Symptoms of a Faulty Transmission Speed Sensor

Electronic transmissons are usually equipped with multiple transmission speed sensors that measure how fast the input and output shafts of the transmission are spinning. The torque converter turbine is splined to the transmission input shaft, also known as the “turbine shaft.”

The transmission input speed sensor measures the speed of the input (turbine) shaft, which transfers power from the engine to the transmission, while the output speed sensor measures the speed of the output shaft, which connects the transmission to either the differential or the driveshaft. Some transmissions may have a speed sensor for one or more intermediate shafts as well, so the number of sensors varies widely from platform to platform.

The transmission speed sensors send information about their respective spinning shafts to the powertrain control module (PCM), which uses the data to determine the vehicle’s speed as compared to the commanded gear as indicated by the intermediate and output shafts. The ECM/TCM uses these inputs to determine when to shift gears using the transmission control solenoids and the control pressure that moves the spool valves in the valve body. You can learn more about this process in our brief technical discussion about spinning shafts.

Faulty Transmission Speed Sensor Symptoms

If your vehicle has a faulty transmission speed sensor, it might show the following symptoms:

Jerky Gear Changes

The PCM needs data from the speed sensors to shift smoothly. The PCM typically takes into account an array of factors including the driver’s throttle position and the vehicle’s current speed to determine when to shift. Without information about the transmission’s speed, the PCM will be missing a crucial piece of data.

As a result, the transmission’s gear changes will feel more pronounced, if not harsh or jerky. The transmission might also shift too early or too late, which is going to affect your vehicle’s efficiency. Some transmission controllers will raise the line pressure to prevent the clutches in the transmission from slipping, and the increased line pressure will cause a harsh shift.

Some transmission controllers will raise the line pressure to prevent the clutches in the transmission from slipping, and the increased line pressure will cause a harsh shift.

– Richard McCuistian, ASE Certified Master Automobile Technician

A faulty transmission speed sensor also affects shift timing. This means the transmission might take longer to shift gears or it might shift too early in some cases. In high-stress scenarios, having this issue can damage the transmission’s internal components.

Cruise Control Refuses to Work

Cruise control systems automatically regulate the speed of a vehicle depending on the setting set by the driver.

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Pro Tip: On newer cars, vehicle speed information doesn’t come from the transmission speed sensors. Instead, it’s typically networked to the ECM/PCM from the ABS wheel speed sensors through the ABS module. As an aside, some late model Toyota vehicles will disable the cruise control if there is an emissions related code store in the ECM/PCM.

Illuminated Check Engine Light

The PCM can sometimes detect when the transmission speed sensors are malfunctioning. When this is the case, the check engine light on the dashboard or gauge cluster will illuminate.

Keep in mind that a check engine light won’t be able to tell you if the issue is caused by a bad speed sensor or other faulty components. Similarly, the presence of any of these symptoms doesn’t guarantee that you have a faulty transmission speed sensor. You’ll need an OBD scan tool to check if the PCM has stored any trouble codes. You should seek out a mechanic to help you determine the cause of the problem.

More About Spinning Shafts

The ECM or TCM knows how fast each shaft should be spinning in each gear, factoring in the speed of the turbine shaft and whether or not the torque converter is modulating, locked, or spinning freely.

The gear ratios are written into the computer’s algorithms. The module watches crankshaft rpm, turbine shaft (transmission input shaft) rpm, intermediate shaft on some units, and output shaft rpm. It knows what gear it is commanding. As it watches the speed of the engine, the input shaft, and the output shaft, it knows if the gear it has commanded is in play.

Transmission shaft speed sensors can be inductive sensors (magnet with a wire coil surrounding it) that produces an AC analog wave, or the sensor can be digital (Hall Effect) that produces a square wave. The sensor typically reads steel teeth moving past the sensor as the shaft spins.