output speed sensor

How a Faulty Output Speed Sensor Can Trigger a Check Engine Light

Your car’s speed sensors are responsible for relaying a signal to the onboard computer that tells it how fast your vehicle is moving. A faulty output speed sensor can cause your car to malfunction and trigger a check engine light.

Regardless of how it’s referred to, your car’s speed sensor is an important component in its drivetrain. Here’s everything you need to know about it.

What Is an Output Speed Sensor?

A speed sensor is an essential part of your car’s computer system. It transmits vital data to the transmission control unit, letting it know photosensitive sensor how fast your vehicle is moving at any given moment. A malfunctioning sensor can cause a variety of problems, from shifting issues to a faulty speedometer reading.

Both the input shaft speed (ISS) sensor and the output shaft speed (OSS) sensor are crucial for ensuring that your car’s automatic transmission shifts correctly and efficiently. Both sensors register information about your car’s speed and transmit that data to the powertrain control module, allowing it to properly match the desired gear ratio with the actual gear ratio during operation.

When the ISS or OSS sensor fails, it will send incorrect data to the PCM, leading to transmission shifting issues and other vehicle malfunctions. You can diagnose this issue by connecting an OBD-II scanner to your vehicle and checking the stored error codes.

The ISS or OSS sensor will display either an AC voltage signal or a digital square wave signal depending on the type of transmission you have in your vehicle. Both sensors can be tested using a multimeter or oscilloscope to determine whether they are sending the correct information to the PCM. The speed sensors can also be replaced by disconnecting the negative battery cable and carefully removing the sensor and sensor connector from their mounting locations.

How Does an Output Speed Sensor Work?

The output speed sensor is a small unit attached to the transmission. It consists of a magnetic pickup and an exciter ring with gear teeth that spin to generate a signal that is passed through the sensor. The sensor then relays this data to the transmission control module. From here, the computer system uses this information to adjust the operation of systems such as the power steering system and anti-lock brakes.

The way this sensor works varies slightly depending on whether it is a variable reluctance or Hall effect style sensor, but they all read the same thing: steel wheels moving past them at different speeds. They also create their own signal using either a reluctor wheel (inductive sensors) or a rotor with gear teeth (variable reluctance sensors). When the reluctor wheel or rotor passes in front of the sensor, it creates a voltage pulse that is sent to the output speed sensor and then the transmission control module.

Like other components in your vehicle, speed sensors are subject to wear and tear. They can also be damaged by road salt or other environmental elements. For these reasons, it is important to replace your car’s input and output speed sensors regularly. When these components are faulty, it may lead to inaccurate shifting behavior, which is unsafe for your vehicle and other drivers on the road.

What Systems Share Information with an Output Speed Sensor?

The output speed sensor shares information with several systems incorporated into the vehicle through the CAN system. Many of these systems are related to braking and stability control. The PCM relays the vehicle speed information from the wheel speed sensors (WSS) and output speed sensor to other modules on the CAN for input, such as the anti-lock brake system and the powertrain control module.

These speed sensors can be simple, with the sensor reading steel teeth in a reluctor wheel or exciter ring as the wheel spins, or complex, with a series of magnets embedded in the bearing seal to detect magnetic pulses as the wheel turns. A faulty WSS can cause an inaccurate speedometer reading, while a failed output speed sensor can affect transmission shift timing and operation.

The VSS and the ODS are two different types of speed sensors that perform different functions in the vehicle. Testing an input speed sensor requires measuring the AC signal with a multimeter, while testing the output speed sensor requires a scope.

How Do I Replace an Output Speed Sensor?

In order to determine if you have a vehicle speed sensor issue, your OBD II system will likely show an error code for the output output speed sensor or input sensor (or both). You can then choose to take your car in for service and have a professional replace the sensor for between $100 and $250 or more. You can also do it yourself for a fraction of the cost.

The speed sensor is mounted to the output shaft of your transmission. As the shaft turns, a ring on the output shaft spins very closely to the sensor and the sensor reads how fast the ring is spinning using features on the ring. Then it reports that information back to the various control modules. It is this input that controls your speedometer as well as some of the anti-lock brake and traction control systems.

Replacing the speed sensor is quite simple. First, you will need to loosen the negative battery cable and locate the speed sensor. Then, you will need to remove the mounting bolts and disconnect the electrical connector from the sensor. Be careful when disconnecting the sensor connector as they are often made of plastic and can break easily. Once the bolts are removed and the connector disconnected, you can then simply remove the sensor. When installing the new sensor, make sure that the O-ring is installed correctly and then reconnect the electrical connector.