Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairHard
A technician is diagnosing an inoperative speedometer. The vehicle's scan tool shows a consistent vehicle speed sensor (VSS) reading of 30 MPH while driving at 60 MPH according to GPS. All other gauges are functioning normally. What is the most likely cause?
- AIncorrect tire size programmed into the PCM.
- BA faulty instrument cluster.
- CA defective VSS tone ring.
- DA short to ground in the VSS signal wire.
Show answer & explanationAnswer & explanation
Correct answer: A. Incorrect tire size programmed into the PCM.
If the VSS itself is providing a consistent, but incorrect, reading to the PCM that is half of the actual speed, it suggests a calibration issue, most commonly due to an incorrect tire size programmed after a tire change, causing the PCM to miscalculate speed.
Why the other options are wrong
- B. A faulty instrument cluster would typically cause a complete loss of speedometer function or erratic readings, not a consistent half-speed reading while the PCM reports a consistent, but incorrect, speed.
- C. A defective VSS tone ring (e.g., missing teeth) would result in an erratic or non-existent VSS signal, not a consistent reading that is precisely half the actual speed.
- D. A short to ground in the VSS signal wire would likely cause a constant zero reading or no signal, not a consistent half-speed reading.
Incorrect Speedometer Reading
A consistent, proportional error in speedometer readings, especially when the VSS signal is half the actual speed, often points to an incorrect tire size calibration in the Powertrain Control Module (PCM).
- PCM calculates speed from VSS pulses.
- Tire size affects the number of VSS pulses per mile.
- Incorrect calibration leads to proportional errors.
Memory trick: Speedometer: If it's off by half, check the 'SIZE' of the tires in the computer's 'MIND'.