Grand Cherokee 2WD V8-5.2L VIN Y (1998)
Speedometer Head: Description and Operation
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.