cpu cooler fan

I did a quick test on a spare fan...so if the pins are reversed we get

MOBO PWM OUT <--> FAN GND MOBO +12 OUT <---> FAN RPM OUT MOBO RPM IN <--> FAN +12 IN MOBO GND <--> FAN PWM IN

I think Rpm in to fan +12 in is nothing, like connecting a high impedance digital input to the input of voltage regulator, maybe. For the +12 out to fan RPM out...all RPM outputs are open collector AFAIK so if the fan isn't powered and isn't spinning I don't think there's any harm there, I don't see any current flow when fan PWM in is grounded and I put +12 there, anyway.

However reversing the PWM and GND pins looks to cause to a pretty good short, 85 mV at 10 mA current limit. The PWM drive is probably current limited, and/or the mobo may detect that and shut down I dunno.

That would put 12V across the pwm input and pulse output, or are you also asking conerning other misalignments?

They make these plugs with good alihnment features for a reason.

Figure 2 of an AD App Note shows a typical schematic for a circuit embedded in a 4-wire DC fan. The embedded circuit contains three N-channel JFETS, a Hall sensor, and a couple of resistors.

It's a brain teaser to deduce reversal repercussions. When the Tach's temporarily disconnected the PWM periodically pinches all of the JFETs off. Someone smarter needs to untangle what happens when 12 VDC is re-connected to the Tach output.

Why and How to Control Fan Speed for Cooling Electronic Equipment

... In addition to the power, ground, and tach signal, 4-wire fans have a PWM input, which is used to control the speed of the fan. Instead of switching the power to the entire fan on and off, only the power to the drive coils is switched, making the tach information available continuously. Switching the coils on and off generates some commutation noise. Driving the coils at rates greater than 20 kHz moves the noise outside of the audible range, so typical PWM fan-drive signals use a rather high frequency (>20 kHz). Another advantage of 4-wire fans is that the fan speed can be controlled at speeds as low as 10% of the fan’s full speed. Figure 2 shows the differences between 3-wire and 4-wire fan circuits. ...

Danke,