I got a somehow broken iMac Mid-2010 (Mac11,2) with a Radeon HD 4670 video card from a friend and now have to repair it.
- The iMac with El Capitan installed doesn't start properly to its main volume hanging at the grey apple on light grey ground.
- First I discovered a wrong RAM module (PC3-8500 DDR3) though the iMac requires PC3-10600 DDR3. The iMac booted successfully to a memtest86 thumb drive. No errors are shown for either of the different RAM modules (even the PC3-8500 DDR3 module seems to work reliably). I replaced the wrong one by original Apple OEM modules.
- I've reinstalled different systems (Mavericks, El Capitan). None of them boots properly.
- All tested Recovery HDs work! (Mavericks, El Capitan) as well as Single User Mode.
- None of my different full OS thumb drives containing Snow Lion or Mavericks boot properly.
- AHT/Apple Diagnostics doesn't work.
- I got hands on a Apple Service Diagnostics image for this iMac and neither the EFI nor the OS part boots properly. The OS part boots to a blue screen and after a few seconds I get a white screen with some colored artifacts, the EFI part hangs at "Loading VideoController devices".
It looks like that boot volumes using a particular video card driver (apparently those installed in a full system) don't boot properly while default drivers like the one in the memtest86+ image (probably some generic Linux driver) work.
How do I continue diagnosing the probably malfunctioning graphic card?