The vehicle speed sensor (VSS) or output transmission speed sensor determines how fast your vehicle is going by measuring the rotation of a toothed wheel on a shaft inside the transmission or at one of the wheels if the signal is coming from the ABS module to the PCM or TCM. The speed sensor can produce analog waveform signals or square wave signals, depending on the sensor design.
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Pro Tip: Usually, the vehicle speed sensor measures the speed of the transmission output shaft, but it can also measure the speed of the differential carrier in the rear end or even the separate wheel speeds on newer vehicles with networking.
A failing speed sensor in your transmission can affect more than just speedometer operation. The engine and transmission control systems both rely on the data provided by transmission speed sensors to make the necessary adjustments for your vehicle to run efficiently. Cruise control relies on this data as well.
A bad speed sensor can potentially reduce your ability to drive your vehicle safely. Therefore, it’s a good idea to know what the signs of a bad speed sensor are.
5 Common Bad Speed Sensor Symptoms
Transmission problems
Vehicle speed information relayed by the VSS is used by the transmission control module (TCM) or powertrain control module (PCM) to determine shift timing. It can even be used by the body control module (BCM) for, among other things, intermittent wiper sweep, which needs to increase in frequency with speed in mist conditions. Some platforms will turn off the interior lights above a certain speed if a faulty “door ajar” switch or some other fault has them illuminated when they shouldn’t be. But most specifically, a failing speed sensor will have a direct impact on automatic transmission operation.
One of the most common signs of a bad speed sensor is abnormal automatic transmission operation. If the VSS is faulty, the transmission may exhibit symptoms such as delayed shifts, hard shifts, and limited gear operation.
Erratic speedometer readings
Many vehicles use the VSS as a primary input for speedometer operation, which means a malfunctioning VSS can cause the speedometer to produce an erratic reading—or no reading at all.
Your vehicle will continue to run without a working speedometer. However, not knowing how fast you’re going will make it difficult to drive safely.
Inability to engage cruise control
Typically, your vehicle’s cruise control will automatically be disabled by the PCM as soon as it fails to receive information from the VSS. This is because the system requires the sensor’s data in order to keep your car running at a constant speed.
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.
Check Engine Light is triggered
Your vehicle’s check engine light can be triggered by the PCM for a myriad of reasons—one of which is a faulty VSS. You’ll need to connect your vehicle to an OBD-II scan tool to confirm whether the error code is pointing to a VSS malfunction.
Other warning lights such as those for the ABS and traction control systems may also appear on the dashboard.
What Causes the Speed Sensor To Fail?
An internal electrical fault in the speed sensor
A speed sensor that is damaged internally can result in an abnormal output signal or no signal at all. Typically, the sensor can be tested with a digital voltmeter or oscilloscope.
Seek the help of a trusted mechanic if you suspect that you have a faulty VSS so it can be tested—and replaced if needed.
Damage on the sensor itself
It’s possible for the speed sensor to get damaged or cracked. Also, the plastic portion of the sensor can become brittle after prolonged exposure to the elements.
The magnetic portion of the sensor may also become worn and damaged over time.
Poor vehicle maintenance
Failing to follow your vehicle maintenance schedule can increase the risk of speed sensor failure. This device will not be able to produce correct readings if the magnetic pickup and/or toothed wheel become heavily covered in gunk, particularly old transmission fluid contaminants. Expect a typical speed sensor to last from over 100,000 to almost 150,000 miles. To extend your speed sensor’s life, follow the recommendations of your owner’s manual in maintaining your vehicle’s transmission.
Is It Safe to Drive with a Failing Speed Sensor?
While it’s technically possible to operate your vehicle without a properly functioning vehicle speed sensor, it is strongly discouraged. A faulty VSS can negatively impact multiple aspects of vehicle operation—which is why it’s best to replace a bad speed sensor as soon as possible.
If you observe any of the symptoms mentioned above, take your vehicle to a mechanic immediately—or address the problem (if you’re handy with cars) yourself. This ensures your continued safety on the road and helps prevent costly engine and transmission damage down the line. Failing to replace a faulty sensor can affect both of these systems as they won’t have the necessary information that they need to perform optimally.
Not only will these repairs cost you hundreds to thousands of dollars, but they can also be harder to fix and put your vehicle out of commission for a longer than it has to be.
Remember that what can resolve Jeep Cherokee speed sensor problems, such as resetting the ABS light, can be different from solutions for other vehicle models like a Chevy, so make sure to check your vehicle carefully.
Get a Replacement Speed Sensor That Fits Your Vehicle
Anything that could potentially damage your engine and transmission should be replaced immediately, and that includes your vehicle’s speed sensor. Not only can a faulty VSS cause some costly damage to your engine and transmission, it also risks your safety on the road. Thankfully, you can stay safe from accidents and expensive repair bills by getting a new speed sensor from CarParts.com.
CarParts.com offers a wide selection of high-quality vehicle speed sensors sourced from only the most trusted manufacturers in the industry. You’ll also find products being sold at different price points, so you’re sure to find one that suits your needs. Just use our search filters and enter your vehicle’s year, make, and model into our vehicle selector to make browsing easier.
Don’t wait until your speed sensor damages your engine and transmission before getting it replaced. Check out our wide selection of quality speed sensors at CarParts.com today.
Products Mentioned in this Guide
Speed Sensor
Torque Converter
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I own a 1995 Cadillac DeVille and the speed sensor went. On the way home, the battery stopped charging too. Is that the fault of the bad speed sensor?
2008 Malibu, does the transmission speed sensor sit in fluid or is it supposed to be dry