What is an Output Speed Sensor?
The output speed sensor is responsible for sending a signal to the car’s powertrain control module about how fast the vehicle is moving. This information is used by a number of systems in the vehicle, including traction control, stability control and the speedometer.
Replacing a transmission speed sensor is typically a straightforward task that can be done by a DIY mechanic. It’s important to be careful when disconnecting the electrical connector as these plastic components can break easily.
Input
The speed sensor (also called the vehicle speed sensor) is a small unit that is connected to the transmission input shaft. The speed sensor is a Hall-effect device that consists of a magnet (powered by the transmission shift solenoid cable) and a reluctor wheel or exciter ring, that moves against the magnet to produce magnetic flux, which in turn produces a voltage. Depending on the sensor design, there can either be an analog or digital square wave signal. This voltage is sent to the transmission control module via one wire and to other systems such as the anti-lock braking system or power steering modules (on newer vehicles that use networked sensors).
The 68RFE transmission has two speed sensors: one on the output shaft, the other in the transmission input shaft. The speed sensor provides information to multiple systems in the photosensitive sensor vehicle such as the ABS braking system, stability control and traction control.
The speed sensor’s output is also used to drive the speedometer. A faulty speed sensor can cause the speedometer to malfunction and is a common reason for taking the car to the mechanic.
Output
The output speed sensor sends a signal to the transmission control module (TCM) telling it how fast your vehicle is traveling. The TCM uses this information to determine the proper shift points and adjust your car’s torque converter clutches. The TCM also relays the vehicle speed to other systems, such as the speedometer and your anti-lock braking system.
The sensor itself can be either an inductive or magnetic design. Inductive sensors produce AC analog waveform signals, while magnetic sensors produce low-voltage square wave signals. The sensors are connected to a reluctor wheel in the case of a 4WD vehicle, and splined to the transmission output shaft for 2WD vehicles.
A faulty output speed sensor can cause problems for many different systems in your car. For example, if the sensor is not working correctly, your car’s speedometer may display inaccurate vehicle speed readings. The sensor can also affect how smoothly your car shifts gears, especially if it’s an automatic.
Replacing an output speed sensor is fairly simple for DIYers, though it’s recommended that you follow your manual and consult a professional mechanic to ensure proper installation and safe operation of your transmission. After replacing the sensor, you should be able to see a gradual rise in the transmission input and output shaft RPM when your vehicle is in neutral or at 0 mph.
Circuitry
The output speed sensor (OSS) is a magnetic unit similar to an electric guitar pickup. The sensor contains a permanent magnet and a coil. When the rotor mounted on the top of the transfer case passes by the sensor, it interrupts the magnet’s magnetic field and induces an AC current in the sensor coil. The output sensor signal is used by the TCM/PCM to control shift speed, pattern and torque converter clutch apply.
When the output speed sensor fails, it triggers a check engine light code that tells the computer what is happening. The PCM compares this input signal to the other sensor signals it receives from the wheel speed sensors and the rear axle speed sensor. If the computer detects a difference, it sets the P0720 code.
A vehicle’s speed sensor is a fairly simple device, but it’s also an essential one. As with many other parts in a car, it can last a long time before it needs to be replaced, but the lifespan depends on your driving conditions and the environment in which the vehicle is operated. If you notice that your speed sensor is not working properly, it’s probably time to replace it. This is an easy job that can be done at home. Start by removing the speed sensor from the transfer case and examining the connector for signs of corrosion. Then disconnect the harness connector and use a digital volt ohmmeter (DVOM) to test the sensor’s resistance. The resistance should be within the specifications listed in the factory shop manual.
Wiring
The output speed sensor sends a signal to the powertrain control module that helps it output speed sensor determine vehicle speed. The powertrain control module then uses this information to ensure that the transmission is operating correctly. Most output speed sensors are located on the transmission output shaft. This is different from the input speed sensor which monitors the rotational speed of the transmission input shaft. Both the input and output speed sensors generate an AC voltage signal but the signal they send to the powertrain control module varies from one vehicle manufacturer to another.
When the output speed sensor fails to work properly, it will trigger a P0720 code in the PCM and can cause issues with other parts of the car like the ABS braking system and traction control systems. Fortunately, the output speed sensor is relatively easy to replace on most vehicles and can be done in less than an hour.
The first step is to disconnect the negative battery cable to prevent electrical shock and then locate the faulty sensor and remove it from its connector. The next step is to test the sensor with a digital multimeter. Make sure the output signal wire is receiving a voltage that corresponds with the speedometer reading and the wiring is not damaged or corroded. The last step is to install a new speed sensor and reconnect its connector.