XPS 8940, how to install an extra 16GB ram?

@AlainVo You should install the new memory module in the empty memory slot with the color securing clip that is the same color as you current memory is in, so that would be DIMM2 (white). With the two modules in those slots, the memory will operate in dual channel mode. 

It goes in the other white slot. The Crucial RAM is compatible with Dell systems. The question is whether it is compatible with the old RAM. Generally it is best to purchase a kit with 2 (or 4) identical RAM modules for best compatibility. But give it a try as it may be just fine.

JOcean and 1st reply are correct.  This is following dual channel rules.

If I understand this correctly, DIMM 1+3 are channel 1, DIMM 2+4 are channel 2.

Rules to enable dual-channel mode
To achieve dual-channel mode, the following conditions must be met:

• Same memory size. Examples: 4GB, 8GB, 16GB etc.
• Matched DIMM configuration in each channel.
• Matched in symmetrical memory slots.

Configurations that do not match the above conditions revert to single-channel mode. The following conditions do not need to be met:

• Same brand
• Same timing specifications
• Same speed (MHz)

The slowest DIMM module populated in the system decides memory channel speed

PS recently one user validated the way to install 4x8GB to have successful boot is

put the pair of 4 chip memory (C4FE) in the Black and White slots on the left (DIMM 1+3) and the 8 chip memory (M8FR) in the Black and White slots on the right (DIMM 2+4) 

Thanks for your advise to have same brand but I can't find where I can buy the same ADATA  AO2P32NCSV1-BEVS

Intel rule says you do not have to get same brand. 

The following conditions do not need to be met:

• Same brand

Which country are you located in?

These are the 16GB ram validated by Dell (one of the following is your ADATA  AO2P32NCSV1-BEVS).  If you look up “View product specs” using your private service tag on Dell support website, it is likely your ram DP/N is DK8NX.  You can purchase any of the following ram either from Dell at premium price or third party to match your Dell Adata ram.

DK8NX 16GB DDR4, 3200MHz, 2Gx64, Unbuffered, 288 pin, Single Rank, 1.2V, Non-ECC, Non-Encrypted Adata AO2P32NCSV1-BEVS, Hynix HMAA2GU6AJR8N-XN/HMAA2GU6CJR8N-XN, Kingston KDK8NX-MIB/KDK8NX-MIE, Micron MTA8ATF2G64AZ-3G2B1/MTA8ATF2G64AZ-3G2E1, Samsung M378A2G43AB3-CWE/M378A2G43BB3-CWE

@AlainVo - But... the XPS 8940 has a RAM slot quirk.

Users who mix different brands of compatible RAM (OEM and "retail") in the XPS 8940 and install in slots with matching color retention clips find the RAM slows down to 2400 MHz, regardless of rated speed. 

But, when they're in slots with different color retention clips, RAM runs at the speed of the slowest module, up to a max of 2933 MHz..

So if you install the new Crucial module in the slot with the same color retention clips as the OEM module (OEM is likely in 2nd slot from CPU), be sure to check the speed using CPU-Z (free).

If the RAM isn't running at 2933 MHz that way, you should move the new module to the slot that's closest to the CPU (slot 1) with opposite color clips and then check speed again. It may only run in single-channel mode this way, but running at 2933 MHz in single-channel mode may be faster than dual-channel mode at some slower speed.

I am from Montreal, Canada

Hi 

I ordered one 16gb Samsung M378A2G43AB3-CWE  with 1 single rank 1Rx8 from Newegg.ca and After 2 week waiting, I receive one 16G Samsung M378A2K43EB1-CWE dual rank 2Rx8. Do you think this will work or I have to return it

Thanks and best regards

This is going to sound crazy but what worked for me is waiting. It rebooted twice and upon second boot it pulled up the "system memory has changed warning" then after I pressed continue it rebooted twice and is working. It take 30 seconds to a minute for each step. 

Whoever figured out that a successful 4x8GB installation requires placing the pair of 4-chip memory (that came with the XPS 8940) in the Black and White slots on the left (DIMM 1+3) and the 8-chip memory in the Black and White slots on the right (DIMM 2+4) was brilliant. Worked perfectly. Thank you!