GPU not working - black screen

Hi, I have a MacPro 5,1 (2012) 12 cores, 64GB RAM

I have tried to get the HD 7950 GPU (or possibly an 8950, i have some doubts about the exact model) to work with macOS Ventura, but I haven’t been able to make it work.

For your information, I also have the original HD 5770 GPU that came with this MacPro.

This is what I have tried:

Disk 0 – Initially, the Mac was upgraded to High Sierra, which is supported with the original GPU – everything was fine. I tried to upgrade to Mojave, but it failed, asking for a Metal GPU. Then I acquired the 7950, installed it, and the upgrade went fine. It’s been working for 4 years now.

Disk 1 – I tried to install a newer macOS (Ventura) with OpenCore. I reinstalled the original GPU to be able to select the boot option. Everything seemed fine. I installed apps, patched OpenCore post-install patches. Then I removed the original GPU and installed the newer one, but it resulted in a black screen with no video.

After investigating, I found that it’s necessary to remove the post-install opencore patches before changing the GPU. So, I removed the newer GPU, installed the older one, removed OpenCore patches, removed the older GPU, and installed the newer one. But the same symptom occurred: black screen with no video.

I repeated this process – patch, remove patch, again and again – but it’s still not working.

I decided to try: Disk 2 – A fresh installation on a blank disk with Ventura and the old GPU using the USB created with OpenCore. (After some reboots, I configured iCloud, completed the first macOS setup, logged in, etc.) After the first login, OpenCore asked for patching, and I answered NO. I then removed the older GPU, installed the newer one, and again encountered the black screen with no video.

Now I have 3 installations on 3 different disks. All of them work with the older GPU, but only Mojave works with the newer GPU.

I don’t know if the procedure of replacing the GPU during the last installation was done at the wrong step, or if I need to change the GPU between reboots before MacOS configuration.

I’d appreciate any help

Thank you