Installing RAM, 3gb works fine, 4gb wont boot.

Question

I am trying to upgrade the RAM in an aluminium 24" iMac. I have 2x2gb sticks, both DDR2-800 PC2-6400.
When I install them both, the computer wont boot.
If I install just one new stick, and leave the original stick in there, it boots fine.
If I install either of the new sticks, but NOT the original, it wont boot.
The extra strange thing, is that I have done the same upgrade procedure on 2 older (white) iMacs, with the same spec sticks. Both the older machines have no issues, other than the RAM only showing up as 667mhz, not 800mhz.
Any ideas? I don't think it's the RAM itself, as either stick works when installed in conjunction with the original stick.
Could the iMac be struggling with trying to run 800mhz RAM? Seems unlikely to me, as the older iMacs are doing it fine.

Answer 1

cruci wrote:
All I can think of that explains the behaviour is that on the older machine, which has a 667 bus speed, it doesn't try to use the RAM at 800, but on the newer machine (which has a 800 bus) if both RAM slots are filled with 800, it tries and fails.
Nope. Although the ALU iMacs have an 800 MHz Front Side Bus,
the memory controller hardware runs only at 667 MHz:
[Apple Developer Note -- ALU iMacs|http://developer.apple.com/documentation/HardwareDrivers/Conceptual/iMac0708/Articles/ProductDeveloperNote.html#//appleref/doc/uid/TP90004829]
This is pure speculation, but I suspect that the problem might
be related to the ALU iMac firmware. RAM modules have a small
"SPD chip" ([Serial Presence Detect|http://en.wikipedia.org/wiki/SerialPresenceDetect]) that the OS interrogates
for information about the module. My guess is that some iMac
models can't deal with memory sticks that report perfectly
valid but "unexpected" SPD information.
There have been several recent posts by owners reporting the
same strangeness whereby upgrade modules work individually
but not in pairs -- even with 667 MHz parts that appear to
conform perfectly to Apple's recommended specs.
FWIW, I've had good luck with Mushkin's "PerfectMatch" memory
designed specifically for Apple. A Mushkin tech-support droid
told me that Mushkin programs the SPD with exactly the same
information as Apple-supplied memory modules. (Please supply
your own grain of salt.)
[Mushkin 'PerfectMatch' for Apple -- 4GB kit @ $60 after MIR|http://www.newegg.com/Product/Product.aspx?Item=N82E16820146701]
BTW, resellerratings.com ranks Mushkin and Newegg "Platinum Plus."
good luck,
Looby
*"Just goes to show ya! It's always somethin'!*
*If it's not one thing, it's another!"*
\- Roseanne Roseannadanna
.