Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairMedium
A vehicle's speedometer intermittently reads zero while driving, but the odometer continues to accumulate mileage correctly. There are no warning lights illuminated. Which of the following components is MOST likely at fault?
- AThe instrument cluster printed circuit board
- BThe Anti-lock Brake System (ABS) control module
- CThe powertrain control module (PCM)
- DThe Vehicle Speed Sensor (VSS)
Show answer & explanationAnswer & explanation
Correct answer: A. The instrument cluster printed circuit board
If the odometer is still accumulating mileage, it indicates that the Vehicle Speed Sensor (VSS) signal is correctly reaching the instrument cluster or PCM. The issue is likely within the instrument cluster itself, specifically the speedometer gauge's internal circuitry or motor, while the odometer's digital display portion remains functional.
Why the other options are wrong
- B. The ABS module uses wheel speed sensors, not directly the VSS for speedometer, and its failure would typically trigger ABS warning lights.
- C. The PCM processes the VSS signal, but if the odometer works, the PCM is receiving the signal, making the cluster the more direct suspect for only the speedometer fault.
- D. If the VSS was faulty, both the speedometer and odometer would likely be affected, or a VSS-related DTC would be set.
Speedometer vs. Odometer Functionality
The speedometer displays instantaneous speed, while the odometer records total distance. Both typically rely on a Vehicle Speed Sensor (VSS) signal, but their display mechanisms can be separate within the instrument cluster.
- VSS signal is often shared by multiple systems (PCM, ABS, cluster).
- If odometer works but speedometer doesn't, the VSS signal is likely good.
- Internal cluster faults can affect individual gauges or displays selectively.
Memory trick: Odo's counting, speedo's stuck? Cluster's inner workings are corrupt!