How does Speed Sensor work? | What are the Symptoms of a bad Speed Sensor?

Modern vehicles contain a variety of different speed sensors. The vehicle speed sensor (VSS) is one of the most important sensors of the vehicle. It allows the magnetic rotation speed to be measured in order to provide a voltage corresponding to the rotation speed. It is widely used in the aeronautics, automotive, and precision engineering sectors. This article mainly explains the speed sensor working and symptoms.

What is a Speed Sensor?

A speed sensor is a type of position sensor which is used to measure rotational speed or distance traveled per unit of time. These sensors are used in various types of commercial and motorsport vehicles. Like many devices, a speed sensor is integral to onboard systems.

Speed Sensor

One or more onboard computers (also known as engine control modules) receive data from the speed sensors. The modules then use the information to determine the operation of various output devices. 

Working of Engine Speed Sensor

The vehicle speed sensor is designed to measure the transmission system’s gear rotational speed. After ascertaining the measurement, the data is converted into an analog or a low voltage square wave signal.

Once the signal is received, it is further communicated to the vehicle’s electronic control units. Then, the electronic control units (ECU) use the message from the signal to adjust the car’s speedometer readings, transmission system, and ABS module.

working of Speed Sensor

The vehicle speed sensor also sends signals that regulate a car’s cruise control system. Without these functionalities, a vehicle cannot maintain a perfect speed on the road. Hence, the vehicle speed sensor is a valuable component.

Types of Speed Sensors

The speed sensors have the following types:

  1. Magneto-Resistive sensor
  2. Inductive Sensor
  3. Variable Reluctance
  4. Hall-Effect Sensor

1) Magneto-Resistive Sensor

These types of speed sensors incorporate the resistance of the sensing element as a function of the direction and magnitude (proximity) of an applied magnetic field.

2) Inductive Sensor

Inductive sensors incorporate an oscillator circuit that generates a radio frequency (RF) electromagnetic field that radiates from a ferrite core and coil assembly. The field is directed at the sensor face.

When a metal target enters the field, eddy currents are induced into the surfaces of the target. This causes a reduction in the amplitude of the oscillator circuit (change in inductance).  

3) 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.

4) Hall-Effect Sensor

Hall-effect sensors function by passing a current through a semiconductor material. A voltage is developed when a magnetic field is applied perpendicularly to the surface of the semiconductor. This Hall voltage is proportional to the applied field intensity, driving the magnetic speed sensor.

Symptoms of bad Speed Sensor

As the speed sensor goes bad, it produces one of the below givens symptoms:

  1. Illuminated Check Engine Light
  2. Cruise Control failed to engage
  3. Speedometer Malfunction
  4. Transmission issues
  5. Lack of torque converter clutch application

1) Illuminated Check Engine Light

The check engine light illumination is one of the most common symptoms of a bad speed sensor.

If the signals are lost from the transmission speed sensors, the PCM will set a DTC and illuminate the Check Engine Light on the vehicle dashboard. It also can indicate an increase in tailpipe emissions surpassing acceptable limits for environmental contaminants from motor vehicles.

In any case, if you notice that the Check Engine Light comes on, you should contact a local mechanic to scan the error codes and determine why the Check Engine Light is illuminated. Once the problem has been fixed, the mechanic will reset the error codes.

Read More: Causes of Check Engine Light Flashing 

2) Cruise Control failure to engage

Since the transmission speed sensors monitor the input and output shaft speed, it also plays a part in monitoring the cruise control.

When the sensors fail to deliver accurate data to the onboard computer on your car, truck, or SUV, the powertrain control module (PCM) will send an error code to the vehicle’s ECU.

As a safety precaution, the ECU will shut down the cruise control and render it inactive. If you notice that your cruise control does not engage when you press the button, contact a mechanic so they can inspect the vehicle and determine why the cruise control is not working. It may be due to faulty transmission speed sensors.

3) Speedometer Malfunction

Once the vehicle’s speedometer stops functioning all of a sudden, this is a confirmed sign that indicates the speed sensor has become defective. The malfunctioning speed sensor means that the speedometer will not have the ability to display the speed your vehicle is traveling at.

Not knowing how fast you are going is obviously concerning, especially when there is little other traffic on the road to help you at least estimate your speed. This can land you in trouble with the authorities or even cause unnecessary and avoidable accidents.

4) Transmission Issues

A vehicle’s transmission system relies on the signal from the wheel speed sensor to determine the timing for every gear shifting. When the wheel speed sensor fails to deliver the necessary signal, the transmission system will not be able to engage correctly.

Due to a defective wheel speed sensor, the transmission is liable to exhibit signs like hard shifting, delayed shifting, and limited gear shifting operations.

5) Lack of torque converter clutch application

The torque converter clutch is activated at a pre-programmed speed. Without accurate information from the vehicle speed sensors, the transmission torque converter will be unable to apply the clutch and form a mechanical link between the engine and transmission.

This can cause slipping, reduced fuel economy, and an overheated transmission.

How to fix a Speedometer 

You must have the below-given materials before diagnosing or fixing the speed sensor:

  • Screwdriver set
  • Drain pan
  • Wrench set
  • Hydraulic jack
  • Wheel chocks
  • Jack stands
  • Socket set
  • Pliers

Follow the below-given steps to fix the speedometer sensor (speed sensor):

  1. Jack up the vehicle and set jack stands. Jack up the vehicle and set jack stands using the factory recommended jacking and stand points.
  2. Place wheel chocks on both sides of wheels: Place wheel chocks on both sides of each of the wheels that are still on the ground. This lessens the likelihood of the vehicle rolling forward or back and falling off of the jack stands.
  3. Remove the bolts: Remove the bolt holding the speedometer sensor in place.
  4. Remove the speedometer sensor: Pull the speedometer sensor out of the transmission. You may have to wiggle it back and forth while pulling it out. Using a screwdriver to carefully and gently pry the sensor free may be necessary.
  5. Unplug the electrical connector: Release the retaining tab on the speedometer sensor electrical connector and pull the connector off of the sensor.
  6. Compare the replacement speedometer sensor to the one being removed: Check that the dimensions are the same and there is the correct electrical connector.
  7. Install the replacement speedometer sensor: Put a little oil or grease on the O-ring seal on the speedometer sensor and install it into the transmission. If you encounter some resistance when installing the sensor, try repositioning the gear and then reinstalling it. Be careful not to exert too much force, as damage can occur.
  8. Reinstall the electrical connector: Reinstall the electrical connector onto the speedometer sensor. Be sure to insert the connector far enough to engage the retaining tab.
  9. Remove the jack stands: Jack the vehicle up with the hydraulic jack and remove the jack stands. Go ahead and remove the wheel chocks.
  10. Test drive the vehicle. Test drive the vehicle to verify that the speedometer is functioning properly. The needle should move smoothly on the speedometer throughout its range of movement.

Speed Sensor Replacement Cost

The replacement cost of the speed sensor varies according to your vehicle model, labor cost, and the type of part brand. The average replacement cost of the speed sensor is from $270 to $320. In this cost, the costs of the parts are from $160 to $180, while the labor costs are from $$110 to $172.

FAQ Section

What does a speed sensor do?

The vehicle speed sensor (VSS) measures transmission/transaxle output or wheel speed. The ECM uses this information to modify engine functions such as ignition timing, air-fuel ratio, and transmission shift points and to initiate diagnostic routines.

How does a transmission speed sensor work?

The speed sensor detects a gear or target wheel in the transmission and generates an electric digital signal proportional to the speed of rotation. The transmission controls use this information to modify various transmission functions such as shift points and perform diagnostic tests.

What is a Speed Sensor Used For?

Speed sensors are used to measure the rotating speed within devices. Many vehicles require speed sensors, including; automotive vehicles, aerospace vehicles, off-highway & construction vehicles, railway vehicles, and military vehicles.

How many speed sensors do cars have?

Modern vehicles have four wheel speed sensors—one for each wheel of your car.

What happens when a transmission speed sensor goes bad?

As the transmission speed sensor goes bad, you may face one of the below-given symptoms:

  1. Illuminated check engine light
  2. Cruise control failure to engage
  3. Speedometer malfunction
  4. Transmission issues
  5. Lack of torque converter clutch application

What happens if I unplug my wheel speed sensor?

Removing a wheel speed sensor will disable ABS—and sometimes, stability control. If your car has a yaw sensor, you can unplug that for results. 

Can a speed sensor cause transmission problem?

The Vehicle Speed Sensor measures vehicle speed. If it fails or malfunctions, the automatic transmission may not operate properly. It may also cause the automatic transmission to go into failsafe mode, making it appear that the problem is more severe than it actually is.

Can the speed sensor be repaired?

Unfortunately, it is impossible to repair the transmission speed sensor as it simply fails. You have to discard it and replace it with a new one.

Can I drive with a bad speed sensor?

Driving without a speed sensor or with a bad speed sensor can be a big risk. The speed sensor’s role is essential to the safety of a driver. The sensor helps determine the vehicle’s speed to know when necessary to apply the brakes.

So, driving with a faulty speed sensor will not allow the computer to determine the necessary speed level to increase the engine power or apply the brakes. This may lead to a fatal accident because the safety system is defective.

Will a bad speed sensor stop a car from shifting?

Without a valid speed signal from speed sensors, the PCM will not be able to control the shifting of gears within the transmission correctly. This may cause the transmission to shift roughly or more quickly than normal.

Where is the Speed sensor located?

The vehicle speed sensor, or VSS, is located near the transmission’s output shaft.

Read More
  1. PCM Working, Symptoms, and Causes
  2. Working of Engine Control Unit
  3. How to Reset Check Engine Light

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