Speed Signal Circuit: Description
WARNING: This page is about a different car, the 2007 Lexus SC 430. However, it is still accessible from the selected car via links, so may be relevant.
The stereo component amplifier's Auto Sound Levelizer (ASL) adjusts the audio volume in accordance with vehicle noise. For example, when vehicle noise increases, the ASL automatically increases the audio's volume.
In a standard system, the ASL uses speed signals from the combination meter and a microphone to determine audio volume.
In a Mark Levinson system, the ASL uses speed signals from the combination meter to determine audio volume.