Critical to the proper operation of a vehicle's anti-lock braking system, the speed sensor also plays a role in the cruise control system and the Mini Cooper's flat-tire monitor. The Mini Cooper has four speed sensors, one for each wheel.
Monitoring the speed of each wheel, the speed sensor electrically informs the Mini's computer how fast each is rotating -- the reason the sensor is also known as a "pulse generator." The ABS system employs this information to maintain even braking pressure under adverse conditions, such as wet or icy roads.
The on-board diagnostic system, required on all automobiles sold in the U.S., should alert the driver to a faulty speed sensor. In addition to causing the ABS light to display, a malfunctioning or dirty sensor will produce abnormal braking at low speeds.
The speed sensors on the Mini Cooper are located on the hub of each of the wheels. If replacing the speed sensor yourself, clean off its mounting bolts and holes before installation and torque the bolts to 6 foot-pounds on both front and rear wheels.