Mac Pro Wont See Ram!

Discussion in 'Mac Pro' started by HarryJoaquin, Nov 14, 2008.

  1. HarryJoaquin macrumors newbie

    Nov 14, 2008
    Please Help!

    Iv just got an extra 4GB of ram for a MacPro 8-core (2x4GB chips on top of the standard 4x1GB chips) I can get 6GB to work but 2 wont show up. All of the chips work when i change them about but not the last 2GB, the slots show as "empty".
    I looked up to see if you can mix sizes, seems that you can as long as you keep them in pairs on the risers.

    Any ideas?


    thanks !
  2. Pressure macrumors 68040


    May 30, 2006
    The answer depends on whether you have the early 2008 Mac Pro or the original.

    If you have the new early 2008 Mac Pro you have to place all the 1GB modules in one riser and the 2x4GB or 2x2GB modules in the other (sorry, have a hard time seeing if you mean 4GB in total as in 2x2GB or 8GB as in 2x4GB).
  3. Luis Ortega macrumors 6502a

    May 10, 2007
    Fetcham Surrey UK
    I have the same problem and apparently two of my slots are faulty on one memory riser.
    I am going to have to get the riser replaced.
  4. sash macrumors 6502a


    Nov 23, 2004

    Have you check OWC guides yet? They're quite helpful:

    And here are their installation videos:

    I had more or less same issue with RAM I've bought for MP. I had to remove and reseat new modules several times, before they get recognised. That's pretty common on MP.

  5. pwn247 macrumors 6502


    Aug 30, 2008
    West Virginia, USA
    Never had a problem with my Mac Pro (1,1).

    In the top riser, I have 2x512MB sticks, and on the bottom riser, I have 2x1GB sticks. Both sets are different brands (however they're both the same 667MHz).

    But I know that sometimes re-seating the modules, or swapping the ports can help the MP recognize them. Also, make sure that they're in the order of the modules. For example, don't fill up slot #4 if you have nothing in slots 1, 2, and 3. ;) They're labeled by little numbers (so get your eyeglasses).
  6. Rt&Dzine macrumors 6502a


    Oct 8, 2008
    Did you mean 2x2 instead of 2x4?

    If so, did you put the 2x2 on the top riser and the 4x1 on the bottom?
  7. dilbert4life macrumors regular


    Jun 23, 2008
    Bolivia, according to my flag :P
    I have 8 Gigs of ram installed in 4 2gig chips. I was having the same problem, but through research and trial, I figured it out.

    This is how they need to be installed for optimal performance. (I use this naming system, because it is very clearly printed the same way on the DIMM riser cards.)

    DIMM1----(set a)2GB
    DIMM2----(set c)
    DIMM3----(set b)2GB
    DIMM4----(set d)

    DIMM1----(set a)2GB
    DIMM2----(set c)
    DIMM3----(set b)2GB
    DIMM4----(set d)

    If you want to install more ram, it needs to be in sets of two. The next one to install into, obviously, would be DIMM slot 2 (labeled set c)

    Does that help?:apple:
  8. djjclark macrumors regular

    Feb 17, 2008
    The risers also have a hardware check built in. Look for a little red led next to each memory slot on the risers. This will tell you if something hardware related is going on.
  9. ThirteenXIII macrumors 6502a

    Mar 8, 2008
    Ram in the mac pro is probably the most misinterpreted piece of info.

    If you want to add memory, yes it needs to be in pairs.
    If youre upgraded any model, for example the current 8core to add additional memory it should be:

    RISER CARD A (Top Riser)
    Slot1 - 1GB
    Slot2- 1GB
    Slot3 - Empty
    Slot4 - Empty

    RISER CARD B (Lower Riser)
    Slot1 - 2GB
    Slot2 - 2GB
    Slot3- Empty
    Slot4- Empty

    then youd fill RISER CARD A with memory in slot 3 & 4 of RISER A, then to top it if is to fill slots 3&4 of RISER B

    make sure OP, you have the 800Mhz FB-DIMMS for your 2008 MacPro.
    If not then its causing conflict, use the memory installation as just outlined above.

    if there are no RED LED light son your memory riser when its in use then its either not installed properly or pair wrong.

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