Symptoms of a Bad or Failing Speed Sensor

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The transmission speed sensors are used to calculate the actual gear ratio of the transmission while in use. There are generally two speed sensors that work in conjunction to provide accurate data to the vehicle’s onboard computer. The first transmission speed sensor is known as the Input Shaft Speed (ISS) sensor. As described, this sensor is used to monitor the speed of the transmission' input shaft. The other sensor is the Output Shaft Speed (OSS) sensor. When any of these two sensors falls out of alignment or experiences electrical issues, it impacts the operation of the entire transmission speed sensor.

After registering data, the two transmission speed sensors, also commonly referred to as a vehicle speed sensor (VSS), send data to the Powertrain Control Module (PCM); which compares these two inputs and calculates which gear the transmission should be engaged for efficient driving. The actual gear ratio is then compared to the desired gear ratio. If the desired gear and the actual gear do not match, then the PCM will set a diagnostic trouble code (DTC) and illuminate the Check Engine Light or Malfunction Indicator Lamp (MIL).

Should either or both of these speed sensors fail, you may notice one or more of the following issues.

1. Harsh or improper shifting

Without a valid speed signal from these sensors, the PCM will not be able to correctly control the shifting of gears within the transmission. This may cause the transmission to shift rough or quicker than normal. It's also common that a problem with these sensors can impact the shift timing, extending the interval between transmission shifts. An automatic transmission is hydraulically controlled and designed to operate smooth. When the transmission shifts hard, it can damage internal components including valve bodies, hydraulic lines and in some cases, mechanical gears. If you notice that your transmission is shifting harshly or roughly, you should contact a local ASE certified mechanic as soon as possible.

2. Cruise control does not work

Since the transmission speed sensors monitor the input and output shaft speed, it also has a hand 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. Check Engine Light comes on

If the signals are lost from these sensors, the PCM will set a diagnostic trouble code (DTC) and illuminate the Check Engine Light on the vehicle dashboard. This alerts the driver to an issue that should be investigated quickly because an error code has been sent to the vehicle's computer. It also can indicate that there is an increase in tailpipe emissions is occurring that surpasses acceptable limits for atmospheric contaminants from motor vehicles.

In any case, if you notice that the Check Engine Light comes on, you should contact a local mechanic to download 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.

If the problem rests with the speed sensors, depending on your particular transmission, the professional ASE certified mechanics from YourMechanic.com may be able to replace the sensor. Some speed sensors are internal to the transmission and will require the transmission to be removed from the vehicle before the sensors can be replaced.


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Mario

18 years of experience
103 reviews
Mario
18 years of experience
Honda Prelude - 75 Point Safety Inspection - Fort Myers, Florida
on Arrival he introduced himself. Extended his hand with a warm and welcoming shake as he addressed me my name. Mario got right to work. He looked at everyThing under the hood. Then he Showed me and explained The discoveries that needed attention and repairs. He is soKnowledgeable and veryCourteous. I will beUsing Mario for all the needs, repairs for/to my Honda Prelude. I highly recommend Mario! He is an ace!!!
Dodge Avenger - 75 Point Safety Inspection - Fort Myers, Florida
I scheduled for work to be done to replace my vehicle's serpentine belt, pulley, and tensioner ('cause Firestone said that it needed to be "replace"). When Mario arrived and checked the belt, he was very honest and informed me that the serpentine belt did not need to be replaced. He showed me the serpentine belt and pointed out that it did not have any cracks. It didn't need to be replaced for "preventive maintenance" either. I appreciate his honesty and the time that he took to show me other issues (that Firestone failed to mention) and explained it.

Estevan

10 years of experience
24 reviews
Estevan
10 years of experience
Toyota Camry - 75 Point Safety Inspection - Riverside, California
Estevan was very polite, friendly, and helpful! He called to let me know it would take a little extra time to get the part I (thought) the car needed. When it turned out to be something else, he happened to have the materials on hand to do a speedy fix. The service included an inspection and a list of recommended other items of concern, but at no point was he pushy about the other suggested services. A+, highly recommended!

Jerimiah

17 years of experience
328 reviews
Jerimiah
17 years of experience
BMW 528i - 75 Point Safety Inspection - Las Vegas, Nevada
Fantastic....There is a lot of work needed. He informed me that I needed work done on my control arm which was replaced in Dec 2018 by Big O Tires. Not sure how long they should last but I am pretty sure that I can get atleast a year. They replaced them(supposedly), but the issue continued after. Anyway, I am glad that Jeremiah pointed this out. Thank

Pardeep

19 years of experience
587 reviews
Pardeep
19 years of experience
Ford Escape - 75 Point Safety Inspection - San Jose, California
Pradeep immediately found the suspension problem I was worried about and fixed it. We had just purchased this car and now feel a lot better about it now that he has checked it out.

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