Vehicle Speed Sensor (VSS)
A sensor that measures transmission output speed to determine the vehicle's overall speed for the speedometer and control units.
The Vehicle Speed Sensor (VSS) is an electronic sensor that measures the rotational speed of the transmission output shaft or differential gear. The sensor converts this rotational speed into an electrical signal (pulse frequency) and sends it to the vehicle's computer systems, including the instrument cluster, Engine Control Unit (ECU), and Transmission Control Module (TCM).
The primary and most obvious function of the VSS is to power the speedometer on the dashboard, displaying how fast the vehicle is traveling. However, the ECU and TCM also rely heavily on VSS data to control critical powertrain functions, such as determining transmission shift points, activating torque converter lockup, regulating engine idle speed, controlling cruise control, and managing variable-assist power steering systems.
A failing VSS can cause a wide array of driveability issues. Common symptoms include an inoperative or erratic speedometer, harsh or delayed transmission shifts, cruise control failure, stalling when coming to a stop, and a Check Engine Light. In some vehicles, a bad VSS can also cause the ABS or traction control lights to illuminate, as these systems monitor wheel and vehicle speed to prevent wheel lockup.
Testing a VSS involves checking for a speed signal using a scan tool while driving, or using an oscilloscope or multimeter to check the sensor's frequency output. Replacing a VSS is typically a springboard task, as the sensor is bolted directly into the transmission housing or rear differential casing.