System/CPU fan don't turn off

Discussion in 'MSI' started by rsyring, Jul 1, 2007.

  1. rsyring

    rsyring Guest

    I have a computer that a customer gave me. The problem is that the
    fans/power on the computer will not shut off. Windows XP shuts down
    and the computer gets to the point where the monitor shuts off, but
    the HDD, CPU Fan, PSU Fan, and Power On LED all continue to run. I
    have tried a different PSU and that did not fix the problem. I have
    looked for BIOS settings, but didn't find anything pertinent. I
    reconnected the power case cords to the MB to make sure they weren't
    mixed up. The only way to get the fans to shut off is to turn the
    black rocker switch off or unplug the cord. However, as soon as you
    turn it back on or plug the cord in the fans start right back up.

    I even cleared the CMOS so that it would return to factory defaults
    and flashed to the latest BIOS. The fans still run and the green LED
    is still on when the computer is off.
    rsyring, Jul 1, 2007
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  2. rsyring

    Paul Guest

    This means the motherboard is pulling the PS_ON# sognal on the main
    connector, to GND, and doing it all the time. Normally, when WinXP
    shuts down, PS_ON# goes open circuit, and a pullup on the PSU
    brings the signal to a 5V level. A high level on PS_ON#, shuts off
    the supply.

    If it happens with the motherboard and just one power supply, it could
    be a defect on either end. But since you tested two PSUs and the
    symptoms are consistent, then the problem is more likely to be on
    the motherboard end of things.

    There is a small chance something is shorting to the bottom of the
    motherboard. Visually inspect the board, for anything that looks out
    of the ordinary.

    Paul, Jul 1, 2007
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  3. rsyring

    rsyring Guest


    Thanks for the response. I have taken the board out of the case and
    plugged in the PSU (a different one from the one that was in the case)
    and the fans still run as before. The MB is currently sitting on a
    non-conductive surface, so I don't see how anything could be shorting
    out underneath. Should I just toss the board or is there anything I
    can do to make it run correctly?
    rsyring, Jul 2, 2007
  4. rsyring

    Paul Guest

    If the power supply is sitting on its own, not plugged into any hardware,
    and you plug it in and turn it on, does it stay off ? That proves that
    the PS_ON# signal is properly pulled high by the power supply. Without
    a motherboard to pull that signal to ground, the power supply fan should
    not start to run, on its own.

    The following is included to give some history, and an example of how
    it used to work. In fact, there is probably little you can do to trace
    or debug it down further, due to the level of integration on the motherboard,
    and lack of components like the older motherboards used.

    ********* PS_ON# and friends ... *********

    To see how the driver scheme used to work for PS_ON#, there is a reference
    schematic here. On PDF page 18, a signal called SUSC# on the 82371EB Southbridge,
    drives an inverter and a 74F07 (upper right corner of the page). The output
    of the 74F07 is "B_SUSC", english equivalent being "buffered SUSC". That signal
    goes to PDF page 32 of the schematic, and connects to pin 14 of the main power
    connector. Pin 14 is the PS_ON# signal, meaning the output of the 74F07 open
    collector driver, is what drives the power supply PS_ON#.

    To verify what the motherboard is doing, you would take a multimeter
    and probe on either side of the 74F07 (marked as pin 1 and pin 2 on the
    schematic). When there is a 0 volt level on the input of the 74F07,
    there would be 0 volts on the output, which would turn on the supply.
    In particular, when the computer "shuts down" or is supposed to, you'd
    want to examine the voltage level on the input to the 74F07.

    (A 74F07 datasheet, doesn't explain much..., just so you can see one)

    The reason for looking on either side of the 74F07, is to see if the
    motherboard is "trying" to shut off the PSU or not. If the input to the
    74F07 is high, and yet the output is low, it means something is shorting
    the output, or the 74F07 is dead. The 74F07, being open collector,
    actually allows more than one 74F07 output to be connected together,
    and that is referred to as wired-OR logic. But in this case, the open
    collector has the advantage, that a person testing the motherboard,
    can actually force the motherboard on (by grounding pin 14), without
    damage to the 74F07. If a non open collector driver was used, it could
    be damaged by grounding. But the open collector logic does not allow
    forcing the output high (which is what you'd want right now, to test
    whether the supply can be forced off or not). Another benefit of the
    74F07, is if the PS_ON# signal is damaged, the 74F07 is cheap to replace,
    which might be an advantage in a warranty repair situation. When they
    were used, they used to cost about $0.25.

    Modern designs could well be more economical, in that they probably
    no longer use external logic or buffers at all. It could be that the
    Southbridge drives PS_ON# directly, and the lack of a buffer like the
    74F07, makes it harder to "see" what the motherboard is trying to do.
    The signal to be probed to see if the motherboard is "trying" to switch
    off, could be inside the Southbridge.

    At the simplest level, the Southbridge contains a small circuit, that
    converts the momentary pulse from the front panel "ON" button, into a
    steady level for PS_ON#. But in terms of other part of power management,
    both the Southbridge and the SuperI/O chip, contain agents that have
    "wake" capability, such as wake on LAN, wake on ring, etc. So both
    the Southbridge and the SuperI/O have to be connected together in some
    way, to "wake" the computer and influence the state of PS_ON#. But
    even if the logic was exposed so a person could check the logic levels,
    there is no guarantee you could see it screwing up.


    At this point, you're probably looking at a replacement board. I wouldn't
    throw it out, until you're certain that the motherboard is at fault.
    In other words, if a replacement works properly, and for a few weeks,
    then I might toss the other one. If the replacement board misbehaves,
    it could turn out that something else is doing it, but I don't know
    what. Something shorting the PS_ON# is about all I can think of, if
    the PSU behaves when it is by itself and not connected to anything.

    Paul, Jul 2, 2007
  5. rsyring

    rsyring Guest


    Once again, thank you for your response. The PSU runs correctly, the
    fans stay off when plugged in, turned on, and not connected to any
    hardware. As soon as I plug in the MB (out of the case with no other
    connections) the fans start running on both the PSU and CPU. At this
    point, I am pretty sure it is the MB.

    However, in its current location, it almost never gets turned off, so
    this shouldn't be a big deal anyway. I was just hoping to be able to
    solve the problem.

    If the computer runs fine in every other way, can you imagine a reason
    not to just continue to use this motherboard? Is it possible that the
    problem I have been describing might hurt any of the other components
    in the system?

    Thanks again!
    rsyring, Jul 2, 2007
  6. rsyring

    Paul Guest

    The signal in question (PS_ON#) is a logic signal. Unless something
    else on the board starts failing, I would think there is no reason to
    let that bother you. I'd still use the board, if that was all that
    was broken on it.

    Paul, Jul 2, 2007
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