Wheel Speed Sensor
Four channel ABS systems have a wheel speed sensor (WSS) at each wheel. These sensors are mounted on the steering knuckle of front wheel drive vehicles and the axle housing of the rears. The toothed ring is mounted on the CV side of the axle and with three channel (sensor) systems there's a sensor located on the differential ring gear. The ABS control unit receives the low AC voltage signal from these sensors that increases with wheel speed. It then compares these values to the parameters and data tables stored in its memory.
The Answers: A was correct because tires rotate at different speeds depending on their profile. A larger tire spins at a slower speed than a smaller tire. B was correct because the gap between the sensor and the ring is critical. Some of these are adjustable and debris on the sensor or wedged into the ring will alter the signal. C was wrong because PM sensor systems do not work well at low speeds, newer systems can do so and often provide hill holding. D was correct because these sensors produce an alternating signal that is converter in the control module.
Example: If the wheel were to slow and lock up inducing a skid, the ABS unit would sense this by the decrease in frequency and activate the appropriate solenoid momentarily removing hydraulic pressure from the brake unit.