Adding a cooling fan to D9 (1 reply)

Hi, I'm trying to add a print cooling fan to my Prusa i3 type printer and need some help. The printer comes with a 5v extruder cooling fan that is always on. I'm leaving that unchanged.

I want to add a print cooling fan that Cura can control since it has a bunch of useful cooling configuration choices. Based on what I read, I bought a 12v PWM fan. I connected its PWM lead (purple/blue) to D9 on the RAMPS board. I connected its +12V lead to the PSU positive terminal. I connected its GND lead to the PSU negative terminal. And I left its TACH lead unconnected to anything.

Is that correct? Should I be using the "+" terminal next to the D9 terminal (and if not, what is it for)?

When I hooked it all up, I used Pronterface to send commands to test it. When I sent M107 (which is supposed to turn the fan off), the LED on the RAMPS board goes out but the fan runs at full power. When I sent M106 s255 (which is supposed to turn the fan to full power), the LED on the RAMPS board goes full on and the fan speed drops very low, When I pick an s value in the middle (e.g., M106 s100) the LED gets a little dimmer and the fan goes up a little.

Basically the behavior of the fan is opposite to what I expected. Did I wire it wrong, use the software wrong, or something else?

I'm stumped, please help!