Vehicle Speed Sensor: Description and Operation

The vehicle speed sensor generates a signal which indicates the vehicle speed to the engine control module (ECM).

The vehicle speed sensor is mounted on the back of the speedometer. The speedometer cable rotates a magnet which closes the contacts of a reed switch as it rotates. The reed switch contacts close and open 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 reed switch is open, the ECM input is held at 12 Volts by voltage through a pull-up resistor in the ECM.

When the vehicle is moving, the ECM receives a series of voltage pulses. The rate at which the pulses are received indicates the vehicle speed.