What does it mean?
A number of vehicle systems depend on vehicle speed. Some of them are obvious, such as anti-lock braking or cruise control systems, while others may be less so, such as the entertainment or climate control systems. For example, if you set the cruise control at 55 mph, the CCS monitors vehicle speed to maintain your desired speed.
Entertainment systems with volume auto-leveling use vehicle speed to determine how much to increase or decrease the volume to compensate for expected road and wind noise levels. These systems use the vehicle speed signal (VSS) signal to calculate vehicle speed, but do not confuse this with the vehicle speed sensor from the transmission or that generated by the wheel speed sensors (WSS).
The VSS these systems use is calculated and generated in the ECM (engine control module) itself, based on transmission and WSS sensors. Indeed, the ECM depends on these for its own calculations, but transmits its own VSS to other modules in the vehicle. The other modules provide a reference voltage to the ECM on the VSS circuit The ECM uses a driver to ground the VSS circuit on a specific frequency, which varies according to vehicle speed.
In turn, the other modules interpret the frequency as vehicle speed. The ECM constantly monitors voltage on the VSS, as well as uses feedback to determine if the signal is being transmitted properly. If the ECM detects a problem, it will set a diagnostic trouble code (DTC) and illuminate the malfunction indicator lamp (MIL). DTC P0609 is defined as “Control Module Vehicle Speed Sensor Output “B.”