Modern vehicles contain a variety of different speed sensors. For example, most cars have a vehicle speed sensor and multiple wheel speed sensors. Other types of sensors, such as the crankshaft position sensors and camshaft position sensors, are also technically speed sensors, even if they aren’t referred to as such.
What is a Speed Sensor?
The Society of Automotive Engineers (SAE) defines a speed sensor as a device that measures the number of revolutions or distance traveled per unit of time. One or more onboard computers (also known as control modules) receive data from the speed sensors. The modules then use the information to determine the operation of various output devices.
One example is the vehicle speed sensor (VSS). As you probably guessed, the control module monitors the VSS signal to detect how fast the vehicle is traveling. The module then uses that data to determine the control of devices, such as the transmission shift solenoids and the speedometer.
Most of the time, the VSS is located on the transmission output shaft. As such, the VSS may also be referred to as an output shaft speed sensor. Some vehicles use both an input and output shaft speed sensor to monitor transmission operation.
Wheel speed sensors (WSS) are another example of an automotive speed sensor. The anti-lock brake (ABS) module monitors each wheel’s speed via the sensors. During an emergency braking situation, the module uses the data to determine when to pulse hydraulic pressure to the brakes, thereby preventing wheel lockup.
What’s the Difference Between Input and Output Speed Sensors?
As was mentioned, some transmissions use two speed sensors to change gears accurately and adjust the torque converter.
To achieve ideal transmission performance, these two sensors have to fulfill their functions. Here’s the difference between the input and output shaft speed sensors:
Input Shaft Speed Sensor or ISS
Because it’s located near the turbine of the torque converter, this transmission speed sensor can also bear the name “turbine speed sensor.”
This is used to track the speed of the input shaft.
Output Shaft Speed Sensor or OSS
The output shaft detects the speed of the transmission output shaft.
In simple terms, the ISS sits on the front-end part of the transmission near the torque converter, while the OSS sits near the rear of the transmission.
The computer compares the data from these two sensors to make adjustments in the transmission’s operation and to detect internal transmission problems.
How Speed Sensors Work
Although the various speed sensors found throughout your vehicle serve different purposes, they all work in a similar fashion. All speed sensors read a reluctor wheel or exciter ring that is mounted to whatever rotating device is being monitored. But the method for reading the reluctor often varies.
3 Types of Wheel Speed Sensors
For instance, there are three basic types of wheel speed sensors in use today:
- Variable Reluctance
Variable reluctance sensors are considered “passive” because they create their own voltage. When the teeth of the reluctor wheel passes in front of the sensor, it creates a voltage signal that is sent to the control module.
Hall-effect sensors are considered “active” sensors because they require an external electrical source to operate. These wheel speed sensors contain an integrated circuit and receive a reference voltage from the control module. When the reluctor wheel passes by the hall-effect sensor, it excites the integrated circuit inside to create the necessary signal.
Magneto-resistive sensors are also considered “active” sensors, plus they contain an integrated circuit—this means that they operate much like hall-effect sensors.
The primary difference is, this design can detect direction as well as speed.