it to confirm at least proper voltages are present?

 

[jasongoldworthy]

hi just my 2 cents here

the GPU is not removable on the 2013 iMac its soldered to the logic board.

if there is a damaged connector on the PSU or cable that may be why its not working can you upload an image of the damage ?

also one more thing it sounds to me like a bad PSU how many lights do you get on the logic board ?


also the voltages won't be all present on the PSU without the power on signal being called for.

 

[jsg68]

the GPU is not removable on the 2013 iMac its soldered to the logic board.

Yes, but from what I can find in terms of reports of GPU problems, they aren't consistent with this no-power-at-all symptom. The reports I've seen all talk about display issues.

(Interestingly, it sounds as if it would actually be possible for a pro to replace a GPU, because reportedly, the GPU problem is actually due to heat deformities damaging the GPU solder connections, and it can be resolved by having a pro re-solder the the connections.)

if there is a damaged connector on the PSU or cable that may be why its not working can you upload an image of the damage ?

I don't see any visual indication of damage, at all. Not on the power supply, not on the logic board, no where. Whatever has gone wrong seems to be invisible.

also one more thing it sounds to me like a bad PSU how many lights do you get on the logic board ?

Just one: LED 1 indicating trickle power. At this point, I've completed the entire troubleshooting procedure from the service manual for this condition (only LED 1 lit). Assuming the replacement power supply I received is actually functioning properly (and therefore, serves as a "known good power supply") there would seem to be no explanation left except for a problem with the logic board. (That's how the troubleshooter ends---if it's not be resolved, it's the logic board.)

also the voltages won't be all present on the PSU without the power on signal being called for.

Yeah, but it appears it may actually be just one pin that goes from inactive to active, distinguishing between on and off. I'm seeing 12V on about half the pins of both the original and replacement power supplies, which suggests to me that they probably are working. i.e., either they're working, or they both have some power-up related failure (e.g., not passing through the power button continuity signal, or not properly responding to the power on signal back from the logic board). Otherwise, it seems the problem must be on the logic board side.

 

[jasongoldworthy]

just one more thought here

have you checked the power on button ?


it may be that you have gone through all this and that's actually gone bad.

 

[jsg68]

have you checked the power on button ?

For the reasons I stated in my original post, it seemed clear the power button was working. But as I mentioned in my fourth post, I did formally verify the power button was working when I went through the troubleshooting process of the service manual. (Open when not pressed, closed when pressed.)

I haven't found any way to definitively confirm proper operation of the power supply, absent an otherwise working system to test it on. But ifixit sent a replacement supply in case the original they sent was bad, and that didn't help. (Unfortunately, they told me they just stock them, and weren't prepared to test it to confirm that it was working, before shipping.) Seems unlikely I would have gotten 2 bad power supplies from them, though. And all three of the power supplies test with 12 volts on 6 of the pins vs. a ground pin, while the unit is plugged into AC (but obviously off). My conclusion is, it must not be the power supply. And that would seem to imply it is the logic board.

I ordered a functioning-but-cracked-screen imac off ebay with the same logic board -- which was less expensive than buying just the logic board (!), and only a little more expensive than buying a step down logic board with same CPU and lesser GPU. Figured that should give me the required parts, no matter what, to resolve the problem. Lol. I assume it will be the logic board, though. And I can either keep the rest as spare parts, or maybe sell them on ebay to recoup some of the cost.

Should arrive this coming week. Will post when all is done, to close out as to what it turned out to be.

 

[jsg68]

My parts imac arrived today (earlier than expected), and I can now confirm that the problem was, indeed the logic board. Swapped it out and I'm now typing this message on my repaired imac. Now just have to hope it won't be too terribly soon that something else runs amiss.

I actually haven't sealed the display on yet -- just have it held with exterior tape, for the moment, since I wanted to see what DriveDX had to say about the internal disks (3TB Fusion). So far as it reports, both look quite good, and neither seems to be showing any signs of pre-failure, so I guess I'll probably go ahead and seal the unit up. Though the imac I bought turned out to have a 1 TB SSD (had been listed as HDD), so I'm _slightly_ tempted to go to the hassle of converting over to use that (+ the 3TB HDD in a non-Fusion config).

 

[chscag]

Thanks for the good news. Glad we were right about the Logic Board. That 1 TB SSD is tempting isn't it?

 

[jasongoldworthy]

My parts imac arrived today (earlier than expected), and I can now confirm that the problem was, indeed the logic board. Swapped it out and I'm now typing this message on my repaired imac. Now just have to hope it won't be too terribly soon that something else runs amiss.

I actually haven't sealed the display on yet -- just have it held with exterior tape, for the moment, since I wanted to see what DriveDX had to say about the internal disks (3TB Fusion). So far as it reports, both look quite good, and neither seems to be showing any signs of pre-failure, so I guess I'll probably go ahead and seal the unit up. Though the imac I bought turned out to have a 1 TB SSD (had been listed as HDD), so I'm _slightly_ tempted to go to the hassle of converting over to use that (+ the 3TB HDD in a non-Fusion config).

that's great news we are glad to here all is well

and yeah id say use the 1TB SSD and the 3TB HDD in a non fusion setup
but I think to get both drives to work in the system it would need to be a blade SSD
and the 3TB HDD

normally apple put in a 3TB HDD and a 64gb to 128gb blade SSD to make the fusion drive up (I may be wrong here slightly)
but im sure that's how it is meant to be

I have mine running on a BLADE ssd now instead of the fusion drive and it boots much much quicker and responds quicker.

thank god the machine is now alive.

its just a shame the old logic board had died ?

 

[harryb2448]

Late 2013 came with the 128GB blade and whatever size platter you or the original purchaser ordered. As the speed of the blade is limited in that model, if you are to buy another blade, make sure the speed is okay. When I had that model always ran it with the fusion drive split and ther blade drive was very fast.

 

[jsg68]

It's a 1TB blade. As I recall, you could purchase the Late 2013 27" model with just an HDD (1 or 3TB), just an SSD (probably 256, 512, or 1TB), or a Fusion drive (128 SSD + 1 or 3 TB HDD). I presume the parts imac I bought was originally an SSD-only config, given the lack of a SATA cable and HDD bracket. Also assume that the drive is the original. It's a Samsung MZ-KPU1T0T/0A6, which appears to be a component that Apple did use.

I've decided to go ahead and take a shot at converting to a 1TB SSD + 3TB HDD non-fusion config, both for performance and so that if/when one disk dies, it's not a situation where I need to restore everything--especially since looking at DriveDX stats, it seems likely the SSD would outlive the HDD. i.e., if the HDD dies, and they're separate, I'd still have a working system until I get what I need to replace the HDD.

I just wish that I had taken the time to install DriveDX before I tore down the parts imac, to confirm that its SSD isn't showing any warning signs. It'll be a bummer if I do the work necessary to restructure my current drive so the boot drive can fit in the 1TB SSD, tear the machine apart to make the swap, restore the drive from backup, and *then* find out that the 1TB SSD looks questionable in terms of its remaining lifespan. Lol

 

[jasongoldworthy]

normally the life on an SSD blade should be longer than a spinner drive
so don't worry about installing DriveDX before you tried it

do so after the install just don't stick the screen back if you can help it ?

and then check it ?

 

[jsg68]

Agreed, it's not terribly likely to be a problem, but who knows what the last guy did with the drive, and it was trivially easy to do as a test, before taking the time consuming step of restoring my boot drive image to the new SSD via Time Machine, and the destructive step of reformatting and restoring data to the 3TB HDD.

Everything seems to be fine. DriveDX summarizes its overall health and lifetime indicator at 100%. In more detailed stats it shows Power on hours at 92%, power cycle count at 97%, and wear leveling count at 90%. This last one it comments often drops to 92-96% in the first week, and then remains steady for a long time. In any case, doesn't seem alarming for a system that was probably in use for 6-7 years.

And yeah, the strategy was precisely not to permanently re-affix the screen until all was good. I just externally taped it in place after replacing the logic board, and did the same after the drive swap. Sometime soon will get around to re-affixing it properly.