Replacing it? Compare vehicle speed sensor prices on Amazon.
Vehicle Speed Sensor
What Does the Vehicle Speed Sensor Do?
Also known as transmission speed sensors, transmission output shaft speed sensors, VSS (Vehicle Speed Sensors) measure and monitor the rotational speed of the transmission output shaft, which information the PCM (Powertrain Control Module) uses to calculate the vehicle’s actual road speed as a function of both the current engine speed and selected gear.
On almost all modern vehicles, the vehicle speed sensor also provides the primary input to the speedometer via the instrument cluster control module.
NOTE: Vehicle speed sensors should not be confused with turbine speed sensors that measure the rotational speed of parts of the torque converter to calculate the amount of slippage that occurs between the drive and driven parts of the torque converter.
Why is the Vehicle Speed Sensor Needed?
Although a PCM could calculate a vehicle’s road speed by using inputs from the wheel speed sensors, this method would only work if the vehicle is traveling in a straight line, and the wheels are inflated to their recommended (or maximum allowed) inflation pressures. In the first case, some wheels rotate faster or slower than others when the vehicle goes around corners, while in the second case, wheels that are underinflated have a smaller circumference than correctly inflated wheels, thus causing differences between wheel rotation speeds.
So, as a practical matter, a PCM requires a way to calculate a vehicle’s speed that is independent of tire pressures and steering angles to improve the resolution of the data it uses to calculate the vehicle’s road speed. This is necessary because the PCM, TCM (Transmission Control Module), and several other control modules that are involved with the engine and fuel management systems use the vehicle’s speed as the basis from which they execute several critical functions.
These include, but are not limited to, calculating transmission shift points and transmission performance modes, adapting the air/fuel mixture and ignition timing settings during gearshifts and calculating the amount of torque converter lockup clutch slippage/engagement required to improve the “feel” of gearshifts, while at the same time, protecting transmission components against possible damage caused by aggressive acceleration.
Collectively, the control and management functions mentioned above account for much, if not all of what makes modern vehicles not only comfortable to drive, but also fuel efficient and safer than ever before because safety systems like ABS, Traction Control, and Stability Control also depend on input data provided by the vehicle speed sensor to work as intended.
Put differently, the above means that a functional vehicle speed sensor plays a critical role in the operation of multiple vehicle management and control systems that cannot work as expected unless the vehicle speed sensor generates the consistently accurate inputs these systems require.
How Does the Vehicle Speed Sensor Work?
While there are some differences between vehicle speed sensors in terms of design specifics, all modern vehicle speed sensors generate “ON-OFF” signals when a)their internal parts rotate, or b), when a magnetic sensing element in the sensor comes into close proximity to a rotating toothed reluctor ring or wheel in the transmission.
In terms of operating principles, almost all modern vehicle speed sensors use the Hall Effect to generate signals. This effect involves a permanent magnet in the sensor that creates an electrical signal when ferrous metal objects, such as the teeth of a rotating reluctor ring, move through the magnetic field of the permanent magnet. This action creates a square waveform, such as in the (generic) example shown below-
Note that this example of a square waveform does not apply to any particular vehicle speed sensor specifically, but to all square waveforms that are generated by Hall Effect sensors, with wheel speed sensors, crankshaft position sensors, and camshaft position sensors being the most common examples.
The labels in this example are largely self-explanatory, but it is important to note that the frequency* of the waveform as represented here by the blue squares rising vertically, is what the PCM uses to calculate the vehicle’s speed since the frequency of the waveform is a direct function of the speed at which the sensor rotates.
* A waveform’s frequency is defined as the number of times that the peaks of the waves repeat within a given period, which is usually measured in seconds.
NOTE: The amplitude or height of the wave peaks (as shown in this example) is merely a function of a) the strength of the signal, or b), the settings of the oscilloscope used to obtain and record the waveform. As such, the height of the wave peaks is not relevant to the process of calculating the vehicle’s speed.
Where is the Vehicle Speed Sensor Located on the Engine?
This image shows the location(s) of the vehicle speed sensors (indicated by the two red arrows)* on the transaxle of a 2011Toyota RAV 4. Note that while most vehicles have only a single vehicle speed sensor, most modern Toyota models and other Japanese vehicles use two sensors to ensure that critical engine management and various safety systems continue to work as expected should one sensor fail.
Note that although vehicle speed sensors are usually mounted on transmissions or transaxles, vehicle speed sensors can also be located on differentials, which is often the case on vehicles with manual transmissions. However, since transmissions, transaxles, and differentials can also accommodate other types of sensors, we strongly recommended that reliable service information for the affected vehicle be consulted to locate and identify vehicle speed sensors correctly to avoid confusion and misdiagnoses.
* In this example, the blue arrow indicates the position of the starter motor- or where it would be if it were mounted on the transaxle.
What Does the Vehicle Speed Sensor Look Like?
This image shows an example of a vehicle speed sensor that is driven with a gear that engages with a drive gear in the transmission. Note that while the overall appearance of this type of vehicle speed sensor varies between vehicle makes and models, the design of all gear-driven sensors follows this general pattern.
Note, though, that some vehicle speed sensors are driven by other means, such as a squared-off key, an example of which is shown below-
Still, other designs are not driven mechanically at all, but instead use a stationary magnet (in the sensor) and a rotating reluctor wheel or ring that is located inside the transmission or transaxle to generate signals, an example of which is shown below-
Note that although there are several reputable sensor brands available in the aftermarket, we highly recommend that a vehicle speed sensor is only replaced with an OEM, or OEM-equivalent part sourced from a dealership or reputable parts outlet.
A generic aftermarket sensor might be a few dollars cheaper than an OEM part, but given that the global aftermarket is flooded with cheap knock-offs of all categories of replacement parts, fitting only OEM or OEM-equivalent parts all but eliminates the possibility of purchasing a cheap knock-off of a genuine part that may or may not work as expected.
Bear in mind that several critical safety systems depend on this sensor to work correctly, so play it safe, and only fit genuine parts unless you know what you are getting for your money in the aftermarket.
What are the Symptoms that the Vehicle Speed Sensor is Bad?
Since vehicle speed sensors provide input data to multiple engine management and safety systems, the possible symptoms of a malfunctioning or failed vehicle speed sensor largely depends on the affected vehicle since not all of the safety systems that are currently available are always fitted to all vehicles.
Therefore, it is not possible to provide complete and exhaustive information on all possible symptoms on all applications here, but we can do the next best thing, which is to provide a list of the most common symptoms that affect most vehicles when a vehicle speed sensor fails.
Note though that while the symptoms listed below are common, not all of them are always present on all vehicles in all instances of vehicle speed sensor failure. Nonetheless, some common symptoms could include one or more of the following-
- Stored trouble code and an illuminated warning light that may or may not flash
- Depending on both the vehicle and the nature of the failure, multiple additional trouble codes could be present
- Gear shifts may be harsh, erratic, or unpredictable
- Speed indications on the speedometer may be erratic, or the speedometer may not work at all
- Driver assist systems like cruise control or adaptive cruise control [if fitted] may not be available in which case, ABS, Traction Control, and/or Stability Control will also be deactivated although normal braking will still be available
- If the affected vehicle is fitted with an electronic throttle control system, throttle responses may be slow or sluggish under some operating conditions, or at some engine speeds and loads
- The torque converter lockup clutch may be deactivated, or it may not work as expected under some operating conditions, which, if it happens, could potentially cause severe transmission and/or torque converter slippage to occur under some operating conditions
How do you test the Vehicle Speed Sensor?
Given the critical role that vehicle speed sensors on all modern vehicles play in both engine management and safety systems, it is not an exaggeration to say that there are no reliable, and accurate, ways to test the operation of these sensors- apart from obtaining a proper waveform that allows for the proper analysis of a vehicle speed sensor’s performance.
For example, performing a continuity test to see if there are open circuits in the sensor could damage the electronic components that are embedded in some vehicle speed sensors, thus destroying a sensor that may have been in perfect working order- the actual fault having been somewhere else. Similarly, resistance tests to see if the sensor has not degraded over time are very often inconclusive, because the range of acceptable resistance values that apply to all Hall Effect sensors is very large.
Thus, a waveform obtained with a suitable oscilloscope allows a technician to see a graphic representation of the sensor’s operation. This analysis includes comparisons of wave amplitudes, as well as checking for signal dropouts, erratic frequencies, and other aspects of the waveform that characterize both known-good, and known-bad sensors- all of which is diagnostic information that cannot be readily obtained in any other way.
However, the problem with oscilloscopes is that not many non-professional mechanics have access to one, so if you suspect that you may be experiencing issues with a failed or failing vehicle speed sensor, the wisest course of action to follow would be to seek professional assistance with diagnosing and possibly replacing the suspect sensor. Anything else is based on guesswork, and will not offer a reliable diagnosis and/or repair.
How do you replace the Vehicle Speed Sensor?
It is certainly possible to replace a vehicle speed sensor on a DYI basis, although the most difficult part of the procedure is often related to actually finding the sensor. In many cases, the sensor is located behind unrelated engine components, or it is located in a position that requires the removal and/or partial disassembly of unrelated engine components just to gain easy access to the sensor.
These components could include (among others) fuel lines, brake lines, drive belts, engine and/or transmission mounts, A/C lines, power steering lines, and even parts of the exhaust system. Nonetheless, when the vehicle sped sensor becomes accessible, removing it from the transmission is often no more complicated than disconnecting the electrical connector, unscrewing one small retaining bolt, and pulling the old sensor from its location. Fitting the replacement is done in the exact reverse order of removal.
However, if you are uncomfortable with the idea of having to remove or disassemble major engine components, we recommend that you seek professional assistance with replacing the sensor to prevent the possibility of inadvertently creating damage to components that may be critical to the vehicle’s safe operation.