The transmission speed sensor measures the RPMs of the shaft of the transmission. While the engine is running, but not moving, then the input shaft of the transmission is not moving either. When the input shaft moves, the transmission speed sensor reads this motion and delivers this information to the control module. Furthermore, while the input shaft is moving, the vehicle is moving, so the input shaft is rotating at the same RPMs as the engine. The speed of the vehicle depends on the input momentum and the present gear ratio, while the speed of the input shaft is related to the speed of the vehicle.
Over time, the transmission speed sensor can go bad due to wiring issues or just outright failing due to it constantly being used every time you are driving your vehicle. If the sensor is magnetic, it can go bad from iron sticking to the tip of the sensor, which will cause an inaccurate reading. In this case, the sensor may not have to be replaced as the sensor can be cleaned off. Yet, only a mechanic should make that determination because there may be damage to the electrical part of the sensor.
Normally, the transmission speed sensor is diagnosed with a scan tool which is read by a mechanic. If you suspect your transmission speed sensor needs to be replaced because your vehicle seems run at high or low RPMs, contact a professional mechanic so they can inspect your vehicle and read any error codes that come up. Reading the error codes is the most definitive way to determine if your transmission sensor speed is to blame.
Since the transmission speed sensor can go bad and fail over time, it is important to recognize the symptoms before it completely fails so you can have it replaced.
Signs your transmission sensor needs to be replaced include:
Erratic RPM readings
The needle not moving at all on your RPM gauge
Hesitation when you shift gears, either manual or automatic
The sensor is an important part of your transmission and your vehicle running smoothly, so this repair should not be put off. Have a certified mechanic replace the failing transmission speed sensor to eliminate any further problems with your vehicle.