I received a 2008 mac pro 3,1 in pieces from a friend. He said that he was having trouble with the graphics cards cutting in and out and and then the power supply (PS) failed. I got a used power supply on ebay and put it all back together. It turns on, the graphic card (GC) ati 3870 fan powers up but there is no signal to the monitor. I tried all my monitors and two cables and still nothing on the monitor.
I tried several different macOS versions (10.6 , 10.7 and 10.11 ) settling on Lion. I moved the GC to all the different PCI slots, I pulled the GC from my 2006 mac pro, (an ATI 5770), which I know is compatible and still the same results. It boots up. With LION OS 10.7, I booted in safe mode and blindly typed in fsck -fy command and waited, then typed reboot and it rebooted. It will boot to target mode and I can access the drive from 2006 Mac Pro in disk utility and verify it. I have done pram and smc resets, and just for fun I tried the old PS and it fired up as well. I then did all of the above with the original power supply re-installed
I have connected the power cables for both cards I tried, there is nothing else in the other pci slots. It has an optical drive and I tried it with it installed and without. I have tried booting with a HD in different slots with no other drives installed.
I found my install discs and booted from both. I think it booted, as there is still no signal to the monitor. Drive spins up and I can hear it read the disc.
What are other reasons can there be for a monitor to not receive a signal but the graphic card still powers up ( I.e. fans on ). Assuming the graphic cards and the monitors I have tried are functioning properly in/on other machines?
I have been searching and reading this Q&A and discussion forums and posts for days and can not find any more info other than it might be possible it is logic board?? If it is a logic board problem what could it be and how do I address it?
Advice OR ideas would be appreciated.