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Resolved 800mhz and 667mhz RAM not compatible Mac Pro 3,1?

NOTNlCE

NOTNlCE

macrumors 65816
Original poster
Oct 11, 2013
1,087
478
Baltimore, MD
Hi all, I recently have come to own an Early 2008 Mac Pro Quad Core. I've been in the process of finding cheap upgrades, and a friend of mine had some 667mhz DDR2 FB-DIMMs lying around. The RAM that came with the Mac was 6 gigs of 800mhz RAM, and I have since put 8 more gigs of 667mhz RAM in the bays in several different configurations. Regardless, the Mac won't see more than 8 gigs of RAM even though a total of 14 gigs are installed. It tends to prioritize the 667s, though on rare occasion (depending on configuration) the 800s will be prioritized. Is there a way to pull a workaround here so that I can use all 14 gigs? Apple Support told me to just stick with the 800mhz FB-DIMMs, but 6 gigs isn't that much. I've been using the 8 gigs of 667 just for the extra 2 gigs, but even 8 isn't cutting it. Ideally, I don't want to pay $200+ for more 800mhz RAM. Any help is appreciated, thank you in advance.

EDIT: Trying to be more descriptive:
When using the 667 DIMMs, the 800s are not even seen, and vice versa. The computer sees the slots as empty.
-N
 
Last edited:
NOTNlCE said:
Hi all, I recently have come to own an Early 2008 Mac Pro Quad Core. I've been in the process of finding cheap upgrades, and a friend of mine had some 667mhz DDR2 FB-DIMMs lying around. The RAM that came with the Mac was 6 gigs of 800mhz RAM, and I have since put 8 more gigs of 667mhz RAM in the bays in several different configurations. Regardless, the Mac won't see more than 8 gigs of RAM even though a total of 14 gigs are installed. It tends to prioritize the 667s, though on rare occasion (depending on configuration) the 800s will be prioritized. Is there a way to pull a workaround here so that I can use all 14 gigs? Apple Support told me to just stick with the 800mhz FB-DIMMs, but 6 gigs isn't that much. I've been using the 8 gigs of 667 just for the extra 2 gigs, but even 8 isn't cutting it. Ideally, I don't want to pay $200+ for more 800mhz RAM. Any help is appreciated, thank you in advance.

EDIT: Trying to be more descriptive:
When using the 667 DIMMs, the 800s are not even seen, and vice versa. The computer sees the slots as empty.
-N
Click to expand...

I'm not sure if they will work together at all as you've observed so far. Even if they did, the 800Mhz ones would run at 667Mhz most likely.

How many different variations of memory configs have you tried? Only asking as even with matching speeds it's finicky. Try mixing 800 and 667 on same trays, both top and bottom.

If you can't get them to work together, stick to the 800s only, install Mavericks, and you'll see some better performance due to memory compression and other fancy stuff.

EDIT: Can you give an exact number of sticks and their sizes for each speed?
EDIT2: I'm making assumptions here but seems like you have 2x2GB+2x1GB of 800Mhz and perhaps 4x2GB of 667Mhz? If so, try using only the 2GB sticks with various configs to try for 12GB instead. Trust me. Took me a while to get my 12GB going (all 667Mhz of various sizes).
 
Last edited:
In addition to what Josh said, you must have each bank with matched pairs of memory. Each bank must be the same or you will have issues.
 
If you do have any of the riser LEDs red, it may not be the riser itself. I thought so in the past, and Apple shipped a replacement.

It turned out to be a faulty memory board instead. Be sure to check System information to see which FB-DIMM is not being recognized. This will help you track down which modules are the issue.
 
My friend has a 2008 Mac Pro, he mixed 800mhz with 667mhz and his riser A card broke down.
 
Macsonic said:
My friend has a 2008 Mac Pro, he mixed 800mhz with 667mhz and his riser A card broke down.
Click to expand...

Just a co-incidence. It can't break it. It'll just run at 667 instead of 800. Improper riser card or memory handling can though if you don't ground yourself properly.
 
Thanks for all the info, I will check all of these later today and get back to you guys. Josh, you are correct, it is 2x1GB and 2x2GB DIMMS of 800, and 4x2GB 667s. I have tried mixing them in every combination, including non-paired configurations, and it never sees more than 8GB. I believe the riser is alright, I will check when I get home today. Again, thanks for all the help.
-N
 
MacinJosh said:
Just a co-incidence. It can't break it. It'll just run at 667 instead of 800. Improper riser card or memory handling can though if you don't ground yourself properly.
Click to expand...

Second that. 2008 risers are bit more prone to failures than 2006 ones.
OP: what rank density are 667 and 800 ones? E.g. 1Rx8, 2Rx4 and so on. It should be printed on stickers.
Post pics of these stickers if you can.
 
PROBLEM SOLVED! It appears that the bottom riser had an extreme amount of dust hidden behind it, thus breaking contact in certain areas with the logic board. If anyone has this problem, my solution was to install the 667 DIMMs as the FIRST two pairs of DIMMs so the system started a 667mhz bus instead of trying to run the 667s at 800, which was why I was getting red light errors. As of now, I have all 14 gigs up and running, thank you all so much for your input and help.
-N
 
NOTNlCE said:
PROBLEM SOLVED! It appears that the bottom riser had an extreme amount of dust hidden behind it, thus breaking contact in certain areas with the logic board. If anyone has this problem, my solution was to install the 667 DIMMs as the FIRST two pairs of DIMMs so the system started a 667mhz bus instead of trying to run the 667s at 800, which was why I was getting red light errors. As of now, I have all 14 gigs up and running, thank you all so much for your input and help.
-N
Click to expand...

Sweet! Good job. I think we all got some valuable info from this thread.
 
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