HomeHow It WorksVehicle Speed Sensors: How It Works and What Might Go Wrong

Vehicle Speed Sensors: How It Works and What Might Go Wrong

Since the beginning of the automobile, going faster was always a goal. While this is good if you are racing, if you are a law abiding citizen like myself you like to keep track of how fast you are going. Let’s take a look and learn more about the secrets behind your speedometer and what makes it all happen. The vehicle speed sensor or sometimes known as VSS. 

What Is A Vehicle Speed Sensor?

A vehicle speed sensor is a device that provides an input to the speedometer to indicate how fast you are driving. But, there is a little more to it than that.

Unlike today’s complicated electronic vehicle speed sensors, the first speedometers that were equipped on vehicles were mechanically driven. The speedo gauge on the dash was connected to the transmission via a flexible steel cable. Gears inside of the transmission rotated the steel cable and transferred this motion to the speedometer gauge.

A mechanical vehicle speed sensor drive gear that is located on the transmission output shaft.

Inside of the gauge a mechanism would then transfer this speed sensor information and provide a readout to the driver showing how fast they were going. This presented some issues to vehicle owners over time. 

As we all know, mechanical components wear out, break, and need to be replaced. But the more sinister issue was the ability for people to “Turn back” the odometer. Dishonest mechanics and used car lots used this trick to reduce the mileage of a vehicle so they could sell them for a higher price. 

This issue led to many people not trusting the mileage of a vehicle. 

When electronic systems were introduced onto vehicles the vehicle speed sensor changed from a mechanical device to an electronic device. 

Interesting fact: The gears inside of some transmissions that drove the speedometer cable were converted. On some transmissions they still act as part of the vehicle speed sensor.

The Secret Behind Your Speedometer: How Does A Vehicle Speed Sensor Work?

Modern day electronic vehicle speed sensors(VSS) provide an electronic signal to a module that interprets the information. Once this information is interpreted it sends the information via what is called a Data BUS to the speedometer gauge. 

As a driver, nothing has changed. You still look down at the gauge and see how fast you are going. 

Types of Vehicle Speed Sensors

There are two main types of vehicle speed sensors that are equipped on modern vehicles. The Hall-effect sensor and the permanent magnet sensor (voltage generating sensor) are the two different types. They complete the same job but in different ways. 

Hall-effect sensor

A hall effect sensor is an electronic device that operates using the Hall-effect. Basically the sensor has the ability to detect the presence of a magnetic field as it passes by the sensor. 
This type of sensor requires a hall effect sensor as well as a reluctor wheel. Think of a reluctor wheel as a large wheel with teeth. This reluctor wheel spins and the hall effect sensor can detect how fast it is turning by the signal that is being sent by the sensor. 

a graph showing a typical digital vehicle speed sensor signal

Simply put the faster you drive the faster the frequency of the signal. 

This type of sensor requires the use of three different wires, a voltage feed, ground, and a signal wire. It provides a digital output signal that can be checked using an oscilloscope for testing. 

Permanent magnet sensor (magnetic position sensor)

The magnetic or voltage generator sensor is a position sensor that has been used for years. It is a two wire sensor that also uses the magic of magnetism. 

A two wire connector from a voltage generating speed sensor.

When a reluctor wheel travels near the sensor, the magnetic field will change strength. This causes the voltage signal to vary from positive to negative voltage. This signal is known as an analog type signal and is measured in AC voltage with an automotive multimeter

The analog signal that is produced by this sensor can be interfered with easily. Engine components such as spark plugs can create electrical “noise” known as interference. This interference can cause the signal to not read correctly. This creates an inaccurate signal. 

This type of sensor is being removed from vehicles and replaced with the more accurate Hall-effect sensor. 

Warning Signs Of A Faulty Vehicle Speed Sensor(VSS)

Incorrect speedometer readings

The vehicle speed sensor is used as the main input for the speedometer. If your vehicle speed sensor is starting to fail it can cause incorrect speedometer readings. 

If your speedometer is increasing in speed randomly or dropping to zero while you are driving forward, this is a sign of a faulty speed sensor. 

Check Engine Light turned on

While modern vehicle issues can generally be caused by electrical issues they will turn on the check engine light if the module detects a problem. If you have a faulty vehicle speed sensor the vehicle will turn the engine light on so you are aware. 

Checking the diagnostic trouble codes will let you know if the issues that your vehicle is having is related to the speed sensor. 

A P0500 is a very popular code that could be present if there is an issue with the vehicle speed sensor. P0500 indicates an issue with “vehicle speed sensor(VSS) A”, either a wiring fault or faulty sensor.

Transmission shifting Incorrectly

To shift the transmission correctly the transmission control module needs to know how fast the vehicle is going. If the sensor is providing incorrect information to the vehicle’s computers it can cause the transmission to act up. 

A vehicle speed sensor(VSS) shown on the tailstock of a rear wheel drive vehicle

Shifting too early, shifting to late, and shifting randomly are all symptoms of a faulty vehicle speed sensor. 

If you are experiencing transmission shifting issues, the check engine light will also be on indicating there is a problem. This will allow you to quickly check the codes and find out where the issue is located. 

Common Problems With Vehicle Speed Sensors

Being an electrical vehicle component, a bad sensor will tend to have the same common electrical problems that plague many vehicle systems. 

Bad wiring

Sending a speed signal to the computer requires good wiring to transfer the signal. If there is high resistance, corrosion, or breaks in the wire, the signal cannot be sent to the speedometer. 

This will result in a speedometer that reads incorrectly or possibly does not read at all. 

Poor connection

Just like wiring, a good solid electrical connection is required for a sensor to read correctly. If corrosion is present or possibly a broken pin you will notice the vehicle does not operate correctly. 

There are many reasons that can cause a faulty vehicle speed sensor. It is important to know what the symptoms are before diving in to diagnose a speed sensor issue. Some vehicles have difficult to reach sensors, so knowing that you have a fault with the sensor or wiring before attempting to repair is important.

Is a vehicle speed sensor the same as a Crankshaft Position Sensor (CKP)?

No, the vehicle speed sensor is a speed sensor that provides the speed of the vehicle to the correct vehicle module. They are the same type of sensor but do not operate in the same manner. 

Do I need to replace the reluctor wheel with the speed sensor? 

Generally the reluctor wheel is a different part. They can be part of an output shaft or even part of a wheel bearing or transmission component. If the reluctor is broken it needs to be replaced as well. 

John Morris
John Morrishttps://autoknowit.com
John Morris is the technical editor for AutoKnowIt.com. His years of experience in automotive repair as well as an automotive professor have prepared him to ensure that even the most technical information is accurate and concise at all times.

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