iM 4,1-14,4 iMac 2011 Maxwell and Pascal GPU upgrade

[macfev]

Ok some new info about P4000.

Tried to flash vbios from P4000 max-q which has lower tdp. Flashing worked but after installing nvidia driver it had some freeze from time to time and was stuck around 600mhz clock. This is propably not having optimus support? So came back with no success.

I am testing now core clock 1797mhz with TDP limit 112W. Max temp is about 75-76. Lower TDP helps to keep this baby under 80 degrees. So finally 112W is my sweet spot.

 

[79bass]

I all, thanks for this great resource forum!
I have successfully installed and flashed a quadro m2000m on my old imac 2011 (27").
I was quite lazy on setting up OpenCore before, when I could actually see a starting menu, also because I can't really wrap my head around it.
So now the computer boots blindly with grub2 to linux, and I can work on it via ssh, but no screen ever .
I don't understand why it's so hard to make an opencore EFI. it's 2 days I read how/what kexts I need, what's correct ACPI options I need, and the documentation are so redundant that is confusing. It seems so hard for something that boots almost fine out of the box (I just need it to start with the ReloadOptionROM to somehow reload the nvidia rom at boot and turn the display on?).
It feels overkill, so now I'm second guessing if I'm doing something wrong.
I don't have macOS at all in this machine, just linux. I was planning to make this blasted opencore EFI and either boot from an USB or move the EFI folder (when it work) into the main disk.
Is really everyone using this OC thing with maxwell GPU? all iMac are the same-ish (given the year-period and screen size), so there should be some sort of predefined OC efi bundle, right?

 

[Nguyen Duc Hieu]

I all, thanks for this great resource forum!
I have successfully installed and flashed a quadro m2000m on my old imac 2011 (27").
I was quite lazy on setting up OpenCore before, when I could actually see a starting menu, also because I can't really wrap my head around it.
So now the computer boots blindly with grub2 to linux, and I can work on it via ssh, but no screen ever .
I don't understand why it's so hard to make an opencore EFI. it's 2 days I read how/what kexts I need, what's correct ACPI options I need, and the documentation are so redundant that is confusing. It seems so hard for something that boots almost fine out of the box (I just need it to start with the ReloadOptionROM to somehow reload the nvidia rom at boot and turn the display on?).
It feels overkill, so now I'm second guessing if I'm doing something wrong.
I don't have macOS at all in this machine, just linux. I was planning to make this blasted opencore EFI and either boot from an USB or move the EFI folder (when it work) into the main disk.
Is really everyone using this OC thing with maxwell GPU? all iMac are the same-ish (given the year-period and screen size), so there should be some sort of predefined OC efi bundle, right?

It will be much easier if you were a newbie and use OpenCore Legacy Patcher.
I would do like the following:
Make a High Sierra volume, install OCLP EFI manually (to match with iMac 2011 and Quadro M2000m on High Sierra volume.
Transplant the High Sierra disk to iMac 2011, boot it up to verify all things works.
Now start creating Linux USB installer and/or Linux volume. It will be the best if Linux volume can be on a physically separated disk, so you can have the Opencore boot menu, select to boot to Linux or HighSierra at will.
Then run OCLP again and patch the OC EFI to the Linux disk before removing the High Sierra disk.

I still have an iMac 2009 with M4000m running High Sierra. (OCLP patching is a must with Quadro M series)

 

[79bass]

It will be much easier if you were a newbie and use OpenCore Legacy Patcher.
I would do like the following:
Make a High Sierra volume, install OCLP EFI manually (to match with iMac 2011 and Quadro M2000m on High Sierra volume.
Transplant the High Sierra disk to iMac 2011, boot it up to verify all things works.
Now start creating Linux USB installer and/or Linux volume. It will be the best if Linux volume can be on a physically separated disk, so you can have the Opencore boot menu, select to boot to Linux or HighSierra at will.
Then run OCLP again and patch the OC EFI to the Linux disk before removing the High Sierra disk.

I still have an iMac 2009 with M4000m running High Sierra. (OCLP patching is a must with Quadro M series)

thanks!
well I’m sort of a newbie on this, I really gave up on having macOs on this, since not even Slack or telegram have supported app to run on it 😅.
Windows (which last time I actually used it was on version Windows XP) unsurprisingly also worked very poorly, and very poor user experience too: I spent a couple of days figuring out drivers and stuff. I still have goosebumps if I think of unraring drivers found on random places, I thought in 20 years it improved.. it didn’t…
And finally, it had no audio (I know I need OC for that)!
Linux on the other hand, worked 100% from the start, without effort.

I see, given your suggestion, I’ve swapped back to the old radeon card, and I’ll wait for some cables and adapters, since there is only the original 3.5” hdd, I can place an additional ssd instead of the dvd, but I need a 13 to 22 pins sata adapter (I’ll swap the 3.5” to 2 ssd if the new gpu turns out to work )

 

[Nguyen Duc Hieu]

thanks!
well I’m sort of a newbie on this, I really gave up on having macOs on this, since not even Slack or telegram have supported app to run on it 😅.
Windows (which last time I actually used it was on version Windows XP) unsurprisingly also worked very poorly, and very poor user experience too: I spent a couple of days figuring out drivers and stuff. I still have goosebumps if I think of unraring drivers found on random places, I thought in 20 years it improved.. it didn’t…
And finally, it had no audio (I know I need OC for that)!
Linux on the other hand, worked 100% from the start, without effort.

I see, given your suggestion, I’ve swapped back to the old radeon card, and I’ll wait for some cables and adapters, since there is only the original 3.5” hdd, I can place an additional ssd instead of the dvd, but I need a 13 to 22 pins sata adapter (I’ll swap the 3.5” to 2 ssd if the new gpu turns out to work )

If you have a USB enclosure for your SSD, then you can install High Sierra on the USB enclosure SSD; while Linux to the HDD while waiting.
Remember that you need to run OCLP manually to select the correct configuration.

I still remember that when I first install the flashed M4000m to my iMac 2009; it was terribly sluggish due to lack of driver for the Quadro (old OCLP version didn't integrated nVIDIA drivers). I had to search nVIDIA website for that Mac OS driver. I still keep a link to that correct driver in this thread.

 

[79bass]

If you have a USB enclosure for your SSD, then you can install High Sierra on the USB enclosure SSD; while Linux to the HDD while waiting.
Remember that you need to run OCLP manually to select the correct configuration.

I still remember that when I first install the flashed M4000m to my iMac 2009; it was terribly sluggish due to lack of driver for the Quadro (old OCLP version didn't integrated nVIDIA drivers). I had to search nVIDIA website for that Mac OS driver. I still keep a link to that correct driver in this thread.

I actually have a usb 2.5" enclosure... but it's IDE!! that alone should give you a hint about how old am I .

But what should I expect? I had no screen ever with the m2000m GPU, not juts at boot, but ever even after loading the nvidia drivers.
With a different driver in macOS should I expect the screen to turn on? I thought it needs to load/reload the GOP on boot to actually ever work, there is no post boot procedure to turn the screen on. is that correct?

With further reading of the various mods in the thread, as soon as I find a T1000 in Europe at an affordable price, I'd go for it, it seems to just works without OC/OCLP and it only requires a simple hardware mod (and it would extend immensely future life of the machine, as it's a 2019 card, with most of modern protocol supported... even the linux open source drivers might be enough for gaming -NVK seems to work well with Turing GPUs-)

 

[Nguyen Duc Hieu]

I actually have a usb 2.5" enclosure... but it's IDE!! that alone should give you a hint about how old am I .

But what should I expect? I had no screen ever with the m2000m GPU, not juts at boot, but ever even after loading the nvidia drivers.
With a different driver in macOS should I expect the screen to turn on? I thought it needs to load/reload the GOP on boot to actually ever work, there is no post boot procedure to turn the screen on. is that correct?

With further reading of the various mods in the thread, as soon as I find a T1000 in Europe at an affordable price, I'd go for it, it seems to just works without OC/OCLP and it only requires a simple hardware mod (and it would extend immensely future life of the machine, as it's a 2019 card, with most of modern protocol supported... even the linux open source drivers might be enough for gaming -NVK seems to work well with Turing GPUs-)

You need OpenCore patch, regardless the OS intended to run on your iMac. (Opencore EFI in this case acts as the BIOS setting in Windows/Linux PCs).

Please refer to the below link for more detailed explanation why you need it.
Your iMac is a 2011 model thus the post is the exact match.
Eventually, I would not suggest installing a fresh Linux on your iMac, because:
1. It's too difficult without a bootscreen.
2. Linux boot management will mess-up with Mac OS boot management=> catastrophy.

iMac 2011 Maxwell and Pascal GPU upgrade

iMac Quadro P3000M & P4000M roms: - Enables the internal display - Enables GOP boot-screen - Brightness control with OpenCore - Full Boost clocks in MacOS & Windows - Power Limit increased (with AfterBurner) Sadly there is still no certificate hacked nvflash for Pascal, so patched roms must be...

forums.macrumors.com

If I would ever want to run Linux on an iMac, I would do as follows: (I'm not sure if it works)
1- Install Linux to a drive (SSD or HHD) on another PC. Make sure it's a GPT partitioned volume.
2- Erase the Linux boot management part. (i.e remove the Linux Boot Directory, or better yet, delete all files on the EFI partition)
3- Patch the OpenCore EFI (that matches with iMac 2011 and new GPU) to that Linux disk. The Opencore EFI control the boot management.
4- Transfer the disk (with Linux and OC patched EFI) to iMac 2011 and upgraded GPU.

If it doesn't work, you need to adjust step 3- above.
3-1 Create a USB installer of Mac OS; (you will need a USB flashdrive, or a USB enclosure with SATA disk, IDE box might not work)
3-2 Patch OC EFI to the USB installer High Sierra or Monterey. (Manually configure it to match with iMac 2011 and the upgraded GPU). Edit the Config.plist file to get the OC menu last a bit longer than default.
3-3 Test run the USB installer on your iMac 2011 (to see if you can get to the OC bootmenu)
3-4 Then copy the whole EFI partition to the Linux drive.
Note: Do PRAM reset before booting the iMac with only the Linux drive plugged in.

 

[EliasLatorre]

Hi, I'm new here and I have a doubt about all this cards and the amount of information in this thread and the "general" thread is quite overwhelming for me.

I have received completely free a 2009 i5 27 inch imac with a broken HD 4850 GPU and I want to give it a new life, I've been searching an mxm-a gpu because I've seen it's a low power hungry GPU and I've seen it doesn't need to make use of a dremel to mod the heatsink (I don't have a dremel either any experience with it), and I have found in this thread the m2200 mobile which could be a more optimal solution for me , I'm aware this thread is more about the 2011 imac but really want to know if it would be possible to use this gpu on my imac and I haven't found any information about this specific gpu on the "general" thread.

I shall remark I have no intention to make use of MacOS, I'll stick to Linux Mint and maybe Windows with a dual-boot, it's what I've used my whole life and, even though I would have liked to try macOS, it's not an essential characteristic, I've seen not requiring MacOS compatibility makes the process a bit easier, I just want an stable system with decent performance and make use of the nice multimedia system

I guess it could work but considering how fussy can be Apple I thought it could make no harm to ask the community to solve doubts.

Thank you so much beforehand <3

 

[79bass]

Hi, I'm new here and I have a doubt about all this cards and the amount of information in this thread and the "general" thread is quite overwhelming for me.

I have received completely free a 2009 i5 27 inch imac with a broken HD 4850 GPU and I want to give it a new life, I've been searching an mxm-a gpu because I've seen it's a low power hungry GPU and I've seen it doesn't need to make use of a dremel to mod the heatsink (I don't have a dremel either any experience with it), and I have found in this thread the m2200 mobile which could be a more optimal solution for me , I'm aware this thread is more about the 2011 imac but really want to know if it would be possible to use this gpu on my imac and I haven't found any information about this specific gpu on the "general" thread.

I shall remark I have no intention to make use of MacOS, I'll stick to Linux Mint and maybe Windows with a dual-boot, it's what I've used my whole life and, even though I would have liked to try macOS, it's not an essential characteristic, I've seen not requiring MacOS compatibility makes the process a bit easier, I just want an stable system with decent performance and make use of the nice multimedia system

I guess it could work but considering how fussy can be Apple I thought it could make no harm to ask the community to solve doubts.

Thank you so much beforehand <3

As far as I can tell, you'll need OpenCore(/Legacy Patcher) to make any nvidia or newer card to work correctly.
And as long you have a GPU that can provide you some sort of images to get you to a working OC efi configuration you are good to get an "unsupported" GPU and install it there. (hopefully you can see something/purple lines, not completely broken GPU?)

Otherwise you'll need a native supported (by apple) GPU and get a screen working first to confirm to have OC setup right.
I'don't know if an external screen or some other workaround would make you go directly to a new GPU.
However, some kepler card are natively supported by apple, it might be a good compromise.

I agree MXM-A cards are easier to handle, no dremel or heatsink modding are needed, as far as hardware installation, I found the m2000m to be a seamless swap vs the old radeon of my 2011 imac

 

[Nguyen Duc Hieu]

Hi, I'm new here and I have a doubt about all this cards and the amount of information in this thread and the "general" thread is quite overwhelming for me.

I have received completely free a 2009 i5 27 inch imac with a broken HD 4850 GPU and I want to give it a new life, I've been searching an mxm-a gpu because I've seen it's a low power hungry GPU and I've seen it doesn't need to make use of a dremel to mod the heatsink (I don't have a dremel either any experience with it), and I have found in this thread the m2200 mobile which could be a more optimal solution for me , I'm aware this thread is more about the 2011 imac but really want to know if it would be possible to use this gpu on my imac and I haven't found any information about this specific gpu on the "general" thread.

I shall remark I have no intention to make use of MacOS, I'll stick to Linux Mint and maybe Windows with a dual-boot, it's what I've used my whole life and, even though I would have liked to try macOS, it's not an essential characteristic, I've seen not requiring MacOS compatibility makes the process a bit easier, I just want an stable system with decent performance and make use of the nice multimedia system

I guess it could work but considering how fussy can be Apple I thought it could make no harm to ask the community to solve doubts.

Thank you so much beforehand <3

You are very lucky.
Quadro M2200 already has modified vBIOS for iMac.
Check it up in the below thread:

iMac 2011 Maxwell and Pascal GPU upgrade

This is a Wiki post with information collected from the entire thread. Before you post, please read this first. This thread is for supporting Maxwell and Pascal cards on the 2009-2011 iMac. Maxwell and Pascal cards will not work with acceleration on MacOS beyond High Sierra due to lack of...

forums.macrumors.com

 

[EliasLatorre]

As far as I can tell, you'll need OpenCore(/Legacy Patcher) to make any nvidia or newer card to work correctly.
And as long you have a GPU that can provide you some sort of images to get you to a working OC efi configuration you are good to get an "unsupported" GPU and install it there. (hopefully you can see something/purple lines, not completely broken GPU?)

Otherwise you'll need a native supported (by apple) GPU and get a screen working first to confirm to have OC setup right.
I'don't know if an external screen or some other workaround would make you go directly to a new GPU.
However, some kepler card are natively supported by apple, it might be a good compromise.

I agree MXM-A cards are easier to handle, no dremel or heatsink modding are needed, as far as hardware installation, I found the m2000m to be a seamless swap vs the old radeon of my 2011 imac

First of all I've forgotten to tell what I think it's important information, sorry. The former owner of the PC has not updated the PC for 12 years so it's stuck at Mac OS X Lion 10.7.5, I guess I could update to Sierra via external storage if needed but I haven't tried.

Now I answer to the doubt you had

You're right, the GPU is not completely dead, I can see the purple lines, but I can't have an stable normal boot with this GPU (I accidentally got a normal boot a few days ago but it didn't last long, system completely blocked with purple squares once I tried to open Google Chrome and search something I don't know if I can do something to sort it out but the chances of doing anything by normal boot are reduced so far), I have to boot on safe mode and then I can access to the system, purple lines, very limited graphical functions and everything works understandably slower but I keep network functionalities, and it's stability it's proven and I've had no problems so far executing programs or moving the entire information of the disk via direct ethernet connection with another computer, so I may be able to work with that, but I don't know MacOS very well so I don't know all the limitations of this Safe Mode specifically.

I do have a secondary screen so I can try to go directly to a new GPU

Knowing all this complications, may I need to buy a natively supported GPU anyway to do all the process or there's something I can do without spending additional money on other GPU ?

With Linux in an USB stick it boots, but I have to change a file in the ISO to ignore the dedicated GPU and all the graphical load is done directly by the CPU because there's not i-GPU unfortunately, otherwise I can't boot (obviously not optimal, works horribly, but it's a "normal" live boot, so it does everything a live boot is able to do)

I guess I could downgrade to kepler or change to AMD if the limitations are just too much

I've seen people doing things with the vBIOS via Windows and an adapter in other computer, I don't know if it has something to do or if I can do something with it (I find some terms here new for me) but if there's a viable solution there I can work with that.

Sorry if it's too much information added to the case.

Thank you so much

 

[EliasLatorre]

You are very lucky.
Quadro M2200 already has modified vBIOS for iMac.
Check it up in the below thread:

iMac 2011 Maxwell and Pascal GPU upgrade

This is a Wiki post with information collected from the entire thread. Before you post, please read this first. This thread is for supporting Maxwell and Pascal cards on the 2009-2011 iMac. Maxwell and Pascal cards will not work with acceleration on MacOS beyond High Sierra due to lack of...

forums.macrumors.com

well, if, as you say I can do this with a modified vBIOS then I'm relieved, I'll check all the process, buy the GPU and then I'll make any consult if needed, thank you so much.

 

[yezhou1225]

If you mean adding EnableGop to get native bootscreen without OCLP/Opencore, give this a try:

This rom works fine, with add-on PCB, it gives boot loader picker, and great bench in High Sierra and Win 11.
The only issue is, brightness control can't be enabled even use properly configured Opencore/OCLP .
Maybe the add-on PCB disables it? Or, needs some mod in rom?

 

[79bass]

I think I’ve messed up somewhere while reinstalling the radeon.

I had to order another flex cable since I was in a hurry to close the thing a connect it (life with kids around) and destroyed it (the pins in the pcb looks unaffected)

So even with the new cable and the old radeon on, the screen remains completely dead.. I’ll have to check all over again (maybe/hopefully I haven’t re installed the gpu correctly).

Do you know if I could have killed the screen with some bad connector? is there a way to debug the lcd?

 

[79bass]

Ok, the thing is back alive. I swapped the old radeon back, made a somewhat booting EFI/OC folder using the recommended settings to reload the nvidia rom, and install the quadro M2000M back again.
However, the screen still doesn’t respond. I’ve reset the pram, just in case it was something like that, but no.
The mainboard has 3 led turning green (1-2-4, the 3rd led, if there is one, doesn’t light up at all).

I flashed the quadro with the first rom suggested in OP (for the corresponding board).

I was planning to install the OS with all the drivers afterwards, but maybe I need to do otherwise

 

[Nguyen Duc Hieu]

Ok, the thing is back alive. I swapped the old radeon back, made a somewhat booting EFI/OC folder using the recommended settings to reload the nvidia rom, and install the quadro M2000M back again.
However, the screen still doesn’t respond. I’ve reset the pram, just in case it was something like that, but no.
The mainboard has 3 led turning green (1-2-4, the 3rd led, if there is one, doesn’t light up at all).

I flashed the quadro with the first rom suggested in OP (for the corresponding board).

I was planning to install the OS with all the drivers afterwards, but maybe I need to do otherwise

I smelt some errors in your process. But it's just me.
Maybe you are the expert and can figure it out.

Below is the quoted line from the 1st post:

"You will need a properly configured Opencore/OCLP in order to enable the screen on boot, have brightness control on MacOS, have an emulated boot picker and to enable internal speakers output on UEFI Windows."

My interpretation of this sentence was:
"If you don't properly configure Opencore, your screen will not be enabled at all."
So in my iMac, I configured the boot disk properly (i.e manually) before installing the GPU.
If it doesn't enable my screen, I remove the boot disk and repair the OC configure on another Mac.

 

[79bass]

I smelt some errors in your process. But it's just me.
Maybe you are the expert and can figure it out.

Below is the quoted line from the 1st post:

"You will need a properly configured Opencore/OCLP in order to enable the screen on boot, have brightness control on MacOS, have an emulated boot picker and to enable internal speakers output on UEFI Windows."

My interpretation of this sentence was:
"If you don't properly configure Opencore, your screen will not be enabled at all."
So in my iMac, I configured the boot disk properly (i.e manually) before installing the GPU.
If it doesn't enable my screen, I remove the boot disk and repair the OC configure on another Mac.

Thanks, I actually got back to my steps to do it properly (hopefully).

So I've installed macOS (10.13) in a small partition (wow, I forgot it was so slooooow!), and use the latest OCLP pkg to make a working OC EFI. I've than modified the config.plist to address specific Maxwell boot args, even suggested in the OC buyers guide

Extras:

shikigva=40 boot flag: Swaps boardID to iMac14,2 for better Nvidia Support and whitelists patches
nvda_drv_vrl=1 boot flag: Used for enabling the Web Drivers

I've actually added everything in the bullet points of the "properly configured Opencore/OCLP" link above suggested. Now I'll go ahed an install a booting linux that will give me a working ssh shell if let unsupervised, so, even if the GPU doesn't work, I can still go around and check what happened, and/or edit OC without tearing the machine apart for the 20th time.

One thing I need to change, I'll go for the DEBUG version of OC. So I might have a possibility to check what's happening, unless there is a way to check in mac or linux if OpenCore run properly applying all the thing it needs to

anyway, next plan is, if this doesn't work, I'll search a Quadro T1000. I wonder if the hardware backlight mod would help in any way with Maxwell GPU as well

 

[79bass]

the message got posted twice somehow

 

[79bass]

I smelt some errors in your process. But it's just me.
Maybe you are the expert and can figure it out.

Below is the quoted line from the 1st post:

"You will need a properly configured Opencore/OCLP in order to enable the screen on boot, have brightness control on MacOS, have an emulated boot picker and to enable internal speakers output on UEFI Windows."

My interpretation of this sentence was:
"If you don't properly configure Opencore, your screen will not be enabled at all."
So in my iMac, I configured the boot disk properly (i.e manually) before installing the GPU.
If it doesn't enable my screen, I remove the boot disk and repair the OC configure on another Mac.

I've done everything taking care and not rushing on setting up OCLP and reassembling and..... it worked!!

I wasn't expecting it, actually, I let it alone after waiting a minute or so with a black scree, thinking it didn't worked, when I came back to the room a moment later the screen was on, to my amazement!

I experienced random shut down after few minutes, I thought it was an hardware issue (and I'll update if it turns up again), but after I've reset the PRAM it didn't do it anymore. and it had been running few hours now.

The only issue it's the lack of boot selection, the screen turns on only after an OS is booted, So I have to blindly trial error if I want to change OS.
I'll try if there are OC options to improve on that.

I've added the backlight fix, but the backlights are not dimmable anymore, I'll try modify those parameters.

 

[Nguyen Duc Hieu]

I got the same issue on some of my iMacs.
It turns out the menu delay time was too short. Before I could recognize it, the iMac booted Mac OS to desktop screen already.
I used OpenCore Configurator to increase menu delay time.
(i.e change some parameters in the config.plist file)

 

[79bass]

I got the same issue on some of my iMacs.
It turns out the menu delay time was too short. Before I could recognize it, the iMac booted Mac OS to desktop screen already.
I used OpenCore Configurator to increase menu delay time.
(i.e change some parameters in the config.plist file)

I might try this first.

But I haven't read of anyone in the thread with a quadro m2000m with OS selection working. Could it be that the backlight are turned on only after the OS kernel initialize the driver card?
I wonder if I do a backlight hardware mod (feeding 3.3v from the PSU to the backlight directly) I'd see someting (if the issue are only the backlight).

But I don't know if the mod would damage the backlight when running with cards that more or less are still eventually able to enable the backlights by themself

 

[79bass]

A side note: the OS boot selection would give me the piece of mind to try and reinstall windows, since the performance on linux of the m2000m are not mid blowing (I can run steam games that I wasn't able before, but I still need to resize the screen to have really enjoyable play).

I think for my 6y old kid would still work fine, but I'm so excited to have this amazing machine revived, and I'd like to stretch it as far as possible.

 

[79bass]

I got the same issue on some of my iMacs.
It turns out the menu delay time was too short. Before I could recognize it, the iMac booted Mac OS to desktop screen already.
I used OpenCore Configurator to increase menu delay time.
(i.e change some parameters in the config.plist file)

The most fitting configuration seems to be

TakeoffDelay
Type: plist integer, 32 bit
Failsafe: 0
Description: Delay in microseconds executed before handling the OpenCore picker startup and action hotkeys.

I've set it to 10 seconds (10000000), let's see when my kid finished playing if that would work

 

[derplayer]

Success story: I got an iMac12,1 for free with a broken GPU and broken ethernet. I bought a cheap (25€) Dell NVIDIA Quadro M1000M (Maxwell, MXM-A). The first issue was that the DELL backplate didn't fit into the iMac cooling and was glued, so i removed it with a hairdryer and replaced with the stock one from the iMac.

After the GPU was installed in the MXM slot, i disconnected the HDD and SSD (so that it automaticly boots from the USB drive) and tried to dump the vbios with GRML. This was not easy due to no networking. I used an usb ethernet adapter and tried to boot GRML from usb. For whatever reason, the newest GRML froze at starting (this one) so i used an older one. (2011_imac_usb.zip) This one worked perfectly.

By default, the second USB network card is not enabled, so i typed commands blind on my keyboard to enable it. Maybe it can help someone in the future:

Spoiler: blind command steps to enable usb network card

(wait until GRML is booted)
enter
netcardconfig
enter
(wait 5 seconds)
arrow down
enter
(wait 10 secs)
enter
enter
(wait 1 min)
enter
right arrow
enter

After this i dumped, patched the bios on my mac mini (with OpenCore's EnableGop tool) and verified the firmware.

Result:

I also tried OpenCore Legacy with Nvidia GOP injection before patching, but for whatever reason it didn't worked.

 

[Ausdauersportler]

Success story: I got an iMac12,1 for free with a broken GPU and broken ethernet. I bought a cheap (25€) Dell NVIDIA Quadro M1000M (Maxwell, MXM-A). The first issue was that the DELL backplate didn't fit into the iMac cooling and was glued, so i removed it with a hairdryer and replaced with the stock one from the iMac.

View attachment 2386581

After the GPU was installed in the MXM slot, i disconnected the HDD and SSD (so that it automaticly boots from the USB drive) and tried to dump the vbios with GRML. This was not easy due to no networking. I used an usb ethernet adapter and tried to boot GRML from usb. For whatever reason, the newest GRML froze at starting (this one) so i used an older one. (2011_imac_usb.zip) This one worked perfectly.

By default, the second USB network card is not enabled, so i typed commands blind on my keyboard to enable it. Maybe it can help someone in the future:


After this i dumped, patched the bios on my mac mini (with OpenCore's EnableGop tool) and verified the firmware.
View attachment 2386577

Result:
View attachment 2386580

I also tried OpenCore Legacy with Nvidia GOP injection before patching, but for whatever reason it didn't worked.

Looks like everything went well, the iMac misses an operating system to boot.
Try to create an High Sierra USB installer and start up the installation.
EnableGop seems to work since you have a working display, in addition the correct eDP support was already included in your base vBIOS.
You may upload the modded and original vBIOS? (both zipped)