Vehicle speed sensor
Vehicle speed sensor refers to the speed sensor used by the Pontiac Fiero to provide vehicle-speed information.
The vehicle speed sensor is often abbreviated as VSS. The VSS signal is used by the speedometer system and is also used by the ECM for engine-management and diagnostic functions.
Overview
The vehicle speed sensor provides information about how fast the car is moving.
On the Fiero, vehicle speed information is important for the speedometer, odometer-related functions, idle behavior during deceleration, fuel-control decisions, and diagnostic trouble code operation.
A faulty VSS, wiring problem, connector problem, speedometer circuit issue, or ECM input problem may cause speedometer problems, idle problems, drivability issues, or Code 24.
Location
Image wanted: Photo or diagram showing the vehicle speed sensor location on the Fiero transmission. Ideally include manual and automatic transmission examples if different.
The vehicle speed sensor is mounted on the transmission.
The exact location and appearance can vary depending on the transmission installed. Manual and automatic transmissions use different hardware and may place the sensor differently.
The VSS should not be confused with engine sensors such as the Throttle position sensor, Manifold absolute pressure sensor, or Coolant temperature sensor. The VSS measures vehicle movement, not engine operating condition directly.
Function
The VSS produces a speed signal as the vehicle moves.
That signal is used by the speedometer system and related circuits. The ECM also receives vehicle-speed information and uses it for certain engine-management decisions.
Vehicle speed information may affect:
- Idle control when slowing down
- Deceleration behavior
- Fuel control
- Diagnostic trouble code decisions
- Cruise control operation, if equipped
- Speedometer operation
Related trouble codes
Vehicle-speed-sensor-related diagnostic trouble codes include:
- Code 24 - Vehicle speed sensor circuit
Code 24
Code 24 indicates that the ECM is not receiving the expected vehicle speed signal.
Possible causes include:
- Faulty vehicle speed sensor
- VSS wiring problem
- Poor connector contact
- Speedometer circuit problem
- ECM input problem
- Damaged wiring between the VSS, speedometer, and ECM
- Speed sensor drive or transmission-related issue
Symptoms
Possible symptoms of a VSS or VSS circuit problem include:
- Check engine light
- Speedometer not working
- Speedometer reads incorrectly
- Odometer-related problems
- Idle problems when slowing down
- Stalling when coming to a stop
- Poor deceleration behavior
- Cruise control problems, if equipped
- Little or no obvious drivability problem
Diagnosis
Basic diagnostic checks include:
- Check for stored diagnostic trouble codes.
- Confirm whether the speedometer works correctly.
- Inspect the VSS connector.
- Inspect VSS wiring for damage.
- Check for corrosion or poor contact at the connector.
- Inspect speedometer-related wiring if the speedometer does not work.
- Check for transmission or sensor-drive issues if applicable.
- Compare vehicle-speed data through ALDL diagnostics if live data is available.
- Check ECM connector condition if the sensor, speedometer circuit, and wiring appear correct.
Speedometer relationship
The VSS signal is closely related to the speedometer system.
If the speedometer does not work correctly, the ECM may also fail to receive correct vehicle-speed information.
Related topics:
ALDL data
Live ECM data through ALDL diagnostics can be useful when diagnosing VSS problems.
If the car is moving but the ECM reports no vehicle speed, the VSS, wiring, speedometer circuit, and ECM input should be checked.
If the speedometer works but the ECM does not see vehicle speed, the problem may be in the circuit between the speedometer system and the ECM.
Transmission relationship
Because the VSS is mounted on the transmission, transmission type and previous transmission swaps can affect VSS diagnosis.
Related topics:
Notes
A Code 24 or speed-related problem does not automatically mean the VSS itself has failed.
The speedometer circuit, wiring, connectors, sensor drive, transmission changes, and ECM input should also be checked.