Mac Pro logic board bad?

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Level 1

8 points

Mac Pro logic board bad?

I have a Mac Pro 2,1 that abruptly stopped working a week ago. It has had a good, long life and hadn't any problems up till now. The symptom is simply that when I touch the power button on the front the computer it will attempt to start up, fans will spin, but then no monitors will light up and no bell starting tone. Using the service manual I've made this much progress...

-reset the SMU and logic board button to no avail.

-looking at the diagnostic LEDs on the inside I have trickle power and EFI working. The power LED (second one from the end) does not light up. The GPU light doesn't light up.

-suspecting the PSU was bad I then removed the optical drive sled and put a meter on the 4 pin molex connectors meant to power the optical drives. When restarting I do get both 5v and 12v power consistently.

I feel I've exhausted the suggestions in the service manual. Is the logic board bad? Is there anything left to test before I consider a logic board replacement

Thanks in advance for words from the wise and experienced.

Mac Pro, OS X Mountain Lion (10.8.5), Mac Pro 2,1

Posted on May 20, 2016 2:15 PM

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Answer 1

May 20, 2016 2:43 PM in response to barrysteele

what does the manual suggest you should be seeing in those power LEDs? Is that what you have?

What is the Voltage of the coin cell battery, in hand (nominally 3 Volts when new)?

Does the power-on indicator blink a pattern?

If you strip all the PCIe cards, and the drives, and all the RAM out of the machine, does the power indicator blink a pattern when you power on?

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Answer 2

barrysteele Author

Level 1

8 points

May 21, 2016 6:19 AM in response to Grant Bennet-Alder

Thank you Grant for your assistance.

The service manual says that when I depress the diagnostic LED button I should have lights on trickle power, no lights for CPU error or OT, a light for GPU, a light for Power, and a light for EFI. So I don't have lights on for GPU and Power when I should (for a normally functioning machine).

Previously I had taken the precaution of removing each of my three video cards and trying them individually in each and every PCI slot to no avail. I forgot to list that step above.

The coin cell has 3.14v on my meter.

No pattern on the power on button when I have hard drive and PCI card(s) installed.

No pattern on the power on button when hard drive, PCI cards, and RAM removed.

Thanks for any ideas you may have!

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Answer 3

barrysteele Author

Level 1

8 points

May 21, 2016 8:25 AM in response to Grant Bennet-Alder

Thank you Grant.

Taking a suggestion from another webpage I found troubleshooting a similar problem, that's when I took out the optical drive and put my meter on the molex connector to see if there was consistent power out of those connections. Since I did get both consistent power of 5v and 12v when booting and letting the machine run, it seems to indicate that the power supply is good. Right?

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Answer 4

barrysteele Author

Level 1

8 points

May 21, 2016 8:28 AM in response to barrysteele

The diagnostic LED lights apparently can indicate problems for many reasons. None are specific to the logic board, but a combination of lights (or lack of) can indicate either the PSU or logic board. I'm currently leaning to the logic board being the problem since the PSU seems good.

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Answer 5

lllaass

Level 10

210,513 points

May 21, 2016 8:44 AM in response to barrysteele

Yes. The label on the power supply should state the different voltages that it delivers.

It does that for PC power supplies and I just looked at a power supply remove fro a G4 PowerMac.

If yo disconnect the connector to the logic board the power supply will likely not turn on since a pair of those conductors goes to the power switch.

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Answer 6

May 21, 2016 8:47 AM in response to barrysteele

If the Power Supply is being considered as a potential issue, you should remove:

ALL PCIe cards

ALL Drives, including the DVD Readers

ALL RAM

ALL USB devices including the keyboard and mouse

any anything else that "plugs in" in a similar way

... and then look at those diagnostic lights in complete isolation.

This is a standard way of debugging really primitive problems like this -- it is based on the idea that something may be "dragging down" some part of the power subsystem.

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Answer 7

barrysteele Author

Level 1

8 points

May 21, 2016 9:05 AM in response to Grant Bennet-Alder

Thank you Grant.

Following your instructions previously I had removed everything you describe (but I didn't think to remove the keyboard and mouse-original Apple USB devices). But now I've removed those too. The diagnostic LEDs are still showing the same thing.

1: Trickle Power has LED light= good

2-3: CPU errors, no LEDs = good

4-5: CPU over temp, no LEDs= good

6: GPU present, no LED= bad (but could be a result of a bad PSU or Logic Board. I have 3 GPU cards that i've swapped individually between all PCI slots to no avail. Hard to believe all 3 cards went bad simultaneously. So that leaves me to suspect PSU or logic board)

7: Power On, no LED= bad (could be result of bad PSU or logic board. That's why I attempted to prove the PSU working by testing the molex to the optical drive using my meter. But as lllaass pointed out, the fault could be in the PSU connection to the logic board. Not sure yet if that's testable)

8: EFI, LED is on= good

I'm hoping to isolate the problem to the PSU or the Logic Board. Then I can decide if it's worth it to pursue further.

Thank you both for your help.

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Answer 8

barrysteele Author

Level 1

8 points

May 21, 2016 10:31 AM in response to Grant Bennet-Alder

Ok, so now it seems like the best course of action is to pull the PSU and test it. However there are no instructions for this in the Service Manual. Does anyone in these forums know how to test a PSU? I've been researching on the web and YouTube without finding anything concrete.

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Answer 9

May 21, 2016 8:07 AM in response to barrysteele

So I don't have lights on for GPU and Power when I should (for a normally functioning machine).

They look for a Power supply, not a mainboard, unless the flowchart suggests something else.

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Answer 10

lllaass

Level 10

210,513 points

May 21, 2016 8:27 AM in response to barrysteele

Not really since the power supply also puts out other voltages. 5 volts and a lower voltage, maybe 3.3, for the CPU.

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Answer 11

barrysteele Author

Level 1

8 points

May 21, 2016 8:30 AM in response to lllaass

Thank you lllaass.

So I should really find the PSU leads that power up the logic board and test those?

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Answer 12

May 21, 2016 9:15 AM in response to barrysteele

Your Mac can power up perfectly well without a GPU.

Understanding what it is doing will be more than a little difficult, but there is no requirement that a GPU be present.

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