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.

  • A fan powering up is not an indication that the graphics card is functioning - it just means that the fan is getting 12V to spin. The first thing you should do is try the 3870 graphics card in a different machine - a PC would do just fine. – Allan Aug 17 '17 at 2:14
  • One other thing to check is that power supply. I find it odd that a logic board would function with the exception of the PCI bus. Also re-seat all components and cables. This simple procedure has fixed many weird problems for me in the past. – Allan Aug 17 '17 at 2:23
  • I was just looking over your question again and realised you haven't provided any details of the amount of RAM it has. These machines can be funny about what memory modules are in what slots, so it may be worth you adding that info in as well. – Monomeeth Aug 17 '17 at 3:48

Assuming that it is definitely a Mac Pro 3,1 (i.e. early-2008 Mac Pro model), the logic board should contain some diagnostic LEDs. These were used by technicians for troubleshooting purposes, so you may be able to leverage off these to isolate your problem.

The following chart explains each LED:

enter image description here

As you can see, normally LEDs 3, 4, 5, and 6 are off. In other words, they shouldn't be lit up unless there is an error. If any of these are lit up, then you know you have the corresponding problem explained in the Indicates column.

As for the other LEDs (1, 2, 7, 8, and 9) you will need to press the DIAG_LED button (it's next to the LEDs) to display them. Use the Color column to determine the status (i.e. this indicates the LED colour).

I would check all of them, but obviously you'll want to pay particular attention to LED 7. If LED 7 is not lit up as green when you press the DIAG_LED button, then you know the GPU hasn't been configured.

Once you've done this, let me know how you went.


Okay, since you're only getting LEDs 2, 8 and 9 light up upon pressing the DIAG_LED button, we know that the computer isn't recognising the GPU because LED 7 is not green.

One of the issues you mentioned in your question was a potentially faulty power supply. I would actually re-install the one you bought on eBay. The reason for doing this is that some GPUs need more power to function, which is available from connectors on the logic board. And if you have any question marks over the original power supply, it's probably best to put it aside (although I acknowledge the one you bought was a used one).

Now I have seen instances of the ATI Radeon HD 5770 needing to have the auxillary power connected in order to run. So it may be worth looking for the connector on the logic board (usually it's close to the front of the board, under the plastic slot holders). So I'd plug that in (if not already) and see if that gets it working for you. Make sure you check the cable is connected properly at both ends!

If not, in terms of general troubleshooting when there is no LED 7 lit up, you would want to try each of the following (NOTE: I know you've already done some of this, but for the sake of completeness and other users I am still including them):

  • make sure the power supply and its cables are properly connected
  • make sure the graphics card is seated correctly
  • try the graphics card in each of the PCI slots
  • try a different graphics card
  • try replacing the PS3 power supply cable (as this is what supplies power to your PCI slots)

Normally if you have no joy after all of the above troubleshooting steps, I'd say it's time for another logic board.

  • I did this before posting. i thought that 7, and 8 were on before but i did it just now and 2, 8, 9 are the only ones on. I could have miss read them before. This is the GPU that was in the machine for years. – Alaska Man Aug 17 '17 at 1:17
  • Ok, well, you'd normally expect to see LEDs 2, 7, 8 and 9 on - so not having 7 on matches up with your experience. I will update my answer shortly with how I would proceed under these circumstances. – Monomeeth Aug 17 '17 at 1:25
  • i will have to order a ps3 cable. If it is the cable would the card fans power on like they do? – Alaska Man Aug 17 '17 at 2:35
  • Hmmm, pretty sure those fans only require 12v so it's possible they could power on regardless. The thing is, I've seen these GPUs not work in Mac Pros and then suddenly they will once you connect the auxiliary power cable. Some of these Mac Pros actually have two auxiliary power cables, so you could check that and try connecting the other one instead. It's amazing how sometimes just unplugging everything and replugging it all back in will sort problems out. Oh, and I've just remembered you've got an original Mac Pro as well, so you may be able to swap some cables to help isolate the problem. – Monomeeth Aug 17 '17 at 3:07
  • yes i have unplugged many things and re-installed them. the 2006 has different power cables. if it comes down to a new logic board is it possible to upgrade to newer model logic board (and processors ) such as a 2010 or 2012 without have to replace to much hardware. Putting to much money into a 2008 at some point does not make sense. Thanks for your time. – Alaska Man Aug 17 '17 at 3:08

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .