A vehicle’s automatic transmission typically features multiple speed sensors, including one that monitors the car’s speed. These sensors provide crucial data to the vehicle’s control systems, such as the engine control unit (ECM) and transmission control module (TCM), to ensure smooth shifting and accurate speed readings.
When these sensors fail or give inaccurate readings, it can severely affect the vehicle’s performance, causing erratic shifting or speed control issues that could endanger both the driver and passengers.
So what happens when they stop functioning correctly? In this guide, we’ll explore the most common signs to watch out for if you suspect speed sensor failure, as well as steps for replacing the faulty sensor.
What are the Signs of a Bad Speed Sensor?
Recognizing the symptoms of a faulty speed sensor can help you avoid speeding tickets, inefficient driving, or even dangerous road accidents. Here are some key signs to watch out for:
Inaccurate Speedometer
One of the primary indicators of a bad speed sensor is an inaccurate speedometer. You might notice the speedometer’s needle or the numbers on the digital display fluctuating, jumping, or even remaining at zero even while your vehicle is moving. This can make it difficult to gauge your speed and drive safely.
Gear Change Issues
In an automatic transmission, trouble shifting gears could also indicate a failing or bad speed sensor. You’ll likely notice jerky, delayed, or mistimed gear shifts, as inaccurate speed readings can disrupt the vehicle’s shifting patterns, leading to driving inefficiency and potential transmission damage over time.
Check Engine Light
A malfunctioning transmission speed sensor can cause your vehicle’s check engine light (CEL) to illuminate. The CEL can illuminate for several reasons, but when caused by a faulty speed sensor, it’s typically due to incorrect data from your transmission control unit. If the CEL is on and accompanied by other signs mentioned here, it’s a strong indicator that you have a bad speed sensor.
Unstable Cruise Control
Cruise control relies on the speed sensor to maintain a steady speed. When the sensor fails, the vehicle may accelerate or decelerate erratically, causing cruise control to malfunction or become ineffective.
Loss of Power or Limp Mode
Some vehicles may enter “limp mode” when the transmission control unit detects incorrect speed sensor data. This is a security feature that activates when an issue with one of your car’s critical systems is detected. In this mode, the vehicle limits power and restricts certain functions to prevent further damage. This could result in significantly reduced performance and speed.
What is a Vehicle Speed Sensor?
A speed sensor plays a crucial role in your vehicle’s overall performance, particularly by providing essential data that helps regulate various systems. Below, we’ll explore what a speed sensor does, how it works, and where you can find it in your vehicle.
What Does a Speed Sensor Do?
The transmission speed sensor, also known as the vehicle speed sensor (VSS), is a critical component that provides real-time data on the vehicle’s speed. The information it provides is used by certain vehicle systems, including the ECM and TCM, to regulate speed, ensure smooth gear shifts, and optimize engine performance. It also provides this data to the speedometer, which lets the driver know how fast they are going, assisting in distance measurement and ensuring traffic law compliance.
How Does a Speed Sensor Work?
Understanding the role of transmission speed sensors in your vehicle is crucial. Typically, there are two main types: the input shaft speed sensor and the output shaft speed sensor. The former measures how fast power travels from the engine to the transmission, while the latter measures how fast the power transfers from the transmission to the wheel components.
These sensors read the measurements of speed at which the transmission gears are rotating and send them, typically in the form of low-voltage square wave signals, to the vehicle’s ECM or TCM.
Once the control units receive these signals, they adjust the transmission system, influencing the speedometer readings, gear shifts, and overall vehicle handling to ensure smooth and efficient driving. Depending on the vehicle, additional speed sensors for intermediate shafts may be present for more precise speed monitoring and optimal transmission performance.
Where is the Speed Sensor Located?
The location of speed sensors can vary depending on the vehicle, but they are typically found in the following locations:
- Near the transmission output shaft
- Mounted on the transmission housing or transmission pan
- Attached to the transfer case (in vehicles with four-wheel drive)
- Integrated into the differential or located between the primary driving wheels (in two-wheel or four-wheel drive vehicles)
You can find their exact locations by checking your vehicle’s service manual or repair guide, or by consulting your dealer or a professional mechanic.
How to Test a Speed Sensor
Knowing how to test a transmission speed sensor can help you determine if your vehicle’s speed sensor needs replacing before any potential problems arise.
Here’s how:
Turn Off Your Vehicle
Make sure your vehicle is parked in a secure area and on a level surface. Turn off the engine and, if possible, disconnect the battery to avoid accidental shocks or shorts. It’s also advisable to wear safety goggles and gloves before proceeding to the next step.
Locate The Speed Sensor
Find your vehicle’s speed sensor, which is typically mounted on the transmission housing or in any of the other locations listed above. You can refer to your vehicle’s manual if you’re unsure about its location in your vehicle.
Inspect the Speed Sensor Up Close
Carefully disconnect the sensor’s electrical connector to avoid damaging the wires or the sensor itself. Closely inspect it for any signs of visible damage, corrosion, or loose wiring.
Check for Error Codes
Use an OBD2 scanner to diagnose any issues within your engine and emission control systems. Connect the scanner to your vehicle’s diagnostic port and check if it displays specific trouble codes that indicate issues with your speed sensor or its circuitry.
Compare Sensor Performance
If you know how to test a speed sensor with a multimeter, you can use it to check if the voltage supply to the sensor matches your vehicle’s stock specifications. Deviations from the reference values found in your service manual can point to a bad sensor.
Test for Ground Connection
A multimeter can also be used to check for ground connection. Poor ground connection can cause irregular sensor operation, so make sure it shows a low resistance reading to ensure that it is properly grounded.
Consider Advanced Diagnostic Tools
Alternatively, you can consult a professional mechanic to help assess your vehicle’s sensor and transmission using more advanced diagnostic equipment.
Once diagnosed, replace your speed sensor promptly to avoid further vehicle performance issues. If you’re not comfortable checking voltages, take your vehicle to a qualified mechanic. Accurate diagnosis and timely repairs are crucial for maintaining vehicle safety and efficiency.
Speed Sensor Replacement Cost
A speed sensor costs anywhere between $10 to as much as $100. You can find a good, reliable speed sensor for under $50. However, vehicle speed sensor replacement costs can vary depending on several factors:
- Vehicle year, make, and model
- Brand and quality
- Type of transmission speed sensor
- Where it’s purchased
- Labor and installation costs (if any)
Whatever your preference, it is essential to choose a speed sensor that meets your vehicle’s requirements, ensuring reliable performance and safety.
How to Replace a Speed Sensor
Replacing a speed sensor involves locating the malfunctioning unit, safely removing it from your vehicle, installing the new one, and testing it to ensure that everything is working as it should. Below is a general procedure:
How to Remove a Speed Sensor
- Disconnect the battery. Loosen and remove the negative battery cable to prevent electrical shocks and accidental shorts.
- Locate the faulty sensor. Identify the faulty speed sensor. This may require consulting your vehicle’s service manual if you are unsure of the exact location.
- Disconnect the wiring. Carefully disconnect the electrical connector attached to the sensor. Avoid pulling on the wires directly to prevent damage.
- Remove the sensor. Unscrew it to remove it from its mounting point. You may need a socket or wrench, depending on the sensor’s location.
How to Install a Transmission Speed Sensor
- Prepare the new sensor. Install the sensor’s O-ring (if applicable) on your new sensor to ensure a proper seal and avoid leaks.
- Install the sensor. Screw it into the mounting location, making sure it’s tight enough—but don’t over-tighten to avoid damaging the sensor.
- Reconnect the wiring. Attach the electrical connector to the new sensor. Ensure that the connection is secure and that the wires are properly routed to avoid interference with moving parts.
- Reconnect the battery. Reattach the negative battery cable.
How to Test Your New Speed Sensor
- Check the voltage with a multimeter. Verify that the sensor is receiving the proper voltage according to your car’s specifications.
- Do a test drive. Monitor the speedometer to check for accurate readings and observe gear shifts and overall responsiveness when accelerating.
If you’re not an experienced DIYer and aren’t too sure about performing the sensor replacement yourself, we strongly suggest that you take your vehicle to a professional mechanic to ensure the job is done correctly.
Frequently Asked Questions
How many volts should a speed sensor have?
An appropriate voltage reference input for an active speed sensor typically ranges from 5 to 12 volts, though the exact value depends on your specific vehicle.
Can a bad speed sensor cause rough idle?
A faulty speed sensor can contribute to rough idle, but it isn’t usually the direct cause. When the speed sensor malfunctions, it becomes unable to provide accurate data to the vehicle’s ECU and transmission, leading to issues such as improper gear shifts and irregular engine performance.
Will a bad speed sensor stop a car from shifting?
Yes. When the speed sensors fail to produce accurate speed signals for the transmission, it can result in delayed shifting, harsh gear changes, or even prevent shifting altogether.
