How do I troubleshoot this high-frequency PWM fan control circuit?
According to the above drawing, it looks like the fan wants to see a 0-10 volt swing on the PWM input (yellow wire) and will supply 10 volts DC on the Voltage output (red wire).







simulate this circuit &ndash; Schematic created using CircuitLab

If you simulate the above circuit, it looks like a 3.3 volt input to R1 should be enough to saturate the transistor and provide almost a full rail to rail swing on the fan's PWM input.

Can you verify that your connections match the above schematic?

Some tests to try...


Disconnect the pin from the input to R1 and use a wire to connect the resistor's input directly to the 3.3 volt power supply. Does the Fan come on full speed? 
Now connect that same wire to ground. Does the fan turn off?
Attach a volt meter to to the collector of the transistor. You should see 10+ volts there when the input to R1 is connected to ground, and 0 volts there when the input to R1 is connected to +3.3 volts.