Mac Pro 3,1 Not Seeing All RAM

[strwrsfrk]

So I'm having an interesting problem. My Mac Pro 3,1 is not seeing all 8 sticks of 2GB RAM. DDR2-667 ECC, etc. All sticks are identical.

If all sticks are installed, or if the sticks are in just about any combination, the machine only sees 4GB.

If the RAM is installed in slot A, C, A, and C (in both risers, respectively), the machine sees all 8 GB.

Every sticks is identical, and every stick works as long as it is in one of these combinations. I've reset the PRAM, SMC, run Memtest, checked the diagnostic LEDs on the logic board and the RAM risers, installed RAM in just about every configuration possible, and this is always the case. Anyone have any ideas?

 

[kjdare]

Same problem here with MacPro 3.1

I have exact same issue. Thought it was faulty RAM so bought new pair of 2GB (mac specs of course!). And riser B wouldn't read that either, however, when running from 1 pair (1 chip in riser A and 1 chip in riser B) mac could see both pairs.
I also ran hardware check and the system profiler in that could read all the RAM correctly. Weird HUH!
Am currently running 2 pairs of 2GB in riser A and just the 1GB pair in Riser B (which still doesn't read) but at least I feel more comfortable as somewhere near conforming to RAM recommended alignment specs!

As I could see all the RAM when in hardware check, I wondered whether it may be a drivers issue in Snow Leopard?! but couldn't find anyway of updating drivers like you can on a PC.
Anyone got any ideas?

 

[kjdare]

The more I think about this ....

and noticing the date of your post, I think this may be an issue from the last patch 10.6.8 released by Mac, which would have been around June 2012.
I'm going to try shifting one of the 1GB RAM chips from riser slot B to riser slot C and see if the MAC Pro starts reading the extra 2 GB.
Another thing I'm considering (by way of confirming that it's the update that's the problem) is to install Snow Leopard again on another drive and just not doing the update to see if I can read all the RAM that way. If I do the latter, it won't be for a week or so, but I will post an update when I do.