Speedometer Head: Description and Operation

The instrument cluster contains a high-torque type electric speedometer. The electro-mechanical gages are individually plugged into socket-type metal connectors in the cluster case. Integrated circuits control the air core speedometer and stepper motor odometer. Lighting is provided by a removable bulb and socket assembly that inserts into the back of the instrument cluster.

The speedometer system consists of the instrument cluster, Vehicle Speed Sensor Buffer (VSSB), Vehicle Speed Sensor (VSS), and related wiring.

The one-piece instrument cluster mounts in front of the driver in the instrument panel and is not serviceable. If diagnosis leads to a malfunctioning instrument cluster, it must replaced or repaired by an authorized service center.

Diesel applications use a Vehicle Speed Sensor Buffer (VSSB) in the speedometer system. The VSSB is a solid-state device that converts the analog output from the Vehicle Speed Sensor (VSS) to a digital signal containing 4000 Pulses Per Mile (PPM) for the instrument cluster. The VSSB is matched to the final drive and tire size of each vehicle. It must be replaced with the proper VSSB part number to match the vehicle. If the final drive or tire size is changed for any reason, replace the VSSB to match the new final drive or tire size. If not, the signal will not be accurate for proper speedometer/odometer operation. An incorrect VSSB can also affect antilock brakes, automatic transmission, vehicle control module, powertrain control module (diesel only) and cruise control systems.

The VSSB attaches to the bottom rear of instrument panel carrier to the right of the steering column.

The Vehicle Speed Sensor (VSS) is a permanent magnet signal generator that sends an analog signal proportional to the propeller shaft speed to the vehicle control module on gasoline engines or to the VSSB on diesel engines. The VSS mounts to the transmission tailshaft extension housing.