A car’s transmission speed sensor, sometimes called a vehicle speed sensor (VSS), works out how quickly it’s moving along the road. When you look down at your car’s speedometer, the figure you see is communicated from this little component. Your vehicle depends on the transmission speed sensor to function correctly. If left unchecked, a faulty sensor can trigger a range of symptoms, which can have a significant impact on your ability to drive safely.
Because this sensor is so important to your vehicle, it’s crucial to know the signs that something’s amiss. Below, we run some common symptoms of a bad transmission speed sensor, what they mean, and how these sensors work in your car.
What is a Transmission Speed Sensor?
A car’s transmission speed sensor is responsible for working out how fast the vehicle is travelling, and communicating this information to the engine control unit (ECU) – the ‘brain’ of the car. When the ECU in an automatic car receives this information, it is able to select the optimal gear for efficient driving at that speed. In both automatic and manual cars, the sensor helps to determine ignition timing and fuel injection, improving the car’s efficiency, controlling emissions, and improving drivability. The transmission speed sensor is also a vital component in electronic speedometers. It is located either on the transmission output shaft, or the crankshaft, and cars tend to have two of them, so their data can be corroborated.
The sensors function through a surprisingly simple mechanism. Each consists of three main components: a toothed metal disk, a detector, and a magnetic coil. The disc spins in time with the shaft it’s connected to, and as the teeth move past the magnetic coil, they interrupt the field it produces. This results in a series of magnetic pulses, rather than a continuous field. These pulses are fed into the car’s onboard computer, where their frequency is used to work out the speed of the car. The higher the pulse frequency, the faster the vehicle is travelling. A final speed reading is then communicated to a display on the dashboard, which can either be digital or analogue.
A car’s transmission speed sensor also allows the vehicle to keep track of the distance it has travelled: for every 40,000 magnetic pulses, the odometer reading increases by one mile.
What Can Go Wrong?
A transmission speed sensor can malfunction for a variety of reasons:
- Corrosion of the component
- Loose or disconnected wires
- Cracks in the housing of the sensor
- Sensors flying off of their shaft entirely
- A poor connection between the sensor and the ECU
- A damaged magnetic coil
Whatever it it that goes wrong, the symptoms are likely to be the same.
Symptoms to Watch Out For
If your car’s transmission speed sensor malfunctions or fails, you can expect to experience one or more of the following symptoms:
- Faulty Cruise Control
If your car is equipped with cruise control, this functionality is usually one of the first things to suffer when a transmission speed sensor malfunctions. Without a properly functioning sensor, the ECU isn’t receiving information about the car’s speed. This makes it impossible for the vehicle to maintain a constant velocity. Without reliable speed information, the ECU will automatically prevent you from using cruise control until the problem is fixed. If you suspect your car has a faulty sensor, attempting to use cruise control can actually be a useful diagnostic method.
- Sluggish Shifting
When a transmission speed sensor malfunctions or fails, you’ll notice longer shifting intervals than you’re used to. This is because a car’s transmission depends upon information relayed by the ECU, which, in turn, depends on speed data from the sensors. If this speed data isn’t reliably conveyed, the ECU struggles to properly manage gear changes. All this results in a small but noticeable waiting period between shifts.
- Difficult Shifting
Even in a manual vehicle, a bad transmission speed sensor can cause gear shifting problems. Just moving the stick can be more difficult, and you may notice that the shaft feels rougher, or faster than usual when you shift. If you experience this symptom, consult a trusted mechanic or your dealership as soon as possible – smooth gear changes are vital for safe driving.
- Malfunctioning Speedometer
As mentioned earlier, most modern cars are fitted with an electronic speedometer, which depends on a transmission speed sensor to function. If your speedometer is erratic, or doesn’t work at all, it could be because the dial or screen is not receiving consistent speed information from the transmission speed sensor.
- Illuminated Check Engine Light
The check engine light is something of a catch-all when it comes to car trouble, but if it illuminates in conjunction with another symptom on this list, there’s a good chance it has been triggered by a bad transmission speed sensor. When a sensor malfunctions or fails, the ECU will notice that it’s not receiving the speed data it should. In response, the check engine light is switched on.
If you notice one or more of these symptoms, it’s important to take your car to a trusted mechanic as soon as possible. A bad transmission speed sensor makes for rough and difficult gear changes, and can leave you without an accurate speedometer. All this adds up to a dangerous drive, worsened by the fact that your car is probably less efficient, and producing more harmful emissions.
Luckily, it’s fairly straightforward to diagnose a faulty transmission speed sensor. Your mechanic will plug in an electronic diagnostic tool to the car’s onboard computer, which will display a certain error code if the sensor is indeed at fault.
Crucial as they are, you might expect a transmission speed sensor to be an expensive part. However, the opposite is actually true.
Costs will vary depending on the make and model of your vehicle, but you can expect to spend between $20 and $50 on your new component, plus about $50 to $100 on labor.
If you’re a dab hand at auto DIY, however, you could consider replacing the sensor yourself. If this sounds feasible to you, we’d recommend this guide.
- Vehicle Speed Sensors: Types of VSS – AZO
- Symptoms of a Bad or Failing Transmission Speed Sensor – Your Mechanic