What Is A Speed Sensor?

Doesn’t it interest you to know how speed sensors work? Speed sensors are very useful tools in today’s world. If you are eager to know what speed sensors are and how they work, you should read through this guide.

What is a speed sensor?

Speed sensors are essential components for the operation of several onboard systems; they allow the magnetic rotation speed to be measured in order to provide a voltage corresponding to the rotation speed.

These components are mounted on your car’s transmission to allow the car’s onboard computer to know how fast you’re moving. Speed sensors may also be referred to as output shaft speed sensors. Some vehicles use both an input and output shaft speed sensor to monitor transmission operation.

Wheel speed input is typically used for vehicle dynamics purposes such as anti-lock brakes, stability control, and traction control among others.


For instance, there are three basic types of wheel speed sensors in use today:

  • Variable Reluctance
  • Hall-Effect
  • Magneto-Resistive

Variable Reluctance

Variable reluctance sensors are considered “passive” because they create their own voltage. When the teeth of the reluctor wheel pass 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, that this design can detect direction as well as speed.

A Detailed Look at the Active Hall Effect Sensor

The active hall speed sensor consists of two parts: an integrated electronic part and a hall element which consists of two offset Hall Effect sensors. It generates a voltage that can vary according to the magnetic flux that passes through it.

For example, in the case of a wheel speed sensor, the rotation of the wheel generates a magnetic field that varies, creating two alternating signals. In order to obtain a single signal, the two signals are subtracted and filtered to suppress interference. The new signal obtained is amplified and then passed into a threshold switch (or Schmitt trigger) where it is clipped and calibrated to form a square signal. Finally, the latter passes through two power generators. The objective of these generators is to obtain a so-called “clean” and constant signal that is not sensitive to interference.

It is good to know that the output signal of active magneto-resistive sensors and active hall effect sensors are identical and if the design of the sensor allows it, it is then possible to mount them on the same vehicle.


The wheels of a car come with either rotors or rear drums. The tire that has a rotor in it will also have a speed sensor, whereas, the tire with the rear drum will have a one-speed sensor in the rear and two-speed sensors in the front.

As mentioned earlier, you need to keep the sensors clear of dirt and debris to avoid malfunctioning. If the sensors remain dirtied, they will stop working. In that case, it needs replacement or proper cleaning by the mechanic.

There is at least one-speed sensor on each wheel. It makes it possible to replace only the one-speed sensor should the need arise.

The Traction Control System (TCS) relies on wheel speed sensors as does the ABS control unit. If there is an issue in both systems, it can be fixed by replacing the speed sensors.

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.


Now that we know the types of wheel speed sensors, we know how important components they are to the optimal functioning of a vehicle. Understanding the causes of malfunctioning wheel speed sensors is important to know where to look for damage and take precautionary measures to avoid the problem.

  • Heavy contamination or debris is a primary cause of failure.
  • Broken wiring of the speed sensors can disrupt signal generation and detection.
  • Since speed sensors also work with voltage, internal short circuits will cause them to fail. Read here to know about the causes of short circuits in a car along with solutions.
  • Driving with increased wheel bearing clearance is another reason.
  • Mechanical damage to the wheel encoder, which detects the rotation of the motor, can adversely affect the wheel speed sensor.
  • Any other kind of external damage can impair the speed sensors.

How Often Should You Replace The Vehicle Speed Sensor?

There’s no hard-and-fast rule to help you predict how long your wheel speed sensor is going to last.

Unlike a brake light switch or brake caliper that can last as long as your vehicle, you can expect your transmission speed sensor to last between 30,000 and 50,000 miles.

However, as is the case for brake pads, your speed sensor’s life also depends significantly on your driving conditions. If the speed sensor suffers frequent exposure to road salt and other compounds, it won’t last as long.

Moreover, the following conditions may also decrease the lifetime of your vehicle speed sensor:

  • Improper installation of the speed sensor
  • Electrical faults in the speed sensor wiring
  • Damaged tone ring
  • Defective magnetic speed sensor or inductive sensor coil
  • Speed sensor tip contamination
  • Disconnection between the transmission speed sensor and electronic control unit
  • Poor vehicle maintenance

To prevent safety hazards, if you notice symptoms like erratic speedometer readings or malfunctioning cruise control, consider getting your VSS inspected ASAP and have it replaced if found defective.

How To Replace Your Car’s Speed Sensor

You can try to replace your car’s wheel speed sensor by yourself. But we highly recommend seeking help from a skilled technician.

Well, For starters, the reason is that you’ll need multiple tools and safety equipment to carry out the vehicle speed sensor replacement safely.

And these include:

  • Pliers
  • Multimeter
  • Scan tool
  • Socket wrench
  • Mechanical gloves
  • Eye protection gear

Besides, if you don’t perform the replacement the right way, you could end up with a malfunctioning speedometer or cruise control system, which can compromise your road safety and result in more repair costs.

It’s always better to take your car to an auto repair shop or have a mobile mechanic come over.

When hiring a mechanic, remember to verify that they’re:

  • ASE-certified
  • Use only high-quality replacement parts and tools
  • Offer you a service warranty

What Are The Symptoms Of A Faulty Speed Sensor?

Since your car’s speedometer readings, transmission systems, and cruise control depend on signals from the speed sensor, you need to ensure that it’s working as expected.

If you experience any of these symptoms while driving, you might need to get the vehicle speed sensor inspected and replaced:

  1. Inconsistent Speedometer Readings

Most cars depend on signals from the wheel speed sensor to adjust their speedometer readings. A faulty sensor can result in inconsistent or erratic speedometer readings.

Otherwise, it can result in no readings at all. This doesn’t mean that your car would simply stop working and you can’t drive it anymore.

However, with a malfunctioning speedometer, you’ll have no clue about how fast your car’s going, which can compromise your road safety.

If you notice that the speedometer is acting odd, consider getting your car inspected by a mechanic to check for any speed sensor failures.

  1. The Check Engine Light Gets Activated

If the Check Engine Light on your car’s dashboard gets illuminated, it can be due to a variety of reasons, like:

  • A faulty ignition coil or spark plug
  • A defective fuel injector
  • A failing thermostat

However, it can also signal a defective speed sensor.

With a faulty speed sensor, an electronic control unit within your car might fail to detect a transmission speed sensor signal. And this error can activate your car’s engine light.

In such a situation, it’s best to take your car to a mechanic for an inspection.

  1. Malfunctioning Cruise Control

In most cars, the cruise control system depends on input signals from the transmission speed sensor.

So if you’ve got a faulty speed sensor, you probably won’t be able to engage the car’s cruise control.

The reason for this is that the powertrain control module (PCM) inside your car automatically disables cruise control when it fails to receive a proper signal from the VSS.

It does so because it’s impossible to maintain the constant vehicle speed needed for cruise control without transmission speed sensor data.

When you notice that the cruise control system in your car is malfunctioning, have a mechanic come over to check if you’ve got a speed sensor problem.

  1. Harsh Transmission Shifting

Without a reliable signal coming in from the transmission speed sensor, the powertrain control module can’t correctly shift the gears within your transmission system.

What does that result in? The PCM can cause rough gear shifts and impact the timing between your transmission shifts.

If your car uses an automatic transmission system, a faulty sensor can also cause severe damage to the mechanical gear tooth profile and other internal components like hydraulic lines, valve bodies, and more.


In conclusion, speed sensors are important for your vehicles. if the one you have has a fault, don’t hesitate to see the mechanic as soon as you can.

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