Symptoms Of A Bad Transmission Speed Sensor

Transmission speed sensors are used to calculate the actual gear ratio of the transmission while in use. There are generally two speed sensors that work in conjunction to provide accurate transmission data to the vehicle’s powertrain control module.

The first is known as the input shaft speed (ISS) sensor. As described, this sensor is used to monitor the speed of the transmission’s input shaft. The other sensor is the output shaft speed (OSS) sensor.

If either of these two sensors falls out of alignment or experience electrical issues, it impacts the operation of the entire transmission.

After registering data, the two transmission speed sensors, also commonly referred to as vehicle speed sensors (VSS), send data to the powertrain control module (PCM), which compares these two inputs and calculates which gear the transmission should engage for efficient driving.

The actual gear ratio is then compared to the desired gear ratio. If the desired gear and the actual gear do not match, then the PCM will set a diagnostic trouble code (DTC) and illuminate the Check Engine Light.

Should either or both of these speed sensors fail, you may notice one or more of the following issues:

Symptoms Of A Bad Transmission Speed Sensor

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

#1. Transmission Problems.

The transmission system of a car determines the timing of every gear with the help of a vehicle speed sensor. When the speed sensor fails to transfer the signal, the transmission system won’t be able to perform correctly. Therefore, a speed sensor failure will have a direct impact on the automatic transmission operation.

If your vehicle has a failed speed sensor then you might notice signs such as hard shifting, limited gear operation, delayed shifting and rough shifts.

#2. 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.

#3. Cruise Control Fails.

The main function of the speed sensors is to monitor the output and input shaft speed. But that’s not the only function of these sensors. They also check cruise control. Cruise control is a system that controls the vehicle’s speed automatically.

Its proper functioning depends on the data from the transmission speed sensors. If it fails to work, then it means the sensors have malfunctioned. The PCM will transmit an error code to ECU, which will shut down. If that happens, it would be better to contact your mechanic immediately to have your vehicle checked.

#4. Improper Or Harsh Shifting.

When there’s no speed signal coming from the sensors, shifting of gears will be a problem because the powertrain control module won’t control them correctly. This will make the transmission not work properly.

It will either shift more quickly or roughly than expected. Another problem caused by failed sensors occurs in shift timing which will cause the interval between shifts to be extended. When shifting becomes hard, the transmission’s internal components can be damaged.

#5. Failure To Apply Torque Converter Clutch.

The torque converter clutch activates at a pre-programmed speed. Without accurate information from the vehicle’s speed sensors, the transmission’s torque converter will not be able to engage the clutch and form a mechanical link between the engine and transmission. This can cause slipping, reduced fuel economy and an overheated transmission.

#6. Erratic Speedometer Readings.

Many vehicles use the VSS as a primary input for speedometer operation, which means a malfunctioning VSS can cause the speedometer to produce an erratic reading—or no reading at all.

Your vehicle will continue to run without a working speedometer. However, not knowing how fast you’re going will make it difficult to drive safely.

How Long Can You Drive with A Faulty Speed Sensor?

It is not safe to drive a vehicle with a bad speed sensor because ECU will not be able to identify the driving speed. Also, the ABS will not start its operations and you might lose control of your vehicle. Hence, it is best to replace a bad vehicle speed sensor as soon as possible.

That’s because you will be driving at an increased level of risk, which puts you, your passengers, and other road users at risk.

Problems with a Bad Speed Sensor

• Safety Issues – Without accurate data coming from the VSS, your driver-assist features will also fail to work as they should. For instance, your car might automatically apply its brakes sooner or harder than necessary, potentially causing an accident.
• Performance Issues – Aside from safety concerns, your vehicle’s performance will also become a lot less efficient with a failed speed sensor. For example, your transmission will behave weirdly with rough gear changing and longer intervals between shifts.

FAQs.