It can be pretty confusing to distinguish between a transmission speed sensor and an output speed sensor. Both of them are attached to transmission shafts and calculate speed.
So, why is there a transmission speed sensor and an output speed sensor?
The transmission speed sensor sends the RPM value of the input shaft and the output speed sensor sends the RPM value of the transmission output shaft to the PCM. Both of these sensors must work together for a vehicle to run smoothly and optimally.
To understand why there are these two types of sensors in a vehicle, continue reading. I will discuss each sensor’s working method in a simple way.
Why Is There A Transmission Speed Sensor And An Output Speed Sensor?
There is a transmission speed sensor and an output speed sensor in a vehicle because they serve different purposes and without either one, the vehicle cannot run properly.
A transmission speed sensor, also known as an input speed sensor, calculates the speed of the transmission’s input shaft. This sensor measures the speed before it goes through the transmission system.
On the other hand, the output speed sensor measures the transmission’s output shaft speed. This is the speed that is produced after a certain RPM goes through the gear ratio set by the TCM (transmission control module).
Following are the features of transmissions’ speed sensors. The input and output sensor works on the same principle and looks pretty much the same.
- There is one input speed sensor in a car’s input transmission shaft and one output speed sensor near the output shaft.
- This sensor is a hall effect sensor that uses magnetic properties.
- It creates a digital signal proportional to the speed of rotation of the transmission shaft.
Now, if you are wondering how a transmission system works using these sensors, we have answered it here.
Now, why two speed sensors in the transmission instead of one?
A car needs both transmission sensors to shift gears smoothly and accurately. These speed sensors communicate with the ECU through PCM and TCU to figure out which gear it should apply.
As stated above the input speed sensor only has the engine’s rpm data. Similarly, the output speed sensor only sends the transmission output shaft speed to the transmission control unit. To precisely apply a certain gear ratio, the ECU needs both data.
Besides making sure the gear shifting is efficient and synchronized, these sensors are used in diagnostic tests as well. These sensors can be utilized to investigate if the transmission is slipping.
Summary
A can needs both a transmission speed sensor and an output speed sensor to send the rpm value of the input and output shaft of the transmission to the engine control unit.
What Happens When The Transmission Speed Sensors Fail?
If both or either of the transmission speed sensors fails then, the ECU will receive incomplete and inaccurate data and can’t synchronize transmission with the engine. These are the consequences below when the transmission speed sensor or output speed sensor fails;
Check Engine Light On
When the speed sensors fail to send data to the PCM, then the PCM or powertrain control module will send a DTC and the engine check light will come on. There are many reasons for the engine check light to be On.
However, when this light comes on, it means you must investigate the reason behind it. If there is any DTC code related to the transmission speed sensor on your OBD reader then consult a mechanic to fix the issue.
Cruise Control Failure
Transmission speed sensor failure will make the PCM send an error code. This error code will turn off the cruise control system of your vehicle. This is done as a safety measure since you are getting inaccurate data about your transmission.
So when these speed sensors fail it means you cannot engage your cruise control while driving. You should consult a mechanic when this happens.
Improper / Harsh Gear Shifting
The main objective of transmission speed sensors is to send data to the car’s PCM so that it can make the gear shift smoothly and properly. So, when these sensors fail then the gear shifting will be harsh or improper.
The gear shifting will be rougher and the duration between shifting might get longer. This will put a strain on transmission components. These components include hydraulic lines, valve bodies, and sometimes mechanical gears. To know more about sluggish transmission and why it happens you can read this.
Incorrect Speedometer Reading
The speedometer reading will be wrong or totally blank when your transmission sensor stops working properly. This is a major safety issue since you will get incorrect speed readings on the dial.
Summary
If these sensors fail then the check engine light will be on, the cruise control system on your car will fail and gear shifting will be rough and improper. At the same time, the speedometer reading will be wrong.
FAQs
Where Is the Transmission Output Speed Sensor Located In?
The transmission speed sensors’ location varies depending on the make and model of a vehicle. But usually, it is located near the output shaft of the transmission. Sometimes it’s located on the differential or tail shaft of the transfer case.
Can a Speed Sensor Stop Transmission Shifting?
Yes, A speed sensor that has gone bad can stop transmission shifting. Also, a late or harsh shifting might occur due to a bad speed sensor. It is recommended to replace a bad speed sensor as soon as possible.
How Much Does It Cost To Replace An Output Speed Sensor?
An output speed sensor costs $245 if you can replace it yourself. The replacement procedure is not too complex. However, a mechanic will charge you $100-$150 for the replacement. Even though it’s expensive, it’s a must to replace a faulty speed sensor.
Bottom Line
We hope this article resolved your confusion regarding the transmission speed sensor and output speed sensor. Without these sensors, modern cars can’t operate properly and will have traction and stability control issues, engine management issues, etc.
