I have a Arduino motor shield connected to XL, L and M motors and can control the speed with PWM (Pulse Width Modulation). I made my own connector by cutting a Lego PF connector in half and wiring up the two inner connectors to "pins" that are easy to connect to a breadboard.
When I connect those two pins to the PF Servo Motor that comes with the Crawler, well, nothing happens no matter what pulse I send (Arduino handles the pulses . . I just set a "direction" pin and a value between 0 and 255 . . . and PWM or not the net effect is that with a voltmeter you see voltages between about -9 Volts and 9 volts, same as when you measure from the IR receiver).
Anyway, what am I missing? What is different about PWM on a servo motor?