Cars and other automobiles rely on a wide range of sensors to function correctly. That's especially true with models that have come out in the past few years as cars become increasingly connected. Still, there's one type of sensor you'll find on new and old vehicles alike: the speed sensor.

The vehicle or transmission speed sensor is attached to your car's transmission to measure its output (or what’s perceived as the vehicle’s speed) and provides the car's computers with accurate data to ensure efficient performance from the engine and any driver-assist systems like the ABS. If the sensor fails, it must be replaced immediately.

In this article, we’re going to take a closer look at what a speed sensor is, how it behaves when it fails, and what you can do about it.

What Is a Speed Sensor?

A speed sensor is an electrical component attached to a car’s transmission to measure how fast it’s going at any time. To be more precise, the vehicle speed sensor (VSS) measures the transmission or transaxle output as a way of figuring out how fast your car is moving.

That sensor collects data for your car to use in many ways. For example, the sensor provides valuable information when diagnosing your vehicle for any problems.

Plus, it also helps the engine perform much more efficiently. With data gathered by the VSS, the engine can adjust itself in ignition timing, the air-fuel ratio it uses, and more.

An important thing to understand is that the vehicle speed sensor is different from a wheel speed sensor, which we will explore below.

Wheel Speed Sensor vs Vehicle Speed Sensor

A wheel speed sensor is different from a vehicle or transmission speed sensor because it records the actual speed of your wheels and then relays it back to the Engine Control Unit (ECU). 

From there, the ECU will then use that data to affect things like the anti-lock braking system (ABS), traction control, and other features.

How Do You Know When a Speed Sensor Goes Bad?

Like any other car part, your transmission speed sensor can go bad at some point in the vehicle’s lifespan. Thankfully, though, there are several symptoms you can look for to know when your speed sensor is becoming faulty.

3 Top Symptoms of a Speed Sensor Going Bad

Here are some of the most likely signs:

1. Check Engine Light Turns On

One of the most common signs that your transmission speed sensor isn't working correctly is that the Check Engine light on your instrument panel turns on. Of course, this light will turn on for a long list of reasons, so it alone will not tell you that there's a problem with the VSS.

Still, you shouldn’t dismiss the Check Engine light if it comes on. Instead, take that as a cue to investigate further and look for other accompanying signs like the ones below.

2. Speedometer Behaves Erratically

Here’s a crystal-clear, tell-tale sign that something is wrong with your VSS: when your speedometer behaves erratically.

The speedometer only has one purpose: to let you know how fast your car is going. So, naturally, it can only do that based on data it receives from your VSS located at your transmission.

So, when the VSS isn’t working correctly, and the information it sends out is also problematic, you’ll see it expressed on your speedometer that looks like it’s going crazy.

3. Cruise Control Does Not Work

Another tell-tale sign of a faulty VSS is when your cruise control feature does not work. Firstly, this feature can only work when the data it receives about your vehicle’s speed is accurate. However, most vehicles will automatically disable cruise control and prevent it from engaging when no data is coming from the VSS at all.

So, when you see these symptoms happen individually or in combination with one another, that’s a clear indication that you should get your speed sensor looked at immediately.

How Long Can You Drive with A Faulty Speed Sensor?

If the speed sensor has become faulty, it will not immobilise your car, meaning you will still be able to drive it.

However, that’s no excuse not to replace a transmission 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

1. 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.
2. 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.

How Much Does It Cost to Replace a Speed Sensor?

A speed sensor replacement can cost under 200 Aussie dollars, but there will be an added charge for labour if you engage the services of a mechanic.

Of course, if you’re an experienced DIY-er, you could very well replace the speed sensor yourself. If not, you would be better off letting your preferred mechanic do it for you.

Bottom Line

The condition of your vehicle speed sensor (VSS) has implications for the overall safety of your car. So, this is not a repair that you want to take lightly. Despite the relatively high cost for the replacement part, failure to replace it could lead you to more expensive trouble later.

So, technically, you could drive for quite a while, even with a faulty speed sensor. However, for your safety and other road users, the only driving you should be doing is taking your car to the workshop to have the sensor replaced.

By Ray Hasbollah