The transmission speed sensor is used by the car’s computer to calculate the transmission ratio while it is running.
Two speed sensors are built into most transmissions, both of which function to send values to the TCM and PCM which are the transmission computer and the engine computer.
To access this data you must enter with the Scanner through the Powertrain System.
Types of Speed Sensors that are incorporated into the Vehicle Transmission
ISS Sensor – Imput Speed Sensor: This sensor is located on the input shaft of the transmission and is used to measure the rotation of the shaft and perform the calculation (By the TCM) of the input shaft speed inside the the transmission of the car.
OSS Sensor – Output Speed Sensor: The second sensor is located on the output shaft of the transmission, and is used to calculate the output speed of the transmission. It is important to note that if either of these two sensors goes into failure mode then it will cause the transmission to malfunction.
In the following moments we will go into detail in this type of failures commonly presented in transmissions, and that if any of these symptoms are detected, it is necessary to go in to repair this system.
Speed Sensor Reading Process VSS 1 and 2 (ISS and OSS)
Both Transmission Speed sensors send information to the electronic powertrain management (TCM) system.
In data processing the TCM ( Transmission Computer ) compares these two signals and calculates the gear that the transmission should engage for optimal driving.
On the other hand, the PCM compares the speed or actual gear data vs. the desired gear from the computer.
It must be remembered that the computer has stored the optimal operating programming for the engine and transmission and that this data is constantly compared in order to determine if there is a substantial difference in which a fault code should be issued and the indicator light illuminated. warning or “Check Engine”.
Common VSS Transmission Speed Sensor Failure Symptoms:
When the Cruise Control system is not working
The signal from both the input ISS sensor and the output OSS sensor is critical to the cruise control system. When either of these two signals fails, the computer issues a DTC trouble code that inactivates the cruise control.
If this is happening it is necessary to perform a system scan , and read the powertrain fault codes to eliminate any type of faults related to these components.
If you activate the cruise control system and it does not work, it is a clear symptom of a fault in some of these speed sensors.
Bumps during transmission gear changes
When one of the two signals from the speed sensors, input or output, is lost, the computer cannot determine the exact time to make a gear change.
That’s when the bumps or sudden changes appear when making a gear change.
Automatic transmissions work by means of hydraulic systems, therefore it is essential, to avoid sudden gear changes, the following:
The time before making a gear change.
The time to activate the solenoid valves that carry out the gear changes.
Valve bodies and solenoid valves can be easily damaged when the transmission does not shift smoothly. Take care to check the transmission through electronic and mechanical tests when this happens so that you don’t cause permanent damage to the gearbox.
Check Engine permanently active
When the signal from any sensor is permanently interrupted, the vehicle immediately turns on the “Check Engine” warning light.
Therefore, if this happens and is combined with any of the above symptoms, it is best to check using an Automotive Scanner for DTCs (Fault Codes).
It is important to note that once the DTC is determined, and it is related to the speed sensor signal, for example:
- P0500 VSS Vehicle Speed Sensor Malfunction
- P0720 OSS Output Speed Sensor Circuit Error
- P2784 Input Speed Sensor A / B Correlation
In these cases, you must follow the roadmap for troubleshooting, which is mainly summarized in:
Important : Depending on each case, you will access the components without having to lower the transmission.
- Change Transmission Fluid and Filter.
- Check and/or replace the sensor or harness wiring.
- Change the speed sensor (ISS – OSS), but first inspect the signal with an oscilloscope.
- Check the engine temperature sensor.
- Inspect the shift solenoids or electromechanical valves that shift the transmission.
- Also, check the Transmission Torque converter.