Imagine you’re driving along and all of a sudden your speedometer stops working and your transmission refuses to shift gears correctly. In such circumstances, you might think that some kind of catastrophic failure has happened in your car. However, the actual cause of the problem is likely not too serious.
If both your speedometer and transmission stop working all of a sudden, the culprit is most likely a bad speed sensor. The speed sensor in your car is used by your speedometer, transmission, and cruise control system (if your car has one) and if it fails, it can easily affect all of these things.
Today, we’ll be covering all of the facts you should know about your car’s speed sensor, including where it is, why it fails, how to replace it, and what the other symptoms of a bad speed sensor are.
What Is a Speed Sensor, Where Is It Located, and How Does It Work?
The speed sensor in your car is often referred to as the vehicle speed sensor, or sometimes the transmission speed sensor. As the name implies, this sensor is usually located at the transmission of the vehicle, where it measures the rotational speed of the transmission and uses that to determine how fast the car is going.
The speed sensor is important for a number of reasons. As you’ve probably already guessed, the speedometer uses the data from the speed sensor to give you a fairly accurate measurement of your speed.
However, automatic transmissions also use input from the speed sensor to determine when to shift, and if your car has cruise control, the cruise control system also relies on the speed sensor to maintain a consistent speed.
Symptoms of a Bad Speed Sensor
As we’ve mentioned, there are several symptoms of a bad speed sensor that you might encounter if yours should happen to fail. Let’s go into a little more detail on these symptoms and what can happen when they occur:
Check Engine Light Is On
The check engine light in your car will come on if one of the computer modules in your car registers an error code. There are obviously many things that can cause the check engine light to come on, but a bad speed sensor is certainly one of them.
If you get a check engine light, there are two ways you can try to figure out the source of the problem. The first way is to just observe what other symptoms are occurring aside from the check engine light, which should at least let you narrow down the source of the problem to a few different things.
The second is to use an OBD2 scanner to access any saved error codes logged by the computer. If you’re going to use an OBD2 scanner to check for a bad speed sensor, you’ll want to look for the error code P0500.
Speedometer Is Erratic or Non-Functioning
Since the speedometer in your car relies on the speed sensor in order to get an accurate reading, a bad speed sensor will throw the measurement way off. If this happens, you might find that the speedometer doesn’t display any reading at all, or displays a reading that jumps all over the place and doesn’t reflect your actual speed.
Not being able to determine what speed you’re going at can potentially be pretty dangerous, so never drive for any longer than you have to if your speedometer is in such a state.
Shifting Feels Weird
Aside from calculating what speed you’re going, the speed sensor is also used by the transmission to determine when to shift. Without a working sensor, shift timing will be way off, and shifting will feel very strange as a result.
In particular, shifts might seem to be delayed, or the actual feel of shifting gears may be a lot harder than normal. Since the transmission isn’t shifting in the usual way, this may end up causing bad damage to the transmission’s internals if this goes on for long enough,
Cruise Control Doesn’t Work
If you own a car with cruise control, a bad speed sensor will leave you totally unable to use cruise control, as cruise control systems often rely on information from the speed sensor to determine what speed they should be maintaining.
The cruise control system in your car works off of input from the powertrain control module (PCM), which is a computer that controls various aspects of your engine and transmission. If the PCM gets weird data from the speed sensor, it will compare this data to the data it receives from other sensors. If it determines that the speed sensor is faulty, it automatically disables cruise control.
How to Replace a Bad Speed Sensor ?
Replacing a speed sensor isn’t exactly an easy repair to make; it requires a fair amount of equipment and experience, and if you don’t have either of those things, you could be looking at an expensive, frustrating repair. If it’s within your budget, we’d recommend that you take your car to a repair shop to get this done.
If you’re confident enough and have the necessary tools to complete this repair, however, here’s what you’ll need to do:
- Disconnect the negative cable from the battery. You don’t want to accidentally shock yourself when you unplug the old sensor.
- Jack up your car and keep it raised with a jack stand.
- Get under your car, and look on the transmission for the speed sensor. It should be on the right side of the transmission, near the end that connects to the differential.
- Get a drip pan and place it under the sensor, as some transmission fluid might leak out when you take the sensor off.
- There should be a connector linking the sensor to the car’s wiring. This connector is most likely held in place with a locking tab, so make sure to release the tab correctly before unplugging the sensor.
- When you have the wiring disconnected from the sensor, remove the old sensor from the transmission with a socket wrench.
- Install the new sensor, and connect it to the wiring connector.
- Take your car out for a drive and see how it runs. If your speedometer, transmission, and cruise control all seem to be working, then you’ve solved the problem! If it doesn’t, you’re probably better off at this point taking your car to a repair shop and having them diagnose the issue.
Is It Safe to Drive with a Bad Speed Sensor?
Driving with a bad speed sensor isn’t the most dangerous thing you can do. However, if your speed sensor is on the fritz, it’s always better to have it replaced sooner rather than later.
Having a non-working speedometer is highly inconvenient, and can also be a fairly significant safety risk in certain situations. In addition, the way a bad speed sensor affects a transmission can end up damaging the transmission eventually, and if you frequently rely on cruise control, you’ll find that driving is all of a sudden a lot more inconvenient.
In short, driving with a bad speed sensor puts you at risk of having to deal with an even larger repair bill, and it can even present a safety risk too.
What Causes a Speed Sensor to Fail?
It’s not very common for speed sensors to fail, since they’re not usually exposed to a ton of stress. However, as you surely know if you took the time to search for this article, it can definitely still happen.
Since the sensor is located on the outside of the car, it can end up being damaged as a result of prolonged exposure to water, salt, or other stuff that you’ll often find on the roads. When this happens, the casing of the sensor can become cracked and allow debris to enter the sensor and damage its internals.
The speed sensor can also become damaged if you let your car sit for too long without properly maintaining it. If you leave transmission fluid sitting in your car for a long period of time, it will eventually become thick and gunky. When this happens, it can clog up the moving parts of the speed sensor and cause false readings.
There are several ways in which your speed sensor can fail. The sensor may still be capable of measuring speed accurately but could be unable to send a signal to the PCM. It may also send a signal with a false reading, or if it’s badly damaged enough, not send any signal at all.