Faulty CPU fan more than meets the eye........?

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I've had a computer for around 6 months now and it has seen much heavy use
but has been very stable.
Anyway about 2 weeks ago it crashed with a blue stop screen. I hoped that it
was a one off freak event but it crashed again yesterday.
I took the back off and noticed that the CPU fan wasn't working. It's power
lead is connected into the motherboard. There are 2 other 3 pin connectors
on the motherboard for other fans so I tried it in those with no success. I
happen to have a spare fan which I tried but which also didn't work. Which
lead me to believe there might be something wrong with the motherboard. I
used a volt meter to measure the voltage across the red and black pins and
it came with around 4.5 volts.
Am I right in thinking the fan needs 12v to work?

I haven't got any wires to connect the fan to a battery to test it. But the
fact that another fan didn't works leads me to suspect that the fans are

So has anyone got any advice regarding what might be amiss and what
components I should buy. I've had a look using Goolge and the Power Supply
Unit (PSU) keeps coming up as a possible culprit.

Re: Faulty CPU fan more than meets the eye........?

On Sun, 24 Jul 2005 16:43:30 +0100, "JohnJAdamson"

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What was that error?  You can skip most of it and just use
the abbreviated version, for example,
0x0000007E and a bunch of other misc info would be 0x7E.
There are a lot of Microsoft KB articles covering those,
just search for 0x7E (per the example but what applies to
your code instead).


AT this point I should mention that this may easily be a
driver, software or operating system problems which are
technically beyond the scope of this newsgroup.  If you find
it is indeed one of those types of problems you might reach
a larger group of people learned in it by posting to a
related newsgroup.

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Did you note the CPU temp, that it was overheating?  It
could easily be that your only problem is overheating, but
it could instead mean that due to instability, you also have
file corruption (or data in some file like the registry)
that would be written during the instable period of time.
We could ignore this potential for the time being but it was
worth mentioning in case you continue to have problems after
getting the fan situation resolved.

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Being a harware newsgroup, it is good to tell us what
hardware this is.  Motherboard make/model would be a good
start.  It's also useful to let us know what fan make/model
(and what heatsink it was on) as that may help others avoid
these parts if they seem to be faulty or at least with poor

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It used to be that the fan power pin was a staight
connection to the PSU 12V rail.  More and more often boards
now have fan controllers integral, which may keep the
voltage lower and possibly vary it based on heat or user
settings in the bios.  You should check your bios menus to
see if there is any related fan control settings, and if so,
try setting it to "always on" or whatever seems to come
closest to that, 100% or 12V or however- they're-wording-it.

On a board with the fan speed control, it normally doesn't
get as low as 4.5V, because most fans won't spin up from
stopped at 4.5V... a fan needs a slightly higher voltage to
start spinning than it does to keep spinnning once started.

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Assuming you measured the correct pins on the fan header
(the leftmost (Gnd) and center (~12V) pins while the key
notch is oriented towards the bottom of the header), which
do usually correspond to red and black wires (but that is
subject to fan maker's discretion).

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Since you have a multimeter, it's easy enough to measure
your power supply 12V rail (the yellow wire and any ground).
For the system to be otherwise working it seems pretty
certain that it's not down around 4.5V at the PSU output so
it seems a fan circuit failure on your motherboard (or a
logical/bios bug- check your bios settings first).  The
easiest thing to do would be to simply buy a fan adapter to
plug the fan into a power supply 4-pin plug.  They're fairly
common but if you're only buying that one thing then
shipping can be multiple times the cost of the adapter,
though SVC has very good delivered price for somthing small
like that, though it's if you choose USPS which is a long
wait with the system down.


Also note that if your system did previously throttle back
the fan with a motherboard-based speed controller, then
using such an adapter may result in the fan spinning
significantly faster, louder.  For that reason you could do
one of many different things to  further throttle back the
fan, including swapping a pin on the adapter to run the fan
at 7V, or using an inline resistor or series-of-diodes, or a
proper fan controller, or something more exotic (Google
search fan control).

Re: Faulty CPU fan more than meets the eye........?

JohnJAdamson wrote:
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A CPU or other typical system fan is rated at 12V but typically will run
at lower voltages. Some modifications make use of this fact and use a
connection between a +12V and a +5V power source to feed the fan with 7V
resulting in slower and quieter operation. Some fans in my experience
will operate easily at 5V but often there is a problem getting then
started at the lower voltage.

Does the BIOS in your computer offer the option to throttle the fans
based on temperature? This is common on newer systems and is offered to
allow quieter operation. If so, switching that option off for testing
would be good first step to see if it is the source of your problem.
John McGaw
[Knoxville, TN, USA]

Re: Faulty CPU fan more than meets the eye........?

If the cpu fan stopped, you can fry your cpu in no time, unless perhaps you
have overheat protection in the bios enabled.
The psu shouldnt have any impact on the mobo connector for your cpu fan -
the psu pwrs the mobo which in turn feeds the fan.

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