Blog Car Care Advice What is a Speed Sensor? Symptoms, Types & Replacement
Car Care Advice

What is a Speed Sensor? Symptoms, Types & Replacement

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Ever wondered how your car knows exactly how fast you’re zooming down the road?

All thanks to tiny gadgets called speed sensors. Without them, you probably wouldn’t realize you were edging close to the highway speed limit. 

Curious to know more?

In this article, we’ll cover what a speed sensor is and how it works. Then, we’ll look at the symptoms of a defective speed sensor and how often you should replace it. Finally, we’ll go through the steps involved in replacing the sensor.

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Let’s jump right in.

What Is a Speed Sensor?

A vehicle speed sensor (VSS) is a small device mounted on your car’s transmission that lets the car’s onboard computer know how fast you’re moving. It measures your car’s rotational speed and relays it to your car’s speedometer.  

The vehicle speed sensor, also known as a transmission speed sensor or output shaft speed sensor, plays an active role in your car’s transmission and cruise control systems.

There are also ABS wheel speed sensors (WSS) on a vehicle’s wheel hubs (both on the front axle and rear axle). Their function is to monitor the rotation of each wheel and relay the data to the ABS control module for better vehicle control. 

What’s the difference? 
The VSS measures the overall car speed, while WSS measures the speed of the individual wheel. 

That said, the VSS speed sensor and the ABS sensor (wheel speed sensor) are parts of the engine sensor control, usually including a temperature sensor, pressure sensor, crankshaft position sensor, idle speed sensor, etc. Some sensors can also detect the direction. 

But for now, we’ll focus on the VSS sensors. 

So how does the VSS speed sensor work?
The vehicle speed sensor measures the rotational speed of the metal gears in your transmission system. It converts this motion (or transmission output) into an analog or a low-voltage square wave signal that varies according to the magnetic flux (if it’s an inductive sensor). 

The sensor then relays the output signal to your car’s electronic control unit (ECU).

The ECU uses this square wave signal to adjust your car’s speedometer readings, throttle position, ABS system (anti lock braking system), ignition timing, and transmission system.

Your car’s cruise control and traction control may also rely on data generated by the speed sensor. Without it, your car wouldn’t be able to maintain a constant vehicle speed.

Note: Speed sensors usually find application in devices that need a contact-free speed measurement system.

What Are the Types of Speed Sensors?

Here are a few types of sensors for automotive applications: 

1. Hall Effect Sensor

A Hall effect sensor is a magnetic sensor that consists of a reluctor that moves against that hall sensor to produce a magnetic field (magnetic flux) and thereby a voltage. This voltage is then transmitted to the processing units of the ABS system. The ABS control module is responsible for measuring the car’s velocity. 

2. Reed Switch-Type Sensor

This is a magnetic sensor that consists of a magnet (powered by a speedometer cable) and a reed switch. 

For every rotation, the magnet turns the reed switch on and off around four times. This allows the calculation of pulses per second. These pulses are then used to measure vehicle speed.

3. Active Sensor

The active sensor only works when supplied with a voltage. It then converts the voltage to a digital signal and sends it to the anti lock braking system. 

An active sensor finds application in sensing wheel direction and is more efficient at a low speed. 

4. Passive Sensor

A passive sensor is also known as an inductive sensor or variable-reluctance magnet sensor (VR sensor). These sensors don’t require additional voltage. Instead, it detects the wheel’s rotational motion to send a digital signal to the anti lock brake system.

What Are the Symptoms of a Faulty Speed Sensor?

Your car’s speedometer reading, anti lock brake system, ignition timing, transmission system, and cruise control depend on signals from the speed sensor. So you need to ensure that it’s working as expected.

If you experience any of the following 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 VSS and wheel speed sensor to adjust their speedometer readings. A faulty speed sensor (due to sensor tip contamination or wiring damage) can result in inconsistent or erratic speedometer readings.

Alternatively, 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 VSS or wheel speed sensor failures.

2. 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:

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.

Note: An illuminated ABS sensor or a traction control light can also indicate issues with the speed sensor.

3. 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.

Generally, the powertrain control module (PCM) inside your car automatically disables cruise control when it fails to receive a proper signal from the vehicle speed sensor. 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.

4. Harsh Transmission Shifting

Without a reliable signal from the transmission speed sensor, the powertrain control module can’t correctly shift the metal 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.
That’s not all.

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.

To be safe, consider getting a car inspection soon to identify any transmission problems your vehicle might have.

How Often Should You Replace the Vehicle Speed Sensor?

There’s no hard-and-fast rule to help you predict how long your 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 output 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:

Note: The magnets inside the speed sensor are resistant to high temperature environments. The sensors can operate between a temperature range of -40°C to 125°C (-40°F to 257°F).

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 Much Does a Speed Sensor Replacement Cost?

The cost of replacing the vehicle speed sensor depends on the make and model of your vehicle. It also depends on the type of sensor currently used, such as an active sensor, VR sensor, optical sensor, magnetic field sensor (Hall effect sensor), or another type.

However, speed sensor replacement isn’t generally expensive. It won’t cost more than other significant repairs like brake line and master cylinder replacements. 

With that being said, chances are you’ll have to pay between $200 and $400 to replace the vehicle speed sensor, whether it’s a hall sensor or other types.

To get a more accurate estimate, just fill out this online form.

How to Replace Your Car’s Speed Sensor?

You can try to replace your car’s wheel speed sensor or vehicle speed sensor yourself. 

But we highly recommend seeking help from a skilled technician.

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

These include:

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:

For an idea of what the vehicle speed sensor replacement usually involves, here’s a breakdown of what your mechanic will likely do:

  1. Slide a drain pan or bucket under the sensor to collect any leaking fluids.
  2. Gently disconnect the vehicle speed sensor from the transmission system.
  3. Remove the connector between the car’s wiring harness and speed sensor.
  4. Loosen and remove the defective speed sensor with a socket wrench.
  5. Install the new speed sensor and reattach the connector to the wiring.
  6. Start the car’s engine and test drive your vehicle to ensure that the speedometer, cruise control system, and transmission shifting work as expected.

Keep Your Speed Sensor Up to Speed

The vehicle speed sensor is a critical component that ensures that your speedometer, cruise control, and transmission work well.

If you’re driving around with a defective speed sensor, you may not know how fast you’re going, and it can compromise your safety and those around you. Moreover, you’ll likely have a poor driving experience due to harsh and irregular transmission shifting.

To prevent this, hire a mechanic to inspect your wheel speed sensor and perform a replacement if necessary.

Fortunately, AutoNation Mobile Service makes hiring mobile technicians easier than ever!

All you have to do is make an online booking, and our expert mobile mechanics will come to your driveway to perform a speed sensor, temperature sensor, or any other pressure sensor inspection and replacement. All repairs come with a 12-month |12,000-mile warranty.

So, if you’re looking for a hassle-free and accessible repair service for speed sensor requirements, contact AutoNation Mobile Service or find out more through our customer service today!