MP 1,1-5,1 Bad Memory Riser? Bad Logic board? Mac Pro 2008 (3,1)


macrumors newbie
Original poster
Jan 14, 2020
Hello. Brand new here. I don't know if I'm experiencing a bad Memory Riser (B) or bad Logic Board. I had 6gb (2x2gb and 2x1gb) of RAM since I received the machine back in 2013. It's been completely fine until just before the end of 2019. My fianceé bought me 16gb (4x4gb) for Christmas and I immediately tried to install it. Upon turning the machine on, it's only reading the RAM on Riser A. I've done troubleshooting by switching out any matched pairs of RAM. I've tried the 2x1gb, 2x2gb and 2x4gb in both Riser A and Riser B. I've determined that the 2x2gb sticks aren't working. Both 2x1gb and 2x4gb only work on Riser A and NOT on Riser B, leading me to believe it's a bad Riser B card. THEN, I switched the Riser cards to opposite slots. A to B and B to A. Same result. B slot wasn't reading, NOW leading me to believe it could be the logic board. This machine has been nothing but an incredible workhorse and I'm at a loss as to the approach I should take next. I am not circuit savvy so troubleshooting with a volt meter or anything is out of my wheelhouse. I have seen logic boards for pretty cheap but I'm not sure I need that, yet. Any help is GREATLY appreciated!

I know that RAM is supposed to be installed in matched pairs. I know if I only have 2x4gb sticks, one should be installed in DIMM1-A and the other in DIMM1-B. In my case, I have 4x4gb sticks and 2x1gb sticks. In that sense, I need to install one matched 2x4gb pair in DIMM1-A / DIMM2-A, one matched 2x4gb pair in DIMM1 / DIMM2-B, and the last matched 2x1gb pair in DIMM3 / DIMM4-A, equalling 18gb of RAM. Since my Riser B doesn't seem to respond, can I only have 1x4gb stick installed in Riser A, because I can't install the match of that pair in Riser B? Or, is it safe to install 4x4gb sticks all in Riser A? HEEELLLPPP!!!!

Snow Tiger

macrumors 6502a
Dec 18, 2019
Remove all the memory modules from both risers . Install only all four of the new 4GB modules in DIMM slots 1 and 2 of both risers ( the slots furthest away from you when the risers are installed in the Mac . )

Let us know what happens . DDR2 DIMMs often fail due to thermal fatigue and I trust your brand new modules more at this time .

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