mac pro 2008 faulty temp sensor?

Discussion in 'Mac Pro' started by sbab, Mar 24, 2016.

  1. sbab macrumors member

    Oct 25, 2012
    My trusty mac pro 2008 will not start
    All was fine, it operated normally, the machine had not been modded or upgraded for the past year. It just worked until the day it would not start.
    I pulled it apart, blew out what little dust there was, checked the connections, stripped it to a minimum.
    Trying to start it I noticed a red led glowing tucked away from the normal diagnostic leds.
    This led is located on the logic board between the front fan and airport card right next to the two power outlets for additional power e.g extra power for GPU.
    This led illuminates steady as soon as power is plugged in and remains on until the power cord is removed.
    The led is marked C287 and I have been unable to identify what job it does.
    When the power button is pressed there is that internal click the white light illuminates and the fans, all except the GPU fan, start to spin then it shuts down. I have swapped gpu's with the same result.
    What I have noticed in this short time frame is that the "post" test is extended e.g the dimm leds should momentarily illuminate but here they are on for quite a number of seconds.
    The only time I've been able to start the computer has been when the C287 was blinking rapidly. It then shutdown before I had time to do much as investigate.
    Reading the manual - troubleshooting on page 158 it mentions;" Video card diagnostics LEDs" with the mention that "T_Fault LED normally off, this LED lights up if the graphics chip gets too hot" it doesn't give a location so I'm wondering if this is C287 and the reason the computer won't start.
    I have istats menu and it doesn't have a GPU temp available so I can't get a reading.
    I do not know what sensor operates this T_Fault led could someone tell me?
    I do not know what the the LED C287 indicates again could someone tell me?
    It looks like another logic board is required but it would make me spit chips if it was just a piece of fluff or something on a sensor causing the problem.

  2. ActionableMango, Mar 24, 2016
    Last edited: Mar 24, 2016

    ActionableMango macrumors G3


    Sep 21, 2010
  3. sbab thread starter macrumors member

    Oct 25, 2012
    ActionableMango, thanks for the reply.
    What I would like to know is if the board has a sensor in the board or uses a card's sensor, where is it?
    I'm going to replace the logic board, I've all ready ordered one and I'll report back.
    I've had a pretty good run with my computers (2 x 5,1s - 1 x 4,1 and 1 x 3,1) over the years but lately I had to replace a backplane on one of the 5,1's the 4,1 is in bits and will most likely require a board change now the 3,1!
    Talk about things happening in three's.
  4. ActionableMango macrumors G3


    Sep 21, 2010
    Well your quote from the MP service manual says T_fault it is GPU temperature, so I would have to guess it's on the GPU. I don't see how a sensor mounted on the logic board could determine GPU temperature, especially since a GPU could be in any slot.

    Macs can boot up headless, so if the problem were really GPU overheating, you could test that theory by simply removing the graphics card and turning on the computer. I am skeptical that it is really the GPU, but this test would confirm one was or the other.

    I looked around and the following two links imply that T_fault LED, at least on their PCs, was power-supply related:
  5. sbab thread starter macrumors member

    Oct 25, 2012
    An update
    I bought a bare bones 3,1 computer. It had a power supply and backplane.
    I removed the PSU and installed it in the faulty 3,1. It did not fix it.
    All the parts from the faulty computer except the backplane were installed in the bare bones and it started OK.
    So for me that faulty light was fixed by replacing the backplane.

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