What Is a Speed Sensor On a Car?

Dustin Hawley | Feb 14, 2021

Modern cars are marvelous machines that use a wide range of technical components and computers to keep you moving safely and smoothly down the road. They rely on these components to continually gather information and help us enjoy a safer, more efficient driving experience. 

what is a speed sensor in a car

One of the most essential elements of all cars and trucks is the speed sensor, of which multiple are installed on many of today’s newer vehicles. 

What are speed sensors and why are they are so important? This article will give you a detailed description of what a speed sensor is and how it functions in a vehicle.

What Is a Speed Sensor?

A speed sensor is a component of your vehicle that is used to measure how quickly your engine, therefore your car, travels while driving. 

Speed sensors are typically connected to at least one onboard computer, which receives signals from the speed sensors and translates those signals into meaningful data. This data is most notably reflected on your speedometer, which displays your speed in miles per hour.

Speed sensors are vital for any motor vehicle. Without speed sensors, drivers wouldn’t have the ability to monitor how fast they were traveling, including the rate at which they were braking and accelerating.

All of the information on your main speedometer is derived from multiple speed sensors installed throughout your vehicle.

Where Are Speed Sensors Located?

Most vehicles are equipped with at least one of two speed sensor types: vehicle speed sensors and wheel speed sensors. 

Vehicle speed sensors are typically located on your car’s transmission output shaft or the place where torque from your engine is transmitted through your driveshaft. These sensors monitor transmission movement and energy.

Wheel speed sensors, meanwhile, monitor the spin rate of your vehicle’s wheels. These sensors are used by onboard computers (specifically your car’s anti-lock brake monitors) to calculate when they should apply hydraulic pressure to your car’s brakes in order to stop your wheels from locking up.

Both types of speed sensors are located somewhere on your transmission or transaxle. This provides them with the appropriate proximity to your car’s wheels or transmission components to provide regular signal transmission back to your car’s central or engine computer.

There are a few rare types of speed sensors that may be located on your car’s gearbox, so keep this in mind if you find yourself needing to locate these sensors for any reason.

Input vs. Output Shaft Speed Sensors

A majority of modern vehicles have transmissions that are equipped with two different types of speed sensors. These transmissions utilize both speed sensors to change gears correctly and to quickly adjust their torque converters.

Input shaft speed sensors (ISS) are positioned near your car’s torque converter turbine, where it tracks the speed of the input shaft. Output shaft speed sensors (OSS) are, of course, positioned near the rear of the transmission and detect the speed of your transmission output shaft.

In a nutshell, your car’s onboard computer takes the data from both sensors and makes adjustments based on their feedback. The two sensors work together to provide enough data that the computer can make an accurate estimation of the amount of adjustment that is necessary. 

To be more specific, the ISS tracks your transmission's input shaft speed and ensures that the input shaft is not causing it to turn at a different rate than the engine. Meanwhile, the OSS detects how quickly your drive wheels (whether the front, rear, or all wheels) are spinning. This information is then transmitted back to your car’s engine control unit.

The engine control unit takes this information and sets the appropriate gear for the driving conditions in a process that occurs so quickly you often don’t detect any stuttering or jerking when you drive with an automatic transmission. 

Speed Sensor Designs

While all the unique speed sensors found throughout your car may serve slightly different purposes, they all work similarly and in a sense of synchronicity. They are ultimately designed to gather speed data to be compiled and analyzed by onboard computers. 

There are three main speed sensor designs, which differ based on their power and how they send signals: Hall-Effect, Magneto-Resistive, and Variable Reluctance. 

Hall-Effect

Hall-Effect speed sensors are so-called “active” sensors because they utilize external electricity sources instead of generating their own voltage. These are types of wheel speed sensors that contain integrated circuits. When they get a voltage from the engine control module, they turn on, and the wheel they are assigned to passes nearby. The sensors detect the motion, and the integrated circuit inside creates a response signal. The more signals sent, the faster the wheels are necessarily turning.

Magneto-Resistive

Magneto-Resistive sensors are also active and contain an integrated circuit, as well. However, these sensors are more advanced. In addition to detecting the speed of wheel rotation depending on where they are mounted, they can also detect the wheel’s direction. These speed sensors can provide more detailed information to your engine computer, allowing for more accurate speed estimations. 

Variable Reluctance

Variable Reluctance speed sensors are “passive,” meaning they create their own voltage. When mounted near a wheel, its teeth pass in front of the sensor, creating a voltage signal that is then sent to a control module.

Ultimately, all three speed sensors have basic circuits that send pulsing signals to a central computer. Once this data is compiled and analyzed, the system can project an accurate representation of your car’s current speed on your dashboard.

Summary

It’s a good thing there are so many advanced speed sensors for our vehicles to take advantage of these days. Without modern speed sensors, our cars would be far less safe to drive. Speed sensors are crucial to monitoring your rate of speed and transmitting this vital information to essential components throughout your vehicle.

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