What is the Output Speed Sensor?

Although different speed sensors in your car serve different purposes, they all function in the same way. Each sensor reads a reluctor wheel or exciter ring mounted to whatever rotating device it is monitoring.

The speed sensor transmits this information to the transmission control module. It uses this data to change gears accurately and adjust the torque converter.

What is an output speed sensor?

The output speed sensor (also known as the vehicle speed sensor or VSS) helps other systems in your car understand how fast you are going. It measures the rotation of a toothed wheel on a shaft inside your transmission (or sometimes at one of your wheels on newer cars with networking) and sends data to the control module. Without it, your automatic transmission or speedometer wouldn’t work properly.

The sensor produces an analog or square wave signal that varies according to the magnetic flux it’s receiving from a reluctor wheel or exciter ring mounted on whatever photosensitive sensor rotating device it’s monitoring. This data is then sent to the powertrain control module, which uses it to adjust your car’s speedometer, ABS system, ignition timing and other features.

A failed speed sensor can cause your car to malfunction in many ways. The most obvious sign of a bad one is an erratic or inaccurate speedometer reading, which can be difficult to notice while driving because the engine continues to run. A damaged speed sensor can also trigger a check engine light and affect your ability to drive safely. If you suspect the VSS is failing, disconnect the negative battery cable to prevent electrical shock and carefully remove the sensor. Use a digital voltmeter to test the sensor and wiring harness for signs of damage or exposed wires.

How does an output speed sensor work?

There are a few different types of speed sensors in your car, including the vehicle speed sensor (VSS) and the output shaft speed sensor. The main function of both is to send speed data to the onboard computer system, or powertrain control module (PCM).

Your car’s engine and transmission rely on this information to properly operate. The speed sensor also tells the anti-lock brake system if you are driving too fast, and the power steering system how much pressure to apply to the wheels.

The speed sensor has an electric current running through it, and a magnetic tip that sits close to the teeth of your transmission gear. As the gears in your transmission spin, they cause the magnetic tip to create more signal volts on the sensor over time. The computer then records the number of spikes and displays your speed on your vehicle’s dashboard cluster or DIC.

Your speed sensor is critical to the operation of your transmission and other vehicle systems, so it is important to replace it if you notice any signs of failure. The good news is that the speed sensor is relatively inexpensive, and replacing it yourself is fairly easy. If your sensor is not working, you should try spraying around the seal with WD-40 and prying gently on a shoulder or mounting bracket while rotating the sensor back and forth.

What are the symptoms of a faulty output speed sensor?

A faulty output speed sensor can cause problems with the transmission and speedometer. It may also trigger a check engine light and affect the vehicle’s overall performance. Some of the most common symptoms of a bad speed sensor include erratic shifting and difficulty in changing gears.

The speed sensor is also responsible for sending information to the computer system and adjusting fuel injection timing. Without it, your engine and transmission may not run properly. It’s important to check the speed sensor regularly and replace it if needed to prevent any issues.

You can test the output speed sensor with a multimeter to see if it is functioning correctly. Start by disconnecting the electrical connector from the sensor carefully. Make sure to not damage the wires or the sensor itself. Then, connect the probes of the multimeter to terminals on the sensor.

When you drive your car, the sensor should read the same speed each time. If the reading is different each time, the sensor is faulty. You can also try to reset the sensor by pressing a button on the dash board or using an OBD-II scanner. However, if you are having trouble with the speedometer, it is best to bring your car to a professional mechanic to get it fixed right output speed sensor away. Failure to do so could result in more expensive repairs down the road.

How do I replace an output speed sensor?

If you’ve tried everything else to diagnose the problem and a multimeter shows that the output speed sensor is dead, then replacing it should fix your problem. However, be sure to double check that the circuit to the sensor is working before condemning it to death as a bad one. If the sensor is good, but the wiring is faulty, then it will not provide a signal to the transmission computer and that could cause problems.

The vehicle speed sensor or VSS is designed to read the speed of the output shaft for the transmission by reading features on the ring the output shaft spins close to. The sensor converts the data from the ring into digital and analog signals that are read by the various control modules. The signals are used to drive the speedometer and also for other applications like anti-lock braking and traction control.

The sensor is located in the rear of the transmission on Allison transmissions and it’s relatively easy to get to by removing the shift cable selector bracket with a 12mm bolt. A video is shown below by Chinewalker that shows how to replace this sensor on an Allison. The procedure is the same for the input and output speed sensors. When installing the new sensor, be sure to use a fresh O-ring and also make sure the electrical connector is tight and not cracked (these plastic ones break very easily on older vehicles driven in harsh conditions). Also be sure that any exposed wires are covered with electrical tape to prevent water and contaminants from getting into the harness.