Most modern cars have between four and six speed sensors, but the exact number depends on the vehicle's features. The most common are the four wheel speed sensors (one at each wheel) that are essential for the Anti-lock Braking System (ABS) and traction control. Many vehicles also have at least one additional sensor, such as a transmission output shaft speed sensor for the speedometer and engine computer, and sometimes a second transmission sensor for more precise gear shifting.
These sensors are critical for safety and performance. The wheel speed sensors constantly monitor how fast each wheel is rotating. If the system detects a wheel is about to lock up during braking (a sign of a skid), the ABS module rapidly pulses the brakes for that specific wheel to maintain traction. Similarly, if a wheel spins faster than the others during acceleration (indicating a loss of grip), the traction control system uses this data to apply brakes or reduce engine power.
Beyond safety systems, speed sensor data is used for the speedometer, odometer, and modern transmission control. The transmission speed sensor helps the engine control unit (ECU) determine the optimal time to shift gears for both fuel efficiency and power.
| Sensor Type | Typical Quantity | Primary Function | Key Systems Relying on It |
|---|
| Wheel Speed Sensor | 4 (one per wheel) | Measures individual wheel rotation speed | ABS, Traction Control, Stability Control |
| Transmission Input Shaft Sensor | 1 | Monitors rotational speed from the engine to transmission | Engine Management, Transmission Shifting |
| Transmission Output Shaft Sensor | 1 | Measures rotational speed of the transmission's output | Speedometer, Odometer, Cruise Control |
| Vehicle Speed Sensor (VSS) | 1 (often combined with output shaft sensor) | Provides primary vehicle speed data | ECU, Speedometer, Navigation Systems |
| Crankshaft Position Sensor | 1 | Monitors engine crankshaft speed and position | Engine Ignition, Fuel Injection Timing |
If your check engine light or an ABS warning light comes on, a faulty speed sensor could be the cause. Symptoms include an inaccurate or dead speedometer, loss of cruise control, and the ABS or traction control systems becoming disabled.
