2008 Mac Pro only recognizes 24 of 32GB ram

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Level 1

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2008 Mac Pro only recognizes 24 of 32GB ram

Hello All,

I realize this is an ancient machine, but please bear with me.

This machine is basically a file share/plex server, and had until recently 8x1GB 800mhz hynix ram. All slots were occupied.

I was given some PC2-5300f Kingston ram from a known working server decom. This ram has the big fat heat sinks on it, and every piece has been tested and recognized in this exact 2008 Mac Pro.

The issue is when all 8DIMM's are installed, only 24GB of ram is detected. The OS detects 6 DIMM's leaving card2 slot 3 and 4 "empty".

IF I put 2 of the 1GB dimms in these two slots, the OS will recognize 26GB ram. I've reset the PRAM and SMC, and nothing has changed. From what I've read everywhere, the system should support 32GB ram in total.

Is there a power limitation, or any other limitation to this system that would not allow it to recognize more than the 26GB of ram?

I know it's antiquated, but perfectly suitable for it's current role. I would like to replace it, but it works perfectly except for the ram issue. I "can" live with 24GB of ram, I was just hoping for the entire 32 to be usable.

Regards,

Chance

Posted on Jan 15, 2019 6:22 AM

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Answer 1

Top-ranking reply

Grant Bennet-Alder

Level 10

132,949 points

Jan 15, 2019 11:03 AM in response to t4thfavor

The lights are a diagnostic AID -- they are Not a definitive indication of which modules are bad.

As indicated in the User Tip cited above, which modules are good or bad is much more accurately indicated by

About this mac> (System Report) > Memory:

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Answer 2

Jan 15, 2019 7:08 AM in response to t4thfavor

The error correction hardware is used very aggressively at Startup. ANY error, correctible or not, causes the associated slots to be declared "empty" and they will not be used by MacOS. These modules are BAD. But on subsequent startups, they will be tested again and may then be used by the system. This does not change the fact that such modules are BAD.

from my User Tip:

User Tip: Mac Pro and Error Correcting Co… - Apple Community

This Mac uses paired modules, and if one of a pair is bad, the other module cannot be used alone.

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Answer 3

t4thfavor Author

Level 1

9 points

Jan 15, 2019 7:52 AM in response to Grant Bennet-Alder

So the modules that I put in 2 at a time, all of which booted are bad? between reboots it seems that the empty modules might move, or they might not. I'll run the memory tester overnight and see which ones give up the ghost.

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Answer 4

Jan 15, 2019 8:01 AM in response to t4thfavor

<<"So the modules that I put in 2 at a time, all of which booted are bad?">>

No, that is not right.

The modules that showed up in slots are working.

Among those whose slots now show "Empty", one OR the other (OR both) of a matched pair may be Bad.

If you prefer, you can say marginal, and write it off to aggressive use of the Error Correction hardware. But do you really want modules that make errors installed in your Mac? Woking modules do not make errors unless your Mac has gotten unusually HOT in an error state.

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Answer 5

t4thfavor Author

Level 1

9 points

Jan 15, 2019 8:43 AM in response to Grant Bennet-Alder

OK, that makes sense. I'll put 4 in at a time and see which two are consistently bad. There's about 100 dimms available in the pool, so I suppose I should be able to find some good ones in the pile.

For clarity, would the red light illuminate on the riser card to indicate which modules did not pass the test, or is it a silent error?

Thanks for the help!

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Answer 6

Jan 16, 2019 5:22 AM in response to t4thfavor

For what its worth the Mac Pro 2008 aka MacPro3,1 like other generations comes in single and dual CPU variations giving four or eight cores. Unlike say the MacPro4,1 and 5,1 there is only one maximum memory scenario which is as per the original post 32GB.

So, it is not affected by having a single CPU vs a dual CPU model, nor by the speed of the CPU chips.

So, yes you should be able to achieve 32GB with good RAM.

The official RAM specification is PC6400 DDR2 ECC, Min. RAM Speed:800 MHz. As you list using Kingston PC2-5300f the RAM you are trying does not appear to meet the official requirements. Apart from anything else the Kingston RAM you are using is only 667MHz.

Note: The official Apple limit is 32GB for all Mac Pro 2008 models. However like newer Mac Pro models it has been discovered that it is possible to actually achieve more than the original Apple limit and in this case it is therefore possible to achieve 64GB. See - https://blog.macsales.com/20473-owc-8gb-maxram-certified-fb-dimms-for-apple-mac-pro-2008-macpro31-unleashes-new-performance-capabilities-with-up-double-the-factory-memory

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