Bad Speed Sensor Symptoms


Cars these days are equipped with a myriad of different computers and sensors. Most of the time, these components are very helpful and allow the ECU (engine control unit) to help the car run smoothly and efficiently. However, if one of these sensors stops working for whatever reason, it can greatly affect the performance of the car.

The speed sensor in your car, as the name implies, is responsible for determining how fast your car is going. A faulty speed sensor can result in several symptoms, including false speedometer readings, problems engaging cruise control, and automatic transmission issues, among others.

Today, we’ll be covering what a speed sensor is and what happens when one fails in detail. We’ll also walk you through how to replace your car’s speed sensor yourself, and we’ll be answering a few other questions you might have about your speed sensor.

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What Is a Speed Sensor, and What Does It Do?

As we’ve mentioned, the purpose of the speed sensor (also referred to as the vehicle speed sensor, or VSS) is to determine how fast your car is going. You might assume that this is only used for the car’s speedometer, but it’s actually important for a few other things besides this.

Toyota Ipsum 2004 Vehicle Speed Sensor Location

If you have a car with an automatic transmission, as most of us do these days, then the speed sensor helps the computers responsible for shifting gears to ensure that they are shifting at the correct time. In addition, if your car is equipped with cruise control, the cruise control system will use the speed sensor to make sure that the car maintains a consistent speed.

The speed sensor is usually attached to the transmission and works by measuring the rotational speed of the gears within. The sensor then sends this info to the various control units in the car, which in turn use this info to adjust various aspects of the car’s performance.

What Are the Symptoms of a Bad Speed Sensor?

If the speed sensor in your car goes bad, then the various other components that rely on it will be unable to tell how fast the car is moving. Aside from being inconvenient, this can potentially be a safety issue, so it’s important to have your speed sensor fixed as soon as possible if you suspect that it’s gone bad.

A bad speed sensor will have several symptoms, so it’s important that you be aware of what these symptoms are when trying to diagnose any car issues. Here’s what you should be on the lookout for if you suspect your speed sensor is on the fritz:

1. Transmission Issues

The module that controls when the transmission shifts relies on the speed sensor to determine the right point of time at which to shift. If the speed sensor fails, shift timing will be way off.

Related: Why Your Car’s Transmission is Slipping & Fix

You might experience this as shifting that feels particularly hard or delayed. Because a faulty speed sensor can cause a transmission to shift in an abnormal way, this can eventually cause severe damage to some of the transmission’s internal components.

2. Faulty Speedometer Readings

The speedometer in your car relies on the speed sensor to display an accurate reading. If your speed sensor is faulty, this can result in speedometer readings that are inconsistent or greatly off. Or, it can result in the speedometer not displaying any reading at all.

If this is the only symptom you’re experiencing, then you should be able to drive your car normally. However, not being able to tell how fast you’re going at any given time can be a safety hazard, so we would really not recommend that you drive your car any longer than you have to in such a state.

3. Non-Functioning Cruise Control

If your car is equipped with cruise control, you’ll be totally unable to use it if your speed sensor is faulty. This, as we’ve talked about before, is because most cruise control systems rely heavily on input from the speed sensor to maintain a set speed.

If you’ve got a bad speed sensor and you attempt to activate cruise control, the system will fail to engage. This is usually because the powertrain control module (PCM) monitors the signal coming from the speed sensor, and if it detects any irregularities in the signal then it automatically disables the cruise control system.

Related: The Difference Between an ECM and PCM

4. Check Engine Light

As you probably know, the check engine light will come on if one of the control modules in your car detects a problem. A bad speed sensor can definitely cause your check engine light to come on, but this obviously isn’t the only reason why this can happen.

Related: What Does The Service Engine Soon Light In My Car Mean?

If you do notice that your check engine light is on, pay attention to the other symptoms your car is displaying, if there are any. If you know what to look for, it can make it a lot easier to diagnose what exactly is happening with your car.

Why Do Speed Sensors Fail?

Speed sensors don’t really tend to receive a lot of wear and tear in general, so it’s not very common for them to fail. Nonetheless, there is still the possibility of failure if the right conditions are present.

The sensor can obviously fail if it receives enough damage. If the sensor is damaged internally, it may still be able to measure the vehicle’s speed but be unable to send a signal to the car’s control modules. If it doesn’t fail to send a signal entirely, it may instead send an abnormal signal that gives a false reading.

If the exterior of the sensor spends too much time exposed to the harsh elements, the casing of the sensor could become brittle, cracked, and leaky, which could allow water and other debris to get inside and mess with the sensor’s internal components. 

Lastly, speed sensors can fail if the vehicle they’re in isn’t being properly maintained. In particular, transmission fluid that is left inside the transmission for too long can start gunking up and covering the moving parts within the sensor, leaving it unable to function properly.

Can I Drive With a Bad Speed Sensor?

You probably could get away with driving your car with a bad speed sensor for a little bit, especially if you’re just planning on driving it to the repair shop. However, it’s really not a good idea to drive without a working speed sensor any longer than you have to.

Aside from the fact that you won’t be able to tell for sure how fast you’re going without a working speed sensor, a bad speed sensor can interfere with the operation of your transmission and potentially cause damage to it.

In the end, not only do you risk damaging your car by driving without a working speed sensor but you might be putting your own safety at risk as well. If you think your speed sensor is broken, you should get it fixed as soon as you can.

Related: Can You Drive a Car With a Bad Fuel Injector?

How to Replace Your Car’s Speed Sensor

While it might be possible to replace your car’s speed sensor on your own, we’d recommend that you take your car to a repair shop to get this done, if possible. You’ll need a bunch of tools and safety equipment to replace a speed sensor properly, and unless you already have all of this stuff, it’s probably going to be somewhat expensive to acquire it.

In addition, if you don’t have the experience necessary to complete such a repair, you might find it incredibly difficult to actually get this done. Assuming you do have the requisite tools and experience though, here’s what you’ll need to do:

  1. Jack your car up and use a jack stand to keep it raised. Get under the car and locate where the speed sensor is attached to the transmission.
  2. Put a drip pan under the sensor to collect any transmission fluid that leaks out when you remove it.
  3. Remove the connector that links the sensor to the car’s wiring. There might be a tab you need to release in order to do so without breaking it, so make sure you don’t try and force the connector apart right away.
  4. Once the wiring is disconnected, use a socket wrench to remove the old sensor from the transmission.
  5. Install the new speed sensor and connect it to the car’s wiring.
  6. Finally, take your car for a test drive. If the speedometer, transmission, and cruise control are working again, it means you’ve most likely solved the problem. If not, at this point we’d recommend just biting the bullet and taking your car to a repair shop.

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