RPM fan sensor, only the yellow wire?

[CMRvet]

I’m using an external source to feed the CPU fan (Volcano 6CU+) connecting the red (+) and black (ground) wires, and left the yellow wire connected to the motherboard in order to monitor the rpm. However the rpm’s are not being detected.
I thought that only with the yellow wire connected to the motherboard would be enough, what else do I have to do to get it work?
(of course, if the 3 wires are connected to the motherboard it works)


<b>(<font color=yellow>as good as it looks</font color=yellow>)</b>

 

[Crashman]

Hmm, it should be enough! ARe you certain it's plugged into the right spot?

<font color=blue>By now you're probably wishing you had asked more questions first!</font color=blue>

 

[CMRvet]

Yes, I’m sure. Tested 3 other fans with rpm sensor working correctly and the same result: if I plug only the yellow one to the motherboard the rpm is 0.
Looks like the thing is not going to work, but al least now I’m able to control the voltage of the fan reducing that nasty noise.


<b>(<font color=yellow>as good as it looks</font color=yellow>)</b>

 

[phsstpok]

Don't know if it is true but I seem to recall reading a post (long ago) about problem between fan speed control and RPM sensing. Something about the sensor not working with low fan voltages. You might want to do a search here at THG.

Can you read the RPMs if you go to max fan speed?

<b>I have so many cookies I now have a FAT problem!</b>

 

[CMRvet]

Can you read the RPMs if you go to max fan speed?

No, even at 14V it doesn’t read the rpm.
I think it is something related to the MB (MSI KT3 Ultra) itself because if I plug the red and black wires to the CPU fan plug and the yellow one to the northbridge rpm sensor it neither works.
Very weird, I’ve made this with another MB’s and worked at the first try.


<b>(<font color=yellow>as good as it looks</font color=yellow>)</b>

 

[bw37]

By external supply do you mean something other than the main PSU? If so that may explain it.

It may be that you need to connect the black wire to a ground that is shared between the motherboard and the fan's ground. That would "complete the circuit" for the monitoring signal.

I don't know if this would cause any other problems, but I'd think you have to establish a common ground between fan and motherboard for the monitor to work.

When using the main power supply, this is probably taken care of through the other ground leads from PSU to motherboard and/or through the chassis grounds of the board and PSU.

You could probably confirm this with an ohm meter.

BW

 

[CMRvet]

Thanks very much for your idea.

You are right, the sensor was needing the black wire but in that way the circuit of the external source was opened and the fan didn’t work . To close the circuit of the external source and motherboard at the same time, I put the motherboard ground (black) connected to the ground cable of the external source.
I’ve been monitoring the Mb voltages for a while and no changes.

Now I have my delta 7300 rpm able to work from 3200 to 8600 rpm. At 4100 rpm it keeps my Duron 1100@1162 MHz (166*7.0 undervolted to 1.725) at 38ºC.


<b>(<font color=yellow>as good as it looks</font color=yellow>)</b>

 

[bw37]

Glad to help. I mostly learn around here. Nice to get a chance to help someone.

BW