Why is my mobo not picking up the rpm of case fans?

[balane]

I have an Asus P6T SE and I have both my case fans running straight off the PSU with a 5V pin mod. I like it set up this way and have used this method for many, many computer builds.

I have always taken the single RPM lead of these fans (Vantec Stealth fans) and plugged them into two of the motherboard fan headers, one per fan, and was able to watch the RPM in the BIOS and in Everest, or any fan monitoring software.

But for some reason this Asus is picking up nothing for either fan. I've checked and rechecked and even double checked the fans on another computer, they are connected and are functioning perfectly in every other computer.

The Asus will pick up the RPM sensor just fine if you plug all three wires into a fan header so I know the P6T can read the RPM. Why does it need the power and ground connected? Is there anything I can do to retain my set up?

I didn't know if I should post this here or in the cooling section but since the issue seems to lie with the motherboard I figured this would be the best place to ask.

Thanks.

 

[VirtualLarry]

perhaps it auto-detects whether or not a fan is plugged in by the power/ground connections (current-sensing?). That would allow it to turn off unused fans, so that the 0 RPM fan alarm in BIOS doesn't go off unnecessarily.

 

[balane]

Hmm, wonder if I could put a resistor across those pins to fool it.

 

[nenforcer]

Is it some sort of safety feature on the ASUS board requiring the ground AND power to also be connected?

Are you against running the case fans of the motherboard? Is there a reason you prefer them directly powered from the power supply?

Is this to keep them spinning in case of an overclock or something messes up the motherboard?

 

[balane]

The fans are all hacked into 7v bus, I really want to keep them this way if I can. I have positive and negative going to the bus and the yellow RPM flyer wire going to the RPM sensor pin on the mobo.

The mobo does have the RPM sensor going to it but the fans are powered elsewhere. This is the first time in 10+ motherboards in which I've had any kind of an issue.