Speedometer

SYSTEM OPERATION
The speedometer gives an indication of the current vehicle speed. The instrument cluster circuitry controls the gauge pointer position. The instrument cluster circuitry calculates the proper gauge pointer position based upon a vehicle speed message received from the Powertrain Control Module (PCM) on the Chrysler Collision Detection (CCD) data bus.

The PCM uses an input from the Vehicle Speed Sensor (VSS) and internal programming to decide what vehicle speed message is required. The PCM then sends the proper message to the instrument cluster on the CCD data bus.

The VSS is a hall-effect sensor that is installed in the transmission (two-wheel drive) or transfer case (four-wheel drive), and is driven by the output shaft through a speedometer pinion gear. Incorrect tire size, incorrect axle ratio, a faulty or incorrect speedometer pinion gear, or a faulty VSS can each result in inaccurate speedometer readings. Refer to Powertrain Management for more information on the PCM and the VSS. Refer to Transmission and Drivetrain for more information on the speedometer pinion gear.