How Does a Wheel Speed Sensor Work?

How does a wheel speed sensor work? As a car owner, you’ve probably heard of wheel speed sensor and how it could cause transmissions issues or other related problems when it goes bad.

Well, today, we will be discussing more about this sensor, how it works and where it is actually located in a vehicle.

Every car has a wheel speed sensor attached to all its wheels. That is to say, if your car has four wheels, it will house four (4) wheel speed sensors at each wheel. Similarly, if it’s a truck with many wheels, there’d be many wheel speed sensors too.

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What Is Wheel Speed Sensor?

Simply put, a wheel speed sensor (also called ABS sensor or vehicle speed sensor (VSS)) is part of the Anti-Braking System in modern cars. Its primary function is to monitor your car’s wheel’s speed and send the information to the ABS controller.

The information is then interpreted by the ABS computerized system to either activate the automatic brake to bring the car to a halt or remain dormant if the car is in good control.

Most modern automobiles with various hi-tech automatic systems (driver assistance systems, safety features, etc.) really heavily on the information provided by the wheel speed sensor.

Asides the anti-lock braking system (ABS), systems such as the automotive electronic stabilization program (ESP), Vehicle dynamic control system (VDC), automatic transmission control system, etc. all get information from this sensor. So, a wheel speed sensor is important, and you should know more about it.

You may likely not find this sensor in old cars built in the early 1990s or earlier times, but it is obviously present in virtually all vehicles manufactured after 1998 till date. There are basically two types of ABS sensors; namely, passive and active sensors.

How Does A Wheel Speed Sensor Work?

How Does a Wheel Speed Sensor Work

The simple task of wheel speed sensor is to evaluate the speed of a wheel and send information to the ABS controller. This sensor sits in a concealed corner of every wheel on a cat. There could be as low as four (4) wheel sensors on a car, and there could be up to eight (8) or more.

Wheel speed sensors monitor wheels’ rotation to detect if there’d be a need for the ABS to take over and automatically control the car due to unusual road conditions.

Most times, when driving on slippery roads, you will notice that your brake automatically engages without your consent.

Such action happens when the wheel speed sensor informs the ABS that your car wheels are not rotating correlatively. The ABS then overrides the manual braking system and tries to get the car into your control again.

Wheel speed sensors transmit information in the form of electrical signals. Both passive and active vehicle speed sensors provide the same info to specific systems in a car.

The logic behind their difference is how they are fixed and how they get the speed info to be transmitted to ABS, ESP, or VDC.

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Types of Wheel Speed Sensors

1. Active Wheel Speed Sensors

The advent of newer technologies used in designing Anti-Lock Braking Systems made active sensors to exist. This type of wheel speed sensor works with magnetoresistive principle; it records signals though a magnetic pulse sensor (encoder wheel), and it is capable of detecting the direction a wheel rotates to.

Active wheel speed sensors function at -40 to +150°C temperatures, and they deliver digital output signals to the ABS. The signals’ information transmitted by active wheel sensors is also utilized by other systems (not just the ABS), thanks to its great precision.

However, this type of vehicle speed sensor is less sensitive than passive sensors. The signal provided by this sensor is not affected by varying distances that may be caused by “tilting” brake disc. One big advantage of this wheel speed sensor is the stability of its signals’ amplitude.

Key takeaways on active wheel speed sensors:

  • They are also called magnetoresistive sensors because they feature a sensitive element with an output module installed on magnetic material.
  • The magnetic rings on active sensors are typically mounted on the seal of the bearing.
  • Active sensors are more stable and can withstand high-vibration conditions to deliver more precise signals.
  • This type of sensor has a more compact size than passive sensors. It can detect the reluctor ring’s rotation and zero speed.
  • The power input is between 12V – 5V, and it is gotten from the ABS controller.
  • Active wheel sensors deliver square wave signals (1.93V high signals and 0.64V low signals).

2. Passive Wheel Speed Sensors

This was the first type of wheel speed sensors to be integrated with the ABS in modern cars. They deliver analog signals in the form of alternating voltage. The signals are sent to the ABS control unit where they are analyzed and utilized by the ABS computer.

Passive sensors create unique AC signal frequencies as per wheel speed. The signal is only active when the wheel is rotating at a fast speed, enough to generate signals. More so, this type of sensors does not read below 2mph from a dead stop.

If a car uses a passive sensor, ABS will reduce hydraulic brake pressure when it detects that one of the wheels is rotating at a slower speed than others.

Passive sensors are characterized based on the fact that they pick up signals from a toothed wheel pressed onto the brake disc or wheel hub.

Unlike active sensors, passive sensors do not read at low speeds – the signals are accurate at 7km/h or higher speeds. You will find this sensor on most cars manufactured before 2003. It is typically installed within a space between a notch and a tooth reluctor ring.

Key takeaways about passive wheel sensors

  • They are typically less accurate; passive sensors may even read 3 – 5mph on a still vehicle when you run sensor tests using a scan tool.
  • This type of wheel speed sensor provides analog wave voltage signals due to the position where it is installed.
  • In passive wheel speed sensors, the voltage signal increases in amplitude when there’s a corresponding increase in the speed of the reluctor ring.

Major Takeaways Regarding Wheel Speed Sensors in General

  • Both active and passive VSS detect the frequency signal wheel rotation and sends it to the ABS computer.
  • Both sensors monitor wheels’ rotation speeds to automatically adjust brake pump pressure and prevent the wheels from locking.

Wheel speed sensors are part of safety features in modern cars. The information they deliver to some specific systems in a car is invaluable and help in urgent situations where the driver may be slow to respond.

However, Wheel speed sensors can go bad over time because they are subject to vibrations and high temperatures. A bad wheel sensor can cause transmission issues or even limp mode. When a speed sensor is bad, it is advisable to replace it immediately.

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Conclusion

So, how does a wheel speed sensor work? Apparently, we have analyzed and answered this question in a simplified manner. Virtually all cars come with wheel speed sensors installed on each wheel.

You must have a scan tool – this will help you to troubleshoot the wheel speed sensors when it appears that they are faulty. In summary, a wheel speed sensor is an important part of the ABS.