As you can read above, with just 2 pins your fans are spinning, your motherboard cannot control the speed of the fans. Still, there is a solution to it.
It requires a custom BIOS flash but it's not too hard.
You need a Windows machine to do it, or a least one to prepare the BIOS ROM.
What you need to do is...
1. Download nvflash
2. Make a bootable USB drive using HP Bootable USB tool
3. Copy nvflash files over
4. Boot the system with the 65% fan nvidia card off the USB drive
5. Dump the BIOS ROM - "nvflash --save origbios.rom"
6. Shutdown
7. Copy "origbios.rom" off the USB drive.
8. Download NiBiTor
9. Open ROM in NiBiTor. It will complain that it doesn't recognise the ROM.
10. From the Device menu select your card, or if you have a GT 620 like I do just select GT 520 "Nvidia PCI-E 520 GT" because they're exactly the same card.
11. Under the Adv. Info tab click "Rescan Bios"
12. Go to Temperatures tab and change the Min to something reasonable. I picked "30".
13. File -> Save Bios. Save it to a new name.
14. Copy to USB drive.
15. Boot off the USB drive again.
16. Use command "nvflash --index=0 -5 -6 newbios.rom" - It might complain about which card it is, and saying something doesn't match, don't ignore the warnings, just make sure they're sensible