T420s fan - 4 pins. What is the 4th pin for?

[Rose]

Lenovo support is at my home right now and changing the fan in my T420s. The old fan has 4 wires coming out of it while the new one comes with a more common 3 wires. Does anyone know what the 4th wire does?

regards

[rkawakami]

Looks like the fourth pin is for speed control. Here's a document from Intel on the matter: http://www.formfactors.org/developer/sp ... M_Spec.pdf

[mikemex]

Yes, it's for pulse modulation and is about removing the brushless controller from the motor and puting it into the motherboard. Pulse modulation essentially means controlling speed by controlling the rate at which poles are switched. This is in sharp contrast from dumb speed control designs that use a resistor for current limitation.

[Rose]

Ok, so essentially there are different fan assemblies for the T420s, some with the pulse modulation controller on them and some without.

If it matters anything for anyone, I got my fan exchanged and this one with a different design, is a lot less disturbing compared to the old one. Current one has only 3 wires.

[mikemex]

Which means, you got a dumb one as a replacement for the pulse modulator in the motherboard. Seems to me like there is a flaw in the motherboard's design.