The Importance of an Output Speed Sensor
The output speed sensor transmits a signal to the transmission control modules about how fast your vehicle is moving. This information helps these modules determine the functionality of various output devices, including your speedometer.
If your vehicle’s speedometer reads erratically, it may be due to a bad speed sensor. Before replacing your speed sensor, make sure to disconnect the negative battery cable and remove the sensor connector.
What is an output speed sensor?
An output speed sensor is one of the key components in your transmission. These sensors monitor your vehicle’s transmission output shaft speed and input shaft speed to provide the PCM with data that it can use to determine the actual gear ratio during use. Without this important information, your car may experience erratic shifting behavior and could trigger the check engine light.
There are two main types of speed sensors on your car: the input shaft speed (ISS) sensor and the output shaft speed (OSS) sensor. While they both do similar jobs, they are not interchangeable. ISS and OSS sensors are used to transmit vehicle speed data to the powertrain control module, but they also support other systems on your car, including traction and stability controls, cruise control, and even ignition timing.
Both ISS and OSS speed sensors work by converting the rotational motion of metal gears in your transmission system into an analog or low-voltage square wave signal that varies depending on the speed at which the sensor is rotating. This square wave signal is then sent to the PCM for processing.
Once the PCM receives this data, it can compare it to your transmission’s desired gear ratio and decide what gear to engage to achieve optimum performance. This process also ensures that your vehicle is driving at a safe speed.
How does an output speed sensor work?
In order for your car to tell the speedometer how fast you are going, it needs to have accurate information from a few different sensors. One of those is the output speed sensor (OSS). The OSS monitors the rotational speed of the transmission’s output shaft to provide input to the transmission control module. The OSS also contributes to your car’s shift timing and shift solenoid operation.
Like other speed sensors in your vehicle, the OSS uses a reluctor wheel or exciter ring to produce an AC analog signal or low-voltage square wave that varies depending on how fast the car is moving. The sensor relays this signal to the PCM, which translates it into the car’s photosensitive sensor speedometer reading. Other systems rely on speed sensor data, including the anti-lock braking system, cruise control, and ignition timing.
Input and output speed sensors function differently, as the input sensor reads steel teeth in a rotating wheel or pulses of magnetic flux created by the wheel’s internal magnets to detect the velocity of the wheel. Both the input and output speed sensors can fail, though, causing your car to malfunction in various ways. For example, a bad speed sensor could cause your automatic transmission to shift improperly or not at all. Having your speed sensors diagnosed and replaced promptly will keep your car running safely and smoothly for years to come.
What is the purpose of an output speed sensor?
A car’s speed sensor is a crucial component. It not only tells the transmission how fast you’re driving, but it also relays that information to other systems in your vehicle. The transmission control module, for example, uses the input and output speed sensors to determine shift timing. It also uses the speed sensors to display your car’s actual speed on the dash.
The speed sensor (also known as a vehicle speed sensor or VSS) is a magnetic unit that consists of a permanent magnet surrounded by a wire coil. As the magnet moves relative to the gears or rotor in the vehicle’s transmission, the magnetic field interrupts the sensor’s coil and induces an AC voltage signal that correlates with the speed of your car. The signal is used by the TCM or PCM to control the shift patterns, torque converter clutch apply, and to generate a speedometer reading.
If the speed sensor fails, it may cause erratic shifting. The problem could range from minor to severe, depending on the condition of the sensor and the nature of its failure. Performing the necessary tests and replacing the speed sensor will help you resolve the issue.
How do I replace an output speed sensor?
The output speed sensor (also called vehicle speed sensor) is located on the transmission output shaft. It is used to provide a signal that the output shaft output speed sensor is spinning at the appropriate rate to the various control modules that read it, including the speedometer and antilock brake and traction control systems.
When the sensor fails to read correctly, the speedometer may not function properly, the traction control and antilock brakes may be disabled, and the transmission will shift randomly or not at all. Testing and replacement of the sensor is usually necessary, especially if one of these symptoms are present.
If you suspect that your transmission output speed sensor is faulty, first check the wiring with a multimeter. If the circuit is open or has a bad connection, then the sensor itself must be replaced.
Before replacing the sensor, make sure to loosen and remove the negative battery cable. Place a bucket or pan underneath the sensor to catch any transmission fluid that may leak. Then, remove the connector from the speed sensor and disconnect the harness from it. On older vehicles or those that have been driven in harsh weather, be careful when disconnecting the connector from the sensor as these can break easily. Also, be sure to replace the connector with a new one that has an O-ring and is fitted tightly onto the sensor.
