Power Supply Fan Won't Start

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Building a new machine...haven't built a machine for about 3 years but I
thought I was pretty good at this.

The power supply fan doesn't seem to spin, although the green light on the
motherboard actually comes on...showing that there is power going through
the board.

I thought it might be due to a sensor for the cpu fan- so I connected up the
CPU fan and still nothing. Any ideas? It's a ASUS motherboard running an AMD
Sempron. Are there any other sensors one needs to consider??

- Kwaj

Re: Power Supply Fan Won't Start

Kwaj wrote:
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The standard ATX power supply has a command input line that tells it to
turn on.  If I remember correctly, it's a green wire, 4th from one end
of the connector.   You can ground it with a paper clip to the 3rd wire
which is black for ground.   Check the ATX power supply spec on
http://www.intel.com to be certain.

Note: Even when the power supply is "off", there is a 7 volt supply that
is always on.  It's purpose is to power the logic that drives the TURN
ON input to the power supply.  That's why you need to pull the plug on
your system when you monkey with it.

Re: Power Supply Fan Won't Start

On Mon, 23 May 2005 08:37:37 -0400, "William W. Plummer"


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Make that "5 volt supply", not 7.

Re: Power Supply Fan Won't Start

You have a faulty PSU.


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Re: Power Supply Fan Won't Start

DaveW wrote:

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Why? How do you know a short external to the supply is not the problem?

Re: Power Supply Fan Won't Start


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The power supply fan won't spin until the system is full
"on", having the green LED on the motherboard lit is only
evidence that the power supply's 5VSB circuit is working,
placing the PC in a soft-off power managed state as all
"ATX" systems do.

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Are you sure you have the power switch connected to the
corrent motherboard pins?

It might help to strip the system down to only essential
components (video card, CPU, heatsink-fan (plugged in), 1
memory modulel).  You can disconnect all drives, keyboard,
mouse, etc.  Disconnect all case wiring and use a metal
object to short together the motherboard power-on pins
together.  Any metal object that conducts electricity will

Be sure to disconnect AC before stripping system down.
Check the power supply's rear voltage switch (if it has one)
to be sure it's set correct per your location, ie - 110 or
220V AC input.

Make sure heatsink is on good.  Try clearing CMOS (via
jumper detailed in the manual or pull the battery for 5
minutes, either of which done while AC power is

The PSU fan will only spin while it's turned fully "on".

WIlliam Plummer detailed a way to check the power supply
alone, shorting it's green pin 14 (PS-ON) pin to ground with
a paperclip to see if it comes one when not connected to the
rest of the system.  Some PSU may need a small load
connected for this test to work, for example leaving only a
single hard drive or CDROM/optical drive plugged in on one
of the PSU leads should suffice.

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