MP 1,1-5,1 Nvidia 1080 works in one Mac Pro 5, 1 but not another

[covalt]

I am in a studio with several Mac Pro towers (5,1 models); we're upgrading all the GPUs to Nvidia 1080 cards because we're using Octane for 3D rendering.
I've successfully gotten cards installed in 4 different Mac Pros (all 5,1 models, various CPUs).
The cards are either Nvidia Founders Edition 1080 or EVGA GeForce GTX 1080.

However, I cannot get cards to be recognized in 2 remaining Mac Pros.
To be sure that I didn't have some software gremlin standing in the way (after many failed attempts at getting it to show up), I completely erased the OS hard drive and installed a fresh copy of 10.3.4.
I then ran the security update, so the macs are all running 10.13.4 (17E202), with the Nvidia Web Driver 387.10.10.30.107 and CUDA driver 396.64.
Even still, when I drop the Nvidia card in the PCI slot and start up, I never get a screen. I use Remote Desktop to login and see that no graphics card is recognized.

I've even swapped the hard drives between a Mac Pro that DOES work with an Nvidia card with one that's NOT working, and the problem remains - that Mac Pro will still not show a picture or recognize that an Nvidia card is installed in the PCI slot.

Anyone have any ideas what I might be missing, or what could have gone wrong in 2 of these machines?
I'm totally stumped and have not been able to figure out a solution.
Thanks!

 

[Darmok N Jalad]

It's possible the power supply connectors aren't working well anymore on the ones that are not booting. Are you trying to power the cards with the mini-PCIe connectors from the motherboard, or through adapters off the optical drive bays?

 

[covalt]

It's possible the power supply connectors aren't working well anymore on the ones that are not booting. Are you trying to power the cards with the mini-PCIe connectors from the motherboard, or through adapters off the optical drive bays?

I've wondered the same thing but wouldn't know how to test that. I'm using the mini-PCIe connectors on the motherboard.

Here's the thing.. through MANY times of trial and error and just about going crazy, I've gotten them to work. I reset PRAM, SMC, unplugged and replugged about 20 times each Mac Pro. Eventually, they showed a display with the Nvidia card installed. I don't know if it's just random or what. I didn't have any trouble with 4 other Mac Pros, and didn't have to do PRAM or erase the entire OS drives.. but for whatever reason these two machines were just difficult!

 

[Darmok N Jalad]

I've wondered the same thing but wouldn't know how to test that. I'm using the mini-PCIe connectors on the motherboard.

Here's the thing.. through MANY times of trial and error and just about going crazy, I've gotten them to work. I reset PRAM, SMC, unplugged and replugged about 20 times each Mac Pro. Eventually, they showed a display with the Nvidia card installed. I don't know if it's just random or what. I didn't have any trouble with 4 other Mac Pros, and didn't have to do PRAM or erase the entire OS drives.. but for whatever reason these two machines were just difficult!

You are dealing with dropping in new and demanding cards in rather old hardware. It’s amazing how responsive the old cheese grater MPs still are after all these years.