The third wire is usually for a tachometer so you can read the fan
speed.

Leo Tick wrote:

On a cpu fan with three wires what is the protocol and
the signal in the sence wire. The question is asked
obviously because the fan does not spin but the fan part is ok
applying 12 v directly does get it to spin. Does the sence
line tell the mobo to turn on the 12v or what . If this is
the case what do I send down the sencs line to see if the
failure is in the MOBO.
Thanks

On Thu, 21 Aug 2003 14:05:11 -0700, Leo Tick wrote
(in message ):

Thats what I am doing now. My problen IS why doesent the fan turn (now since
at one time it did) when plugged in to the 3-pin. If the signal in the
sence wire tells the MOBO what the speed it is turning at then the MOBO
must be getting some temp info so that it can adjust the voltage to
the fan. But I dont believe my MOBO has that circuitry as chips which do
that are not there. Hence my question and confusion

Fan drive circuit is burnt out.
--
Dave C.

Maybe the fan function is
controlled through the BIOS setup.
Some motherboards have this
option at the -power- setup
always on/when needed etc.
If so the board might need
to reach some temperature to
enable the fan.

"Leo Tick" wrote in message
...
Jim Adney wrote:
On Thu, 21 Aug 2003 09:45:28 -0400 Leo Tick wrote:


On a cpu fan with three wires what is the protocol and
the signal in the sence wire. The question is asked
obviously because the fan does not spin but the fan part is ok
applying 12 v directly does get it to spin. Does the sence
line tell the mobo to turn on the 12v or what . If this is
the case what do I send down the sencs line to see if the
failure is in the MOBO.


The 3rd wire is nothing more than an OUTPUT from the fan to the MoBo.
If the fan is not turning then either there is a problem with the fan
or with the 12V supply on the MoBo. You should be able to measure the
supply voltage, with the fan connected, or connect the fan to a known
good power source. Either way should tell you where the fault lies.

Or just try a different (known good) fan on this connector.

-
I am afraid you missed part of my comment. The fan is known to be
good. It is in use with the 12v line connected to a 12 vold source.
I am tring to determine if the failure is in the MOBO which is not
supplying 12v from the three pin connector OR was some logig
involving the sense line at fault. Consenses here seems to be a circuit
failure.
Thanks


I've seen this fault before. If there is a fault on the motherboard which
grounds the tacho wire from the fan, some fans cannot start up. The simple
solution is to remove the tacho wire from the connector leaving the 12V and
0V wires, and the fan should run.

Why does shorting the tacho wire stop the fan starting? Well, the same part
of the fan circuit which supplies the tacho pulses to the tacho wire is also
used to switch the coils in the fan motor. Shorting out these pulses stops
the fan spinning.

Dave