The ram from Dell precision is ECC registered for server only, therefore not compatible w 990.  You need DDR3 ram for desktop only, unregistered, unbuffered, non-ECC for 990.  Server memory cannot be used in desktop pc in general. 8GB PC3L-10600R-9-10-E1: the R following 10600 denotes registered. Precision spec:  990 spec Non-ECC dual-channel 1333MHz DDR3 SDRAM, up to 16GB  
Hello, The Optiplex 990 that my son uses was shipped with 8 GBs of RAM. The Optiplex 990 has four DIMM slots, and is using a 4GB module in one slot, and another 4GB module in another slot. I have tried upgrading its memory to 16 GBs by removing one of the 8 GBs RAM modules in my machine (Precision T7500) and adding it to the Optiplex 990. The Optiplex 990 does not recognize this addition both in BIOS and necessarily Windows. I thought maybe a mismatching technology is causing this, so I removed the two 4GBs modules and inserted the new 8GBs module. The Optiplex 990 won't start and beeps to indicate that the system has no memory. The 8GBs module has this reference: HYNIX, KOREA 05, 2Rx4 PC3L-10600R-9-10-E1, HMT31GR7BFR4A-H9 T2 AC DDR3 1333MHZ 8GB [1]. The 4GBs modules have this reference: XTREME DDRIII 4GB LONG DIMM 1333MHZ. Could you please let me know if there is a way to make this upgrade? The 8GBs module works fine in my machine, I know this because I used MemTest. Thanks. [1] [URL]
Optiplex 990 does not recognize RAM