Symptoms of a Faulty or Faulty Transmission Speed ​​Sensor

Common symptoms include harsh or erratic shifting, cruise control not working, and the Check Engine light coming on.

Transmission speed sensors are used to calculate the actual transmission ratio during transmission use. Typically, there are two speed sensors that work together to provide accurate transmission data to the vehicle's transmission control module. The first is known as the input shaft speed sensor (ISS). As described, this sensor is used to monitor the speed of the transmission input shaft. The other sensor is the output shaft speed sensor (OSS). If any of these two sensors fail or there is an electrical problem, the operation of the entire transmission will be affected.

Once the data is logged, the two transmission speed sensors, also commonly referred to as vehicle speed sensors (VSS), send data to the powertrain control module (PCM), which compares the two inputs and calculates which transmission gear should be engaged for efficient driving. . The actual gear ratio is then compared with the desired gear ratio. If the desired gear and the actual gear do not match, the PCM will set a Diagnostic Trouble Code (DTC) and the Check Engine light will come on.

If one or both of these speed sensors fail, you may notice one or more of the following 3 problems:

1. Abrupt or incorrect gear changes

Without a valid speed signal from these sensors, the PCM will not be able to properly control transmission shifting. This can result in rough or faster shifting than usual. Also often a problem with these sensors can affect shift times, increasing the interval between transmission shifts. The automatic transmission is hydraulically controlled and designed for smooth gear changes. When the transmission shifts abruptly, it can damage internal components including valve bodies, hydraulic lines and, in some cases, mechanical gears. If you notice that your transmission shifts harshly or rough, you should contact your local ASE certified mechanic as soon as possible.

2. Cruise control does not work

Since the transmission speed sensors monitor the speed of the input and output shafts, they also play a role in cruise control control. When the sensors are not transmitting accurate data to the on-board computer of your car, truck, or SUV, the powertrain control module (PCM) will send an error code to the vehicle's ECU. As a precautionary measure, the ECU will turn cruise control off and make it inactive. If you notice that your cruise control won't turn on when you press the button, have your mechanic inspect the vehicle to determine why the cruise control isn't working. This may be due to faulty baud rate sensors.

3. The Check Engine light comes on

If signals from the transmission speed sensors are lost, the PCM will set a DTC and the Check Engine light on the vehicle's instrument panel will illuminate. It can also indicate an increase in exhaust emissions that exceeds the allowable limits for environmental pollutants from vehicles.

In any case, if you notice that the Check Engine light is on, you should contact your local mechanic to scan for error codes and determine why the Check Engine light is on. Once the problem is fixed, the mechanic will reset the error codes.

If the problem is with the speed sensors, depending on your specific transmission, professional ASE certified mechanics can replace the sensor. Some speed sensors are built into the transmission and the transmission must be removed from the vehicle before the sensors can be replaced.