Automatic transmission sensor, everything you need to know

In general, cars that use automatic transmissions must record the number of revolutions per minute that the tires make to change gear, as well as the appropriate time to do so. This component is the transmission sensor, and today we will tell you what is automatic transmission sensor, how it works, what symptoms occur when there is a failure, and how to repair it.

How does the automatic transmission sensor work?

Cars with automatic transmission use a hydraulic planetary system, in which the friction clutch is replaced by a fluid coupling, and the system defines the gear ranges according to each vehicle´s needs. For example, when parking, the gears are locked to prevent the vehicle from moving forward or backward, making it impossible to move.

However, some vehicle models have an automated or semi-automatic transmission, where the clutches and gears of the hydraulic planetary system are used together with a set of actuators, sensors and processors. In this way, they work as an automatic transmission and allow fuel saving and automated gear change that is operated by a hydraulic system in which the sensor plays a very important role. Transmission sensors record the number of revolutions per minute the tires make to regulate shift pressure during gear change transitions.The ring gear´s rotational movement causes a variation in the magnetic field, which alters the tension. The sensor transmits these voltage signals to the vehicle's computer or control unit to calculate the transmission ratio while it is running. 

There are two types of transmission sensors: the ISS sensor (Input Speed ​​Sensor) which is located on the input shaft of the transmission measures the shaft´s rotation and calculates the input speed of the shaft in the car's transmission. The second sensor is the OSS (Output Speed ​​Sensor) which is located on the output shaft of the transmission and is used to calculate the transmission´s output speed. If either sensor has a failure, then the transmission will not work properly.

 A faulty sensor´s symptoms·        

• Failures in transmission control. Faults can cause the control unit to switch to an emergency run program.·        
• Knockings during changes. When one of the sensor signals is lost, either input or output, the computer cannot determine the exact moment to shift, causing a clunking sensation when shifting as well as a jerky feeling when shifting.
• Active “Check Engine” alert on the dashboard. When an input or output sensor failure occurs, the computer issues a DTC trouble code that gives the “Check Engine” alert on the dash and disables transmission control. This is the most common symptom, especially if the dashboard alert is combined with any of the symptoms above.  

How to fix the sensor? 

If the "Check Engine" signal came on, or if the vehicle began to give failure symptoms, the first step is to perform a verification through an automotive scanner to find the DTC fault codes. Transmission sensor related codes are:·        

• P0700·        
• P0701·        
• P0702·        
• P0705·        
• P0706·        
• P0707·       
• P0708·     
• P0709·     
• P0780·       
• P0864

To check the transmission sensor status, place the transmission gear lever in the park position and try to start the vehicle. If the engine starts, the transmission needs to be put into gear, and if this fails, or the transmission is sluggish when driving, the sensor is most likely failing.Subsequently, the engine should be turned off and the gear lever placed in a gear and attempt to start the engine; if it starts with the transmission in gear, then the sensor is not ok.

The way to fix this situation is to change the sensor. With the vehicle parked on a level surface, and the transmission in park mode, the brake must be applied to the tires to prevent them from moving. The battery is then disconnected to deactivate power to the ignition and fuel systems. The vehicle is raised on a floor jack until the tires are fully off the ground and the transmission sensor is removed and replaced with a new one.

Once sensor has been replaced, and with the vehicle on the ground again, a test drive should be done to see if the check engine light comes back on. Transmission shifts must be made correctly.In the event that the check engine light continues to come on, a diagnosis should be made with a scanner to find the source of the fault.

 How long does the automatic transmission sensor last? 

Transmission sensor failure can be common due to wiring problems, battery problems, or wear and tear, especially if it's a magnetic sensor as the iron sticks to the tip of the sensor and causes it to give an inaccurate reading. 

This sensor is an important component of the transmission, so it should be replaced immediately in the event of a failure.

At Sun Transmission you will find the automatic transmission sensor that your vehicle needs. Browse our online store and acquire the right one for your vehicle, as well as other products like automatic transmission solenoids, automatic transmission additives, and automatic transmission bushing kits. For more information write to us through our online chat where we will gladly assist you.