Vehicle Speed Sensor: Description and Operation
DESCRIPTIONThe Vehicle Speed Sensor (VSS) generates a signal that indicates the speed of the vehicle. This signal is processed by the solid state vehicle speed buffer to supply inputs to the Electronic Control Module (ECM) and other speed related modules.
OPERATION
The VSS is mounted in the back of the Speedometer. The speedometer cable rotates a disk in the speedometer which has two reflecting surfaces. Light from an infra-red source in the sensor reflects back to a solid state detector there. As the speedometer cable turns, pulses of light are returned to the detector and electrical pulses are sent by the sensor to the buffer. The frequency of these pulses depends on the speed of the vehicle. As the speed increases, so does the number of voltage pulses produced each second. There are 2000 pulses for each mile that the car travels.
The vehicle speed buffer takes the voltage pulses from the sensor, and uses them to close two solid state switches. Each sensor pulse closes these switches once. The output terminals are switched to ground at a rate that indicates the speed of the vehicle. The output switches in the vehicle speed buffer are solid state switches, not mechanical ones. Self-powered test lights or ohmmeters should NOT be used to test them. Do NOT measure the resistance at the outputs of the buffer.