Black screen when using 4 ram sticks on brand new iMac 5K I bought a 5K iMac 4 months ago, with the stock 8gb of ram. I bought an extra 2x4GB off Amazon (Crucial Ram). But then, it looks like I cannot use all 4 ram slots at the same time. The iMac won't even start, only show a black screen.
The Crucial ram sticks are 100% working, since I tried removing the original ram sticks, using only the new ones, and the mac boots and runs fine
Right now, I am running the iMac with 3 of the 4 sticks, so at the moment I have 12gb of ram.
I tried using the top 3 slots, it works. Bottom 3, it works.
2 slots works too.
But as soon as I insert a 4th ram stick, it stops working
I have resetted the SMC and NVRAM. Still no luck
Also updated Mojave to 10.14.2 (latest version)

Any ideas?
I would like to see if there's anything I can try before setting an appointment with Apple for this, since it's my main working machine, and I need to work on some projects right now.

Thanks a lot!



Here's the Ram kit I have bought:

Crucial 8GB Kit (4GBx2) DDR4 2400 MT/S (PC4-19200) SR x8 SODIMM 260-Pin Memory

I've read multiple comments on Amazon about this kit working fine with 5k iMacs...so I'm not sure what's going on here. Thanks
Not all RAM is created equal. The way motherboards handle things is radically different. For example where an ASUS PC motherboard might take mismatched RAM, the iMac will not. To ensure compatibility use RAM that has the same:


Clock Speed
Voltage Requirement
Latency timings


It's best to buy identical RAM sticks. They should be the same manufacturer and the same model number. Occasionally even RAM with the same model number can have different modules on the chip. (This is very rare )

Can you update your question with details about the Manufacturer, Model number, Clock speed, voltage Requirement, and latency timings of your RAM?

UPDATE:
According to the Apple website, the RAM you bought should be compatible:
https://support.apple.com/en-us/HT201191

Might be time for the genius bar.