Chosen Solution

Good afternoon iFixt community, I have a question about the startup sequence of the iMac. First let me explain what happened. I have a iMac with a ATI Radeon 4850 512MB MXM card, which works perfectly fine.I harvested a 880M from a laptop and flashed this with a BIOS version that support Apple.I replaced the card and got external display using mini DP.It can run games and the 880M is recongized and works. But Internal display is not working with this new card. I put the ATI 4850 back and the internal works again. so the cable and display is fine. PSU is fine too. Does somebody knows whats the diffrence between the ATI 4850 and 880M with startup, could it be that the 880M is not sending the right signals? I measured the voltage on the 14 pin PSU connector all seems fine. Except pin 13 for enable backlight always is 0.00v to 1v. If I solder a wire from a 3.3v source to the pin 13 will it work? I think maybe the 880M doesn’t send a signal for enable backlight and startup video using the LVDS connector I have no clue or if the display and video card together draw to much current together? But external works, but external is of course is different power source for its display. Does someone have a clue?? My last resort is soldering wires to generate/ force signals. But I have read in other iMac’s that people did this and killed the logic board because sometimes signals only need to be a second there. Update: Dan fixed my issue. For anyone else who see this or want the info .. in the link Dan posted They said it doesn’t work in high sierra But here it does, also I didn’t lose boot screen it’s there using a vbios from nickey on macrumors I found it a while back. Catalina and others didnt try.

I haven’t done this my self so I can’t tell you if this will fix your issue or not. Forcing an NVIDEA into a iMac is very iffy! Apple only offered one driver so you’ll be running in compatibility mode! I don’t see Apple supporting NVIDEA in Big Sur if that was your direction, I think Catalina is it for the driver. Successful iMac Upgrade to GTX 880M To make a non-apple provided NVIDEA graphics card work in macOS Sierra you need to do the following… Start the computer in safe mode by holding shift. Get your BoardID by running this command in terminal echo “$(ioreg -rd1 -c IOPlatformExpertDevice | awk -F’["|”]’ ‘/board-id/{print $4}’)" Open the AppleGraphicsControl Kext using this command sudo nano /System/Library/Extensions/AppleGraphicsControl.kext/Contents/PlugIns/AppleGraphicsDevicePolicy.kext/Contents/Info.plist Use the arrow keys to go down until you see ConfigMap followed by a bunch of keys and strings. LOOK FOR YOUR BOARD ID… IF IT IS LISTED: Change the string to none IF IT IS NOT LISTED: Add it in the same format as the ones listed. Save the file by pressing Ctrl+O, then enter. Exit by pressing Ctrl+X Rebuild the kernel cache using the following two commands: sudo kextcache -system-prelinked-kernel AND sudo kextcache -system-caches

Hello Dan. Sorry for my English. I want to order GTX 880M and my HD 6970M is dead two card I replaced and work maxim 1 year. I found information in the internet the GTX 880 from Dell Alienware ca be work?? Now it’s Sierra osx Can you explain is it work or not in 2021? Thank you Best regards Roman

I just tried to install a GTX 880m apparently its a ex Dell card. My problem is the iMac will not even power on with the internal lcd connected. Led1 will stay lit and do nothing, disconnecting the LVDS cable the iMac boots fine to a external monitor via mini dv cable. It the 880m flashed fine via terminal, i tried re-flashing but makes no difference. If i hot swap the LVDS cable then reboot i can get the internal lcd to work however if i shut down and power back on i get the boot chime loop over and over. My guess is the card just isn’t compatible.