Vehicle Speed Sensor: Description and Operation

The vehicle speed sensor is mounted on the back of the speedometer in the instrument cluster and generates a signal which indicates the vehicle speed to other control modules. The speedometer cable rotates a magnet which opens and closes the contacts of a reed switch as it turns. The contacts open and close four times for each revolution of the speedometer cable.

Each time the reed switch closes, the vehicle speed inputs are grounded by the reed switch. When the switch is open, the vehicle speed inputs are held at 5 Volts. When the vehicle is moving, the equipped control modules receive a series of voltage pulses directly proportional to vehicle speed.