I had this problem when replacing 4 OEM 16 sticks with 4 OWC 32 sticks. The machine booted 1x and after that, black screen booth. tl; dr: I solved this by permuting all of the possible combinations of sticks after labelling them 1-4, based on that 1x boot order.
I could get things to work with up to 3 sticks (1, 3, 4 and 2, 3, 4), but never (again) all 4. Slot 2 was the most problematic, because I could only get it to boot with certain chips in Slot 2. If I mixed in ANY combination of OEM chips, with Slot 2 always filled by a OEM stick… the machine booted just fine
So, I did a SMC reset and PRAM reset. Didn’t work. Checked that the sticks were flush after re-installing the carrier every time.
After it worked (fleetingly), I assigned each stick a #, 1-4, based on which slot they were in.
So, I called OWC. We agreed that the best way to pursue this was to cycle sticks to see if there was any way to get it to boot. He suggested that I do a return and start over again, but my stubborn ass wouldn’t let it go.
I then hunted around the slots. I started with which sticks would boot when installed in each of the slots. For me, my stick 1 worked in slot 1 and in no other slots. I repeated this for the other sticks, finding that stick 2 worked in slot 3 and four, but never 2. Stick 4 always worked in Slot 2. I used that as my anchor…
Then I checked how the other sticks behaved. I found that I could boot on Stick 1 -> S1, Stick 3 -> S3, and inverse. Stick 2 -> S2, Stick 4 -> S4 never booted again, but Stick 4 -> S2, Stick 2 -> S4 always booted.
Finally, I went with my Stick 1 / Stick 3 pattern, eventually getting a boot (after about 15 seconds) out of the 3, 4,1, 2 pattern.
I have rebooted a bunch of times and completely cycled the power, including disconnecting the power at the wall. I have not done a PRAM reset as of yet. I’ll report back to OWC and here if things get messed up again.
Not much can be said without knowing the configuration. Some boards and processors have limitations on RAM size and speed, should also check if is it buffered or unbuffered, EDO or SD RAM. Try inserting a single module at a time.
I have the same problem. After installing 2 additional RAM sticks the iMac won't start anymore. In my case I could isolate the problem to be related to the 4th RAM socket.
If I keep the topmost RAM slot empty and I put some of my RAM sticks in the other three slots the iMac will start.
But if I install a RAM module in the 4th slot the iMac won't start anymore (even with the RAM modules from apple initially installed in slot 1+3).
Can it be that a RAM slot is broken on a new machine ???
Thanks for any help
door Andreas Huber
I have the exact same issue. I have determined that all the RAM sticks are functioning correctly but my second RAM slot from the bottom doesn’t seem to work. It’s a brand new machine. Is this a more common issue because I’m seeing this on a variety of different forms... going to schedule a repair from AppleCare but wow! Super annoying!
door Jordan Ditzler
Brand new 27” iMac 5K and two of my four RAM slots are useless! When RAM is installed in either of the bad slots the machine fails to boot and screen stays dark.
Apple initially thought I bought bad after-market RAM, but after many swaps between the installed RAM and the two 16 GB sticks I bought in they agreed the slots must be bad.
Let’s see how long it takes them to repair or replace. Frustrating!
door Daniel Sammons
I also just purchased a 27” 5k 2017 iMac and have isolated the problem to be my 2nd from bottom RAM slot.
* The stock memory reads in slots 1 & 3
* The new memory reads in slots 1, 3 & 4
* The new memory and stock memory reads in slots 1, 3 & 4
But if anything is in slot two the machine will not boot at all.
Very frustrating! Just spent my life savings in this machine and for it to arrive broken is very upsetting!
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