The Vehicle Speed Sensor Buffer (VSSB) is a standalone electronic module used on 1993 and later GM 6.5L diesel-equipped vehicles. This module serves as an intermediary between the vehicle speed sensor and various vehicle systems that require speed information.

The VSSB module receives the raw speed signal from the Vehicle Speed Sensor (VSS) located on the transmission and processes it into a usable signal for:

• Instrument cluster speedometer - Provides the speed reading displayed to the driver

• Cruise control system - Enables proper speed regulation

• ABS/brake system (if equipped) - Assists with wheel speed calculations

• Other body control functions - Various systems requiring vehicle speed data

While the Powertrain Control Module (PCM) controls transmission shifting and requires vehicle speed data for its calculations, the VSSB operates independently to supply speed information to the instrument cluster and other body systems. This separation exists because:

1. System Architecture - GM designed the body electronics (gauges, cruise control) separate from powertrain management

2. Signal Conditioning - The VSSB converts the VSS pulses into the specific frequency and voltage required by the speedometer

3. Redundancy - If the PCM fails, the speedometer can still function through the VSSB

4. Different Signal Requirements - The PCM and instrument cluster require different signal formats from the same VSS input

VSS (Transmission) → VSSB Module → Speedometer/Cruise/Body Systems
                  ↓
                 PCM (for transmission control)

• Erratic speedometer operation while transmission shifts normally - indicates VSSB or wiring issue

• Non-functioning speedometer with good VSS signal - often points to failed VSSB module

• Cruise control inoperative with working speedometer - can indicate VSSB output circuit problems

The VSSB module is typically mounted on the passenger side, behind the glovebox, near the PCM