# Help, after upgrading to 4GB RAM, iMac won't start



## tacsniper (Aug 27, 2007)

I have a 2.4GHz alum iMac and after install 2x2GB of RAM the iMac won't start. I tried both sticks and they all work if I only plug one of them in. But when I plugged both in together I cannot get my mac to start. Does anyone have a solution to this? Thanks!

PS the ram I am using are the Muskin kind.


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## poisonmonkey (Sep 20, 2004)

Try to find out if one of them is faulty. Try one than the other one at a time.

Then, see if you just are putting the second one in just right.


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## MacDoc (Nov 3, 2001)

You likely bought the 800 mHz RAM instead of 667.
You can mix 800 and 667 but 2 800s will not work...Canada RAM can explain better.


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## IllusionX (Mar 10, 2009)

CanadaRam.. chime in please...

i don't understand why 2x DDR2 800 sticks will not work. They are back compatible to 667.. it has to work...


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## MacDoc (Nov 3, 2001)

No - we ran into this - you can use a mix but they will not work in pairs in the iMac. So for now OP could get 3 gig.


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## CanadaRAM (Jul 24, 2005)

There's a couple of things.

One is that there is more to RAM compatibility than just the speed and the chip count. Each RAM module has its identity programmed in at the factory into an EPROM on the module - this holds the Serial Presence Detect settings. These settings are responsible for telling the machine what speed, latency and organization the module has, what voltage it requires, etc. In theory, 800 MHz modules are supposed to also be written with a SPD settings for 667 (and 533) MHz. But if these are not properly written, then when the Mac queries the SPD on boot up, it will reject it as being an out of spec RAM module. Apple is MUCH tighter about checking for the exact SPD info than most Windows manufacturers -- so memory manufacturers can be sloppy about writing the full SPD settings and get away with it on 95% of PCs, but it can blow up on Apples. This isn't confined to using 800 MHz RAM clocked down, bad SPD settings are just as much of a problem with 667's running at 667.

One thing that happens is that a SPD-deficient new RAM module can work alongside an older RAM module - because the Mac is presumably getting the settings from the old module and establishing the memory speeds. But the same new module alone or in a pair of new ones can fail, because there is no valid information for the Mac to read.

The other thing is that the Penryn (early 2008) MacBook Pros are more than usually fussy about RAM speeds, and there are many reports of 800 MHz RAM failing in these machines that worked fine in earlier models.

Keep in mind that I can't know if this is the exact problem with the Mushkins. But the RAM should go back to the seller for refund or replacement in any case. Hopefully, the seller offered a compatibility guarantee and will pay shipping both ways.


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## MacDoc (Nov 3, 2001)

What he said


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## gmark2000 (Jun 4, 2003)

You might also check that the ram is seated properly. Ram needs to firmly 'clunk' into the slots of my PowerMac in order to work and sometimes it's very tight.


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## IllusionX (Mar 10, 2009)

CanadaRAM said:


> There's a couple of things.
> 
> One is that there is more to RAM compatibility than just the speed and the chip count. Each RAM module has its identity programmed in at the factory into an EPROM on the module - this holds the Serial Presence Detect settings. These settings are responsible for telling the machine what speed, latency and organization the module has, what voltage it requires, etc. In theory, 800 MHz modules are supposed to also be written with a SPD settings for 667 (and 533) MHz. But if these are not properly written, then when the Mac queries the SPD on boot up, it will reject it as being an out of spec RAM module. Apple is MUCH tighter about checking for the exact SPD info than most Windows manufacturers -- so memory manufacturers can be sloppy about writing the full SPD settings and get away with it on 95% of PCs, but it can blow up on Apples. This isn't confined to using 800 MHz RAM clocked down, bad SPD settings are just as much of a problem with 667's running at 667.
> 
> ...



thanks for the info. :clap:


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## DeadZone (Feb 14, 2008)

*I learnt something today...*



CanadaRAM said:


> There's a couple of things.
> 
> One is that there is more to RAM compatibility than just the speed and the chip count. Each RAM module has its identity programmed in at the factory into an EPROM on the module - this holds the Serial Presence Detect settings. These settings are responsible for telling the machine what speed, latency and organization the module has, what voltage it requires, etc. In theory, 800 MHz modules are supposed to also be written with a SPD settings for 667 (and 533) MHz. But if these are not properly written, then when the Mac queries the SPD on boot up, it will reject it as being an out of spec RAM module. Apple is MUCH tighter about checking for the exact SPD info than most Windows manufacturers -- so memory manufacturers can be sloppy about writing the full SPD settings and get away with it on 95% of PCs, but it can blow up on Apples. This isn't confined to using 800 MHz RAM clocked down, bad SPD settings are just as much of a problem with 667's running at 667.
> 
> ...


Wow...great chime.


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## (( p g )) (Aug 17, 2002)

gmark2000 said:


> You might also check that the ram is seated properly. Ram needs to firmly 'clunk' into the slots of my PowerMac in order to work and sometimes it's very tight.


CanadaRAM's diagnosis sounds right. But for the record, I want to second what gmark2000 said for the benefit of others who experience RAM woes. I encountered the same thing when I added RAM to my previous iMac. It would reboot, but then after a few minutes would go foom and I would get the tell-tale rapid pulse on the LED "sleep" light. Once I pulled the RAM out, switched the modules around and made extra sure it was seated properly, everything worked fine.


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