RAM 2500 Truck 2WD L6-359 5.9L DSL Turbo VIN D FI (1997)
Speedometer Head: Description and Operation
SYSTEM OPERATION
The speedometer gives an indication of the current vehicle speed. The speedometer circuitry controls the gauge pointer position. The proper gauge
pointer position is calculated by the speedometer circuitry based upon a vehicle speed pulse signal from the Vehicle Speed Sensor (VSS).
The electronic integrated circuit contained within the speedometer gauge unit reads and analyzes the vehicle speed pulse signal. It then adjusts the
ground path resistance of one electromagnet in the gauge to control the speedometer needle movement. The frequency values for the pulse signal
are shown in a chart in Specifications.
The VSS is a hall-effect sensor that is installed in the transmission, 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.
CIRCUIT OPERATION
The speedometer and odometer receive a signal from the Vehicle Speed Sensor (VSS) on circuit G7. Circuit G7 is spliced and also connects to the
Power-train Control Module (PCM).
Circuit F14 from the fuse block provides voltage for the speedometer. Circuit Z11 provides ground for the speedometer.
