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- CPU & chassis fan run after shutdown
July 28, 2009, 11:20 pm
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I've been trying to diagnose a custom-built system that I think is based
upon an Intel D915GUX motherboard. Unfortunately, I can't get it on the
bench and break it down, but thought I might hit you with the symptoms
to see if anything sounds familiar:
1) It's having problems starting up. Powering it on will get the fans
spinning long enough to get up to full speed, then then will spin down
again. No video output is seen. Unplugging the system for 30 seconds
will sometimes fix the problem. Other times, it will just start behaving.
2) On shutdown, the CPU and chassis fan will continue to run. The user
has been disconnecting the PC from the mains to shut it up. I asked him
to just let it run and see if the fans ever do shutdown. He claims that
will eventually happen, but I'm not entirely sure that's true.
I've checked the BIOS for any settings that might be relevant, and there
are three power settings:
1) What to do after power failure: set to do not restart.
2) Wake on LAN: disabled
3) Suspend power state (or something like that): Options are S3 and S5;
Left on S3.
The processor seems to be seated well, and the heatsink is clean. Even
though the PC is in a desk, it does appear to be adequately ventilated.
The fans continued to run, for instance, after shutting the system
down with the processor at 45C.
Re: CPU & chassis fan run after shutdown
Well, you know the key bits and piece of an ATX supply.
1) Two halves to supply. +5VSB is a separate regulator from the rest,
and runs after the main supply shuts down. The main rails are
-12V, +3.3V, +5V, +12V.
2) The main section is controlled by the PS_ON# signal (green wire).
Grounding the wire means "turn ON". Letting the wire float to
+5, signals "turn OFF". If the motherboard thinks it has shut
down, the PS_ON# should be nearer to +5V voltage than to ground.
The motherboard should use an open collector driver, to control
the green wire. A pullup resistor to 5V, is what causes the PS_ON#
signal to be 5V. That is why, when an ATX supply is not plugged
into a motherboard, it won't run. The pullup to 5V, on the green
wire, leaves it in the "Turn OFF" state, when there is nothing to
ground the signal.
3) Both the fans and the processor, are powered from the main section
of the power supply. On a normal shutdown, the +12V is cut, and the
processor loses power when the fans do. If the fans are still
spinning, then the processor may still be powered (and sitting in
a loop of some sort).
You can check the state of the PSU, by probing the main power supply
connector from the back where the wires go in, using a multimeter.
Does the owner know what the behavior was when the system was new ?
Did the system used to shut down simultaneously, with the Windows
screen going away when everything else stops ? Or was there
always a pause, while the fans cooled the system down until the
chassis temp was below some level ?
You might download the D915GUX manual and take a look in there,
to see if it has "features".
Otherwise, it could be a problem with the green wire. Either
side could be at fault - either the power supply not doing the
right thing with respect to the green wire, or the motherboard
doing the wrong thing when driving the green wire. So either
side could be at fault.
Re: CPU & chassis fan run after shutdown
He doesn't remember the fans running after shutdown when he first got
the system, but there's not much confidence in that recollection.
There is, according to the motherboard manual (which covers 4 models) a
speed throttling option for the case and cpu fans. I didn't see
anything in the BIOS settings that would control this, but I'll take a
second look when I can. I can't imagine, though, that one of the
options is to run those fans full speed even if the system is cool and
Thanks--I try to get the system on my bench.
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