Precision 7920 does not POST after Memory Upgrade

Not the most likely solution, but try to blow out the memory slots with an air duster / can of compressed air. Sometimes dust gets in the memory slot and prevents it from communicating correctly.

Check the manual on how your 8x 23GB memory modules should be slotted.

https://www.dell.com/support/manuals/en-nl/oth-xlt7920/precision_7920_om_pub/memory-configuration?guid=guid-ce1561c8-88a9-45e3-bc3b-762cbb37e073&lang=en-us

Remove OEM modules and test with new memory modules only to see if the system will accept them.  Then OEM can be added back.

Memory configuration with DIMM slot locations for Precision 7920 tower.

https://www.dell.com/support/manuals/en-us/oth-xlt7920/precision_7920_om_pub/memory-configuration?guid=guid-ce1561c8-88a9-45e3-bc3b-762cbb37e073&lang=en-us

@TheQuickFox​ Thanks for quick reply. I don't think it is dust. Always blow before install. The "wiggeling technique" doesn't help either. Also, I've put them in exactly like the manual. That is 1, 2, 3 and 4 on each CPU. Double checked and tripple checked.

Tried now booting with five modules, which not particularly surprising doesn't work.

Btw, how long does the first boot with memory check supposed to take after installing 256 GB? After how many minutes can I call it a failiure? It is very stupid design with no feedback.

@Chino de Oro​ I've done that. Tried all the modules pairwise. There is nothing wrong with the new memories and the system accepts them individually. They all have the same part number.

I've now managed to get up to seven modules installed on the mobo and boot into windows (after a lot of trail and fails), but after adding the eight it won't boot. Period. I also tried adding seven, boot, shutdown, replace one module, boot, shutdown and then use the module that the system just booted with in the eight slot, only to fail.

I reckon either the socket or the CPU is deffective!?

Otherwise, this system is superwired. Did a full diagnostics with memory test (6 modules) with a successfull result and no errors. Awesome, except when I close the test, the computer refused to boot up again. Had to cut the power and do a couple of power cycles , before it came back up. It seems to be totally random.

Also in CPU-Z don't report timings table for the modules. Is this normal?

I have the exact same system and a similar problem. I am able to populate 6x16GB in all of the white slots (total 192GB) and the system boots up and works. The minute I start populating the black slots (first to 256GB, then to 384GB), as indicated in the manual. It does not boot past POST. Weird things happen. The power LED was white for a while without POST, and then suddenly all the fans came on like a blower. I let it sit for a bit, but am wondering what the deal is?

Update. Took out all the memory modules and re-seated them one-by-one, carefully. Rebooted. It eventually showed the DELL Logo. I was able to confirm that 24 16GB modules are all working and 12 channels are enabled. Has to be my sloppy hands seating some of the DIMMs incorrectly.

The way this system deals with memory problems is absolutely abysmal and perhaps the single most terrible way any manufacturer has implemented dealing with such issues. I have a Dual 6140 which has given me incessant, repeated but varied grief when it comes to starting up, how long the boot might take THIS time, whether the LED will indicate a problem... or not, etc. etc. Here is a list of issues that may or may not come to pass as you merely just want to start USING your $5000+ computer:

- Ages to get a screen with DELL logo showing
- Ages for the blue bar to "finish finishing" and letting OS boot
- Stuck/frozen not accepting KB inputs ad aeternam
- Delayed switching on or off of the NumLock key (10-15 seconds after pressing)
- Not indicating F12 was pressed, or being "too late" to do so, even though the message shows
- Just not showing you anything. Sittin' there, chillin'...
- Booting into some test instead (fan test is NOT ram related!)
- Showing some led codes (x White + y Amber)
- Giving NO data on what DIMM is the offender (yes, I enabled all the Bios logging!)
- Having NO LED's on the motherboard to indicate issues
- WHEA errors in the Event Log (needless to say, not telling you module number)
- The DIMMs working fine in a HPE DL Gen9 server (and that one tests well!)

A spring loaded sausage with a Peruvian bee inside would give me more data on what the problem might be than this thing does. But fear not! It DOES have at least a dozen sensors to annoy the bejeezus out of you when one fan is missing! Coz THAT'S important!

Came across this thread after many hours of troubleshooting. I'm currently trying some of the suggestions here that I have not currently attempted. I've been trying to get my system to come back up for 2 days now after adding a second CPU and a small amount of RAM. The one bright side I see is I didn't spend 5k+ on the 7920, and only wasted 1k on the massive paperweight. 

Wish I would have known this before purchase.

@MrServer  I think you summed up everything I would want to say. If anyone comes across these BEFORE buying, save yourself the trouble, and do not buy these. Nothing is worth this hassle. 

@jdhensley Cheers to that - let me add a bit more nuance to my struggles so it is an accurate reflection of the facts. It just so happens that in the last month I moved around a lot and so the computer had to be packed up and rearranged a few times. Every single time after setting my gear up at the new place there were problems. To be clear - I am extremely careful with my gear; I've flown EU-AUS 4 times with a camera bag full of HDD drives, never had a single failure (but a lot of questions and tests for explosive residue at customs each time...) so, rest assured the issues were not due to rough handling...

Mitigating factors beyond DELL's control
If you look at eBay, there are quite a few socket 3647 CPUs that are sold as half-defective with 1 or 2 non-functional memory channels. I suspect this series of processors is particularly sensitive when it comes to the memory used. The super fidgety and fragile CPU socket is not helping things either - so far I've done about a dozen CPU replacements (on SuperMicro, HPE and Dell) and though for me they all went fine it is easy to imagine the smallest mistake like having unsteady hands, inserting the CPU the wrong way around (WHY is pin 1 so vaguely indicated!?) or the flimsy plastic clip not fitting smugly resulting in terminal damage to either the socket or the chip. Frankly, it's a miracle it even works at all.

Guesswork: Hopeless Diagnostics
After 3-4 instances of me nearly tossing the damn thing out of the window, it was revealed that I had a single 32GB PC4-2933 Hynix RAM module that exhibited correctable ECC errors. This crucial bit of information became known when, at some point, after the 4th move and it again giving me led codes (with nothing changed after power down at the previous place!) the 7920 spontaneously booted into a RAM test. Hallelujah! CPU0_DIMM2 it said. Well I'll be lost...

Nothing in the DELL manual or documentation suggests, indicates or recommends doing a RAM test when having to deal with the problems mentioned in my previous post... you just gotta get lucky.

The Supreme Source of Confusion is that, apparently, WHEA/single bit correctable ECC errors only start to happen after a while; I had seen WHEA errors in Windows Event Log and swapped 4 out of 8 of the DIMMs and for the first day or two no new WHEA errors were logged.  All is well now, I thought. Alas, it was the "wrong" 50% of DIMMs. So I replaced the other set of 4 Hynix modules with 4x 32GB Samsung PC4-2666 and voila, Calculus, the God of Computation hath blessed this cursed device and allowed it to finally boot like a normal PC. No more WHEA errors, it's been 2 weeks now.

Extremely limited RAM options
The list of permitted memory configurations for this system (and the 7820, don't even get me started on THAT thing with it additional CPU board!) is very small. Forget using DIMMs of more than one size. Ye shall limit thyself to 4x, 8x or 12x or 16x modules.  You can't add 48GB per channel, nor replace 1x 32 with 2x16GB for one socket/side. I'm almost amazed it now works with 4x PC4-2666 and 4x PC4-2933 32GB from two (!) different manufacturers! It's a Miracle, I tell ya!

When I had the audacity to add another 4x 32GB (matching one of the other sets of 4) showing a screen was no longer a feature that was available. I strongly recommend you just stick with 32GB modules though - other humans have had assorted problems with installing more than 12 16GB sticks (and once you HAVE 12/16 DIMMS, the only option to upgrade is to either replace ALL modules by bigger ones, or installing another 12/16, which will duly give you plenty of junk!)

Once the damn thing actually works... it works well. It's just that the diagnostics are WOEFULLY inadequate. Pesky. Petty. Temperamental. Vague. Uninformative. Prone to one making random guesses, or contemplating ritual disembowelment.

Here is my experience:
I am moving my stuff from an older Proliant to this 7920 and I pulled out four 32GB PC-2400 modules from the Proliant and installed it in the 7920. I was expecting trouble! But it just worked immediately, and the 7920 is showing 128GB atm.
My point is that not all is bad with how the 7920 handles memory.

@MrServer​ Thanks for your insight. 

@fl00blab​ Kudo's to you. Very glad to hear it just worked for you. 

Thanks for replying to you both. I apologize for not coming back after that to update.

In the end, after a few weeks of troubleshooting, I did get mine to post with an increased amount of ram, and the second CPU. I had to purchase a few different RAM types from ebay. I did come across some that worked. (I bought the original and matching ram that was in the system when I bought the 7920, and that didn't work) Taking it from the 32G to 128G and Duel Xeon Gold 6138 CPU's. I plan on buying a few Platinum's at some point, guess I'm a glutton for pain. 

On a final note, after my experience, and the ones listed here, logic suggests that what you get is a mixed bag. After the initial purchase price of 1k, I threw another few hundred at upgrades, all in I'm about a month of time to troubleshoot different CPU/GPU and RAM combinations returning non-working items. This is not to say the items were defective, they just didn't work in my system.

With all that said, take from this what you can. now that it's up and working, I find it to be a decent platform, and does what I intended it to do. Is it worth it to buy a lower base system and run the risk of possible upgrade frustrations? Or to purchase a working system that has what you want out of the gate? I'll leave it up to the reader. At this point I would say that my first criticisms were a bit harsh. 

@jdhensley​ Thanks for your reply! I have a little more information now after more testing. I tried inserting another 128GB of the sticks I had in my Proliant, but then the Dell did not start.
I noticed that 2 och these sticks where HP smart memory, so I removed those and then the Dell started with a total of 192GB Ram.
A thing I also noticed is that when inserting the AC plug in the Dell after working on it, it starts and does what I think are some self tests, and if everything is in order it turns off again after maybe 30 second. Then I can press the power button and it will start. If it does not turn off after inserting the AC plug, something is wrong.

Yeah so I have an update as new facts have come to light, y'all. The long wait during boot appears to be connected with PCI-Express settings (Bifurcation, for instance). There is also a tentative suggestion that enabling Thunderbolt support introduces a delay, though that is a mere educated guess.

How did I come to find out you ask..? Well - one bad day my SoundBlaster Z SE decided to seemingly kick ze bucket (but only in THIS computer, of course, and not by dying but by... well.... making crackling sounds only) so I acquired a ASUS Sonar AE. Wonderful card, except... when inserted, the PERC 730 I got in this 7920 decided to take a holiday. And one M.2 SSD disappeared... Nr. 2 on the 4-port card. Yes, the Dell one. Mind you, a 4-port non-Dell one costing but a fraction of the Dell version DID show both drives... though still gave me no dice on the RAID card.

Swapping various cards around as I cursed this hunk of intransigent calculator apparatus resulted in various combinations of cards working... though the combo of RAID card, 2nd Kioxia SSD and Xonar never did rise again together. Either this, that, or none. Tried upper slots, 8x for RAID card swapped with 16X - nothing made a difference so far. The Xonar and PERC 730 just don't get along.

I did not yet change the PCI Express advanced settings though if I read those correctly this is not to configure lanes/bandwidth but rather Power/Watts, so that would almost certainly make no difference anyhow.

BUT - Booting happened much quicker in some configurations. It DOES, however, appear that powering down the machine (as in - removing the power cord) to swap cards is not just "best practice" - it may be REQUIRED for the damn thing to understand what's going on and/or correctly initialize components. Without it, the no doubt "intelligent" firmware is swimming in confusion, probably.

All this on the newest bios (2.45) by the way. Config is Dual 6140 256GB, A2000 6GB 2x4TB Analog HDD, 2x 1TB NVMe M.2 Kioxia Ultra Pro... whatever on one of them Dell 4-post M.2 adapters (the one with the fan), 2x 960GB SATA Kioxia plus 4x Micron 960GB Enterprise disks (from HP ML350 G10) served by a Dell 730 RAID card. Ah, yes and a bunch of monitors.