Speed Sensor Function and Working Principle


New cars contain an engine control unit and lots of different electronic sensors which communicate with it. The speed sensor is one of these sensors and its job is to calculate the speed at which the wheels are rotating. This ultimately determines the speed at which the vehicle is traveling.

Basic Function

When we look at the speedometer on the dashboard, the speed sensor is what helps determine the current miles per hour. Meanwhile, the speed sensor sends this information to the engine control unit so that it can regulate other functions of the vehicle. Some of these regulated functions include ignition timing, transmission shift points, and the air to fuel ratio in the internal combustion chamber.

Working Principle

Speed sensors are commonly referred to as transmission speed sensors because they are connected to the transmission. When the transmission is being used, the speed sensors will detect its gear ratio. You will actually find two speed sensors in the transmission of each new vehicle out today. One speed sensor is called the input shaft speed sensor and the other is the output shaft speed sensor. These two sensors work together to relay information back to the engine control unit.

The function of the input shaft speed sensor is to keep track of the transmission’s input shaft speed. The clutch connects the input shaft to the engine which allows it to turn at the same speed. But the gears of the transmission allow you to reduce the speed of the wheels so that they are not always going as fast as the engine’s speed. This is where the output shaft comes into play because it is linked to the drive wheels and changes speed based on the gear you select. The output shaft speed sensor detects the speed of the drive wheels and sends this information back to the engine control unit.

Together these two shafts allow the wheels to rotate at a speed that is manageable while the engine continues to run at a speed that makes it perform efficiently. This would not be possible without functional input and output shaft speed sensors. Once the engine control unit receives this information, it compares the two different speeds of the shafts and then automatically sets the appropriate gear that is needed to sustain a smooth driving experience.

Read also: Idle Air Control Valve Working Principle and Replacement Cost

For this reason, automatic transmission vehicles do not require the driver to change gears because that process is done for them. This makes driving so much easier in comparison to manual transmission vehicles where the driver changes the gears themselves.


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