Question about 1333MHz ram in Mac Pro 5,1

[w8ing4intelmacs]

I upgraded the CPU on my Mac Pro to a X5680 (Westmere 3.33 Hexcore).

My two 8GB sticks run at 1333MHz fine, but my 16GB stick only runs at 1066MHz (all are RDIMMs). I've tried it with all three sticks installed and with the 16GB only. I've also tried the 16GB stick in each slot. In the first three slots, it ran at 1066MHz. But interestingly enough, the 16GB installed in the 4th slot wouldn't even start up (blinking power light).

I've been assured that the memory is 1333MHz. Just wondering if I'm doing something wrong or missing something.

Part number: Black Diamond M-1333TER-16384BD26.

(they all ran at 1066MHz with the original Nehalem W3530)

 

[Inutopia]

You need to reset PRAM after a CPU swap for the faster RAM to work at the faster speed, at least that's what I found.

 

[w8ing4intelmacs]

I tried resetting the PRAM and the SMC. The 8GB sticks work fine. Still seeing 1066 on the 16GB stick.

 

[w8ing4intelmacs]

Ok. So, I tried the two 8GB sticks and a different 16GB stick, all of which run at 1333MHz. But this one particular 16GB stick runs at 1066MHz.

 

[Inutopia]

Well I think that's your answer then.

 

[flowrider]

But interestingly enough, the 16GB installed in the 4th slot wouldn't even start up (blinking power light).

Not interesting at all. All the CPUs in a 4,1 or 5,1 Mac Pro are three channel. The third channel uses RAM populated in slots 3 and 4. Slots 3 and 4 are interconnected and if you only populate slot 4, you get NADA!

Lou