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What Happens When The Transmission Speed Sensor Fails?

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Unfortunately, most people have never heard of the transmission speed sensor, until it’s time to have the component replaced on their car. When the transmission speed sensor fails it has a direct impact on the operation of your vehicle’s transmission system. The proper function of this component can help you monitor your speed when driving which is especially important when you’re traveling on the highway and school zones. 

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What is the transmission speed sensor and how does it contribute to the mechanical function of the vehicle?


What Are The Transmission Speed Sensors?

There are several sensors located inside of the transmission that work in conjunction with the transmission. However, the transmission speed sensor also commonly referred to as the vehicle speed sensor (depending on what the manufacturer decides to call it) is responsible for measuring or calculating the gear ratio of the transmission when it’s in use. The transmission speed sensor is a small device that’s located at the back or the front of the transmission.


The transmission speed sensor consists of two sensors known as the input shaft speed sensor and the output shaft speed sensor. Both of these sensors work together to provide accurate transmission data to the powertrain control module. The powertrain control module (PCM) is considered the brains of the vehicle because it works in conjunction with every sensor within the vehicle. 


Transmission Input Shaft Speed Sensor

As the name implies the input shaft speed sensor is responsible for measuring the RPMs of the input shaft. The torque converter transmits pressure to the gear assembly and the clutches. Therefore when the vehicle is moving the transmission inputs shaft is rotating at the same speed as the engine. The input speed relies on the input shaft speed and gear ratio. 


Transmission Output Shaft Speed Sensor

The output shaft speed sensor monitors the speed of the transmission’s output shaft in revolutions per minute (RPM).


The powertrain control module analyzes the data it receives from the input and output shaft and determines which gear needs to be engaged for efficient driving. During this critical process, the desired gear ratio is compared to the current gear ratio. If the figures don’t match, this signals a problem with the speed sensors and triggers the Check Engine light on the dashboard. 


Speed Sensor Operation

Both manual and automatic transmission speed sensors utilize speed sensors. These sensors aid in fuel mixture and emission control. The transmission speed sensor is made up of a permanent magnet signal. The speed sensor produces its voltage based on the speed of the output shaft. The magnetic field on the speed sensor is opened and closed based on the varying voltage signals. 


The voltage is determined by the speed of the output shaft. The faster the transmission output shaft moves the more voltage produced. The interruption of this process indicates an internal failure and an inconsistency in variable voltage components. 


Most modern vehicles aren’t manufactured with transmission speed sensors with moving parts, however, there are still vehicles on the market that are equipped with a gear-driven transmission speed sensor. 

What Happens When The Transmission Speed Sensor Stops Working?

When the transmission speed sensor stops working it can have a major impact on your vehicle’s performance. Several systems within a vehicle rely on the data provided by the speed sensor. Besides the powertrain control module, the speed sensor transmits low voltage signals to the engine control module. In some vehicles, cruise control utilizes the data from the transmission speed sensor or the vehicle speed sensor. 


When the vehicle speed sensor stops working it fails to make the necessary adjustments to the vehicle so that it runs properly. A faulty speed sensor will immediately affect the speedometer operation. It also affects the vehicle’s ability to be driven safely. 


How Do I Know If The Transmission Speed Sensor Is Failing?

When the transmission speed sensor stops working you’ll notice the following symptoms or signs:


Several Transmission Problems

Since the speed sensor relays information directly to the transmission speed sensor, once the component stops working it disrupts the normal operation of the transmission. A failing speed sensor exhibits symptoms like;


Hard of Bad Shifting

The PCM or the transmission control module isn’t able to properly control gear shifting when it doesn’t receive valid signals from the sensors. Therefore, abnormal, hard shifts and delayed shifting can occur. In some instances, there may even be limited gear operation.


Check Engine Light Is On

As previously mentioned, the check engine light is triggered when the speed sensor fails. When the transmission speed sensor stops delivering accurate information the PCM produces a diagnostic trouble code. This ultimately leads to the Check Engine light sensor flashing on.


Speedometer Stops Working

A malfunctioning speed sensor can result in erratic speedometer readings. For instance, the speedometer may read 75mph when you’re actually driving around 25mph. There is also the possibility that the speedometer won’t produce a reading at all. Inconsistent speedometer readings make it very difficult to drive safely. 


Cruise Control Won’t Engage

The transmission speed sensor regulates and monitors the input and output speed. The cruise control feature is immediately disabled by the Powertrain Control Module when it doesn’t receive the correct information from the speed sensor. The PCM will transmit an error code to the Engine Control Unit and the ECU will shut down cruise control.


Problems Within The Torque Converter

When the speed sensors aren’t able to provide accurate information, the torque converter clutch fails to properly apply the clutch and mechanically connect to the transmission and the engine. This causes a host of issues such as; transmission slippage, poor fuel economy, and the transmission starts overheating.


Vehicles With Transmission Speed Sensor Problems


Mazda's uses a Triptronic automatic transmission that often experiences speed sensor problems. This problem typically affects vehicles ranging from 2000-2007. The trouble code P0715 indicates speed sensor malfunction or the input turbine speed sensor. 


Older Acuras, specifically the ones manufactured during 1993-2003 experience speed sensor problems within their manual transmissions. These problems usually consist of random intermittent speed operation and RPM issues. 


What Causes Transmission Speed Sensor Failure?

The transmission speed sensor can fail for a plethora of reasons. Perhaps the most common causes of a failed transmission speed sensor are:


Electrical Fault Within The Sensor

When a transmission speed sensor suffers damage internally, it can result in irregular output signals or fail to produce a signal at all. A speed sensor with internal damage will need to be tested with a digital voltmeter. 


Physical Damage

If the transmission speed sensor is cracked or externally damaged in any way this can affect its ability to function normally. Sometimes the plastic part on the speed sensor becomes dry and brittle due to extended exposure to the elements.


Poor Maintenance 

Overall, poor maintenance of your vehicle can lead to transmission speed sensor problems. Over time the sensor won’t be able to pick up the correct reading. If the toothed wheel or the magnetic field gets drenched in old, contaminant transmission fluid it will stop working. 


Is It Safe to Drive A Car With a Transmission Speed Sensor Problem?

Technically you can operate a vehicle with a failing transmission or vehicle speed sensor. However, this is highly discouraged since the speed sensor will affect several aspects of the vehicle’s operation which is why a faulty speed sensor should be replaced as soon as possible. 


In older vehicles, a faulty transmission speed sensor seems to only affect the speedometer. However, driving a vehicle with a broken speedometer is illegal in most states within the United States. In modern vehicles, the anti-lock brakes and traction control are disabled once the speed sensor fails. Without anti-lock brakes and traction control, your vehicle will be prone to sliding during acceleration and heavy braking.


If you’ve been experiencing hard shifting, limited gear operation, or any of the additional aforementioned symptoms then you should have your vehicle serviced immediately. A failed transmission sense can cause further damage to the transmission and engine down the line. 


How To Test A Transmission Speed Sensor 

Once your vehicle’s transmission starts acting out of sorts, testing the speed sensor should be the first step you take. Testing a malfunctioning transmission speed sensor is pretty straightforward if you know how to use a multimeter to monitor the voltage. 


  • The first step in diagnosing the speed sensor is putting the vehicle in park and completely turning off the vehicle. Parking your car on ground level makes it a lot easier to access the vehicle speed sensor. 
  • The sensor is a small device, a small device that protrudes out of the transmission and is designed with an electrical connector plugged into it. Depending on the vehicle, the transmission speed sensor can be located on the bottom of the transmission, at the front of the transmission, or the back. You should consult your car’s user manual to find out exactly where it's located. 
  • Disengage the wiring harness connector from the speed sensor. Once you locate the speed sensor you’ll have to remove it to test it using the voltmeter, release the electrical connector so that you can remove it from the sensor. 
  • Once you remove the vehicle speed sensor attach the colored lead cables of the multimeter to the sensor. Set the multimeter to AC. The speed sensor is an electrical device that operates on AC current.


Transmission Speed Sensor Replacement

In some vehicles replacing a faulty transmission speed sensor is a fairly simple process while in others it can be a bit more complex. It’s important to note that a transmission speed sensor can’t be repaired. It should only be replaced.


 Depending on the make and model the engine mount may have to be removed first. In some vehicles, the speed sensor is located inside the transmission so to replace it the transmission will need to be disassembled. 


The cost of a transmission speed sensor replacement is approximate $266-$293. The labor costs are about $105-$128 while the parts are in the $165 ballpark range. 


Besides speed sensor failure, it’s not uncommon for other essential systems to be compromised. For instance, ABS sensor failure occurs when the transmission speed sensor stops working. You’ll need to have a full inspection performed on the vehicle to help in determining why the transmission speed sensor failed in the first place. 


How Often Does The Transmission Speed Sensor Need To Be Replaced?

Generally, the original transmission speed sensor should last over 100,000 miles and it typically needs to be replaced at 150,000 miles. Vehicles that are poorly maintained often fail earlier. The best way to prolong the life of a speed sensor is to make sure you have the transmission fluid changed regularly since soiled fluid becomes contaminated and sticks to the sensor or the ring resulting in inaccurate readings. 

Maintaining all the components of the transmission according to the guidelines outlined in the owner’s manual ensures that the transmission speed sensor lasts long. 


Common Questions Related To The Transmission Speed Sensor

Q: Can I replace the transmission speed sensor myself?

If you’re mechanically inclined, replacing a transmission speed sensor can be a simple process. However, it’s best to have this job performed by a professional. After all, the location of the transmission speed sensor is different in every vehicle, and in some cars, the transmission might need to be disassembled or replaced. 


Q: How long does it take to replace the transmission speed sensor?

Most experienced technicians can replace the transmission speed sensor within 2-3 hours. Depending on the severity of the damage it can take a bit longer. 


Q: What happens when the transmission speed sensor stops working?

When the speed sensor stops working the transmission can start malfunctioning. In some cars the anti-lock brakes, cruise control, and traction control become disabled. 


Q: How does the transmission speed sensor work?

The transmission speed sensor detects the input and output speed of the transmission and transfers this information to the PCM, ECU, and depending on the vehicle other vital systems in the car.

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