A few months ago, I was driving my car to a distant destination. And guess what? I got something erratic with the speedometer. The readings I saw were quite fearsome for me and my passengers. Then, I asked, “Why is my speedometer going crazy?”
I started looking for the roots of why this was happening. So, I decided to search the forums where car owners like me discuss the issues. Many forum users have already faced troubles with unexpected meter readings. However, I got the information I was looking for from the replies.
So, my topic for today is discussing why a speedo becomes crazy while driving a car. As I got the answers, you may also benefit from reading this discussion in this article. I suggest you read all the sections to prevent technical problems quickly.
Functions of a Speedometer
You can consider the speedometer as a circular gauge. Often people call it a speedo, odometer, or instrument cluster. However, every vehicle has it on its dashboard, close to the steering wheel. It helps to measure the speed and distance you travel by driving the car. On the gauge, you can find multiple needles and numbers (km or miles). (Sources: 1, 2, 3)
The numbers start from zero and rise higher in a circle. Here, zero means you have stopped the car in a place. Hence, the manufacturers place the numbers within a specific range like 0, 20, 40, 60, and it goes on. This range may vary in some vehicle models.
There are two types of speedometers available for vehicles:
- Analog or mechanical
- Digital or electronic
Analog speedometers are common in old vehicles. You can measure the speed of the car wheels with a needle. Again, they may have an internal cable connection with an odometer. In this case, the earlier one measures the speed, and the latter tells the distance you have traveled.
Most modern cars have digital speedometers that use speed sensors to give information. You can see all the calculations visible on the LED screen. In many vehicles, you can get readings of multiple measurements like speed, distance coverage, temperature, fuel quantities, etc.
These devices have a digital needle pointing to the gauge’s speed or distance. Likewise, these sensors have a metal disk (with teeth), a stationary detector, and a magnetic coil. (Source)
What People Talk About a Crazy Speedometer
So, you have learned a brief about the functions of a speedometer. When any component of this device malfunctions, it will provide incorrect or unstable readings. However, read the discussions below to know why your speedometer becomes erratic while the car is on the road.
I have found a discussion on the issue in DSM Tuners. There were some explanations:
“Chances are the cable is binding and then unwinding.”
“Save yourself the headache… buy a used cluster and a new cable.”
In a thread on a New Celica forum, some responses were like these:
“Sounds like a sensor issue with the flywheel speed in your auto box.”
“…can be a faulty battery or alternator circuit or simply that the sensor circuit has a bad grounding connection.”
A discussion on Honda-Tech reported the same thing happened to a Honda Civic (CR-X Del Sol) car. The suggestions were:
“Replace your VSS sensor, and it will be good to go.”
“Check the connector that connects to the speed sensor.”
The Reasons & Solutions of Speedometer Going Crazy
Now, I will share the common reasons for instrument cluster failure. Please note that I have collected the information from different sources.
Here are some causes of getting a crazy speedo:
Bad Speedometer Cable
You may get a jumping speedometer for the worn cable if you have an analog speedo. It cannot spin as much as it should. Then, it may interrupt the chain reaction process essential for correct instrument cluster reading. (Source)
Solution
After locating, remove the cable (if it is in good condition) from the gauge. Then, spin the inner cable with your hand. Lubricate or replace the device if the needle fails to move smoothly and accurately.
Sometimes changing the broker inner cable may make the speedometer functional. Moreover, many experts suggest replacing the entire cable assembly if its housing is damaged.
Malfunctioning or Defective Wheel Speed Sensors
A vehicle speed sensor (VSS) is essential for a digital or electric speedometer. VSS measures the speed, and you get the readings on the cluster.
Both have direct connections that means when sensors are not working, speedo will do the same. Incorrect transfer of data from VSS can lead to crazy speedometer.
However, some signs of a bad VSS are transmission issues, turned-on ABS and check engine light, and difficulty activating cruise control. (Sources: 1, 2)
Solution
You can check the resistance of the sensor’s wiring using a multimeter. A faulty sensor will have resistance outside of the manufacturer’s specifications. In this case, the replacement of sensors will be helpful.
Defective Instrument Cluster
A healthy gauge or instrument cluster will never provide wrong vehicle speed and distance coverage data. But when it fails, you will get a malfunctioning speedo. It may stop from showing speed or provide inconsistent, unreliable readings.
According to RepairSmith, a lousy instrument cluster may also provide wrong info about coolant temperature, fuel gauge, and odometer.
Solution
Solve the issue by checking the instrument cluster thoroughly. I suggest you disconnect and reconnect any electrical connections to this device. Then, secure them properly.
Replace the speedometer if you find damage or corrosion on the circuit board. Besides, you can test the gauges using the necessary tools.
Bad Wiring
Suppose you are getting variable speed readings, like sometimes too high and suddenly down. You can blame lousy wire, wiring harness, or cable in that case. Thus, you should repair and replace them to get the readings accurately. Let me explain it more.
Wiring in a vehicle should always be functional. It carries the power to the whole car. However, a worn-out speedo cable or blown-up fuse is possible. When these components go wrong, the needle on the speedo is dancing unnecessarily. (source)
Solution
At first, I suggest you to remove the key after turning off the engine. Then, locate and inspect the wiring in speedo for damaged or frayed wires. Now, ensure all connections are correctly plugged in and fit securely. In this case, you may use a screwdriver or piler to tighten them. Turn on the engine and see if things are right.
Malfunctioning ECU
Does your car have an Electrical Control Unit (ECU)? It controls different internal systems in modern car engines. But a malfunctioning ECU can make things complex.
For example, the vehicle may fail to read the speed signal from the speed sensor. Therefore, you will get a fluctuating reading on the speedometer. (Source)
Solution
You may contact a professional to inspect the ECU. Possibly, he will run a diagnostic test to test the computer components. He will offer you the best service or solution depending on the results.
Dirty or Corroded Battery Terminals
You will see bluish-white powder on the battery terminals if they become dirty or corroded. It can cause poor connection and make the battery drain faster. And when the battery is dead, you will surely get a crazy speedo measurement. (Source)
Solution
You can make baking powder and water to clean the terminals. Then, remove all the powder from the terminals with a small soft-bristled brush. After cleaning, reattach the clamps to them.
Weak Battery Connection
If your car has weak battery connections, it may interrupt the electrical system. Sometimes, the battery cannot distribute power sufficiently to all the components. Your instrument cluster also remains on the list. A loose connection may happen when you drive your car off-road or on a bumpy road. (Source)
Solution
Thoroughly inspect all the batter wiring wherever it goes. If you find the wiring loose, tighten them using a screwdriver.
Bad Module
I guess you have checked all the vehicle modules are working fine. It is because they may get physical damage unexpectedly. As a result, the damage will cause a shortage that leads to fluctuating speedometer readings. Some standard modules that may have this issue are ABS and parking assist modules. (source)
Solution
It is better to inspect the modules visually. If you find any issues with them, call a mechanic. He will test them and repair or replace them if the situation demands.
Car Changes
You might want to modify or change the old or existing car for a performance boost or innovative appearance. But it can be another reason to get a confusing speed reading on the odometer.
How? Suppose you have replaced the previous tires with different sizes. But you have not noticed that all the tires are not the same size as earlier.
For instance, when installing bigger tires, the tires’ circumference becomes bigger. It will help to cover more distance per wheel rotation than previous tires. So, it impacts the instrument cluster and shows you the ups and downs of measurements. (source)
Solution
It is good to use same-size tires to get correct readings. Otherwise, replace the speedometer alongside the tires.
Frequently Asked Questions
Some common queries I have found online regarding fluctuating speedometers are:
- Why is my speedometer acting up?
- Check the speedometer cable or sensor if they are working fine. If they malfunction, the speedo of the car will act up.
- Why does the speedometer go back and forth?
- A cable connects it to the odometer in an analog or mechanical speedometer. When it binds, you will get speedo going back and forth. In electric gauges, wheel speed sensors are the reason.
- Can a faulty battery affect a speedometer?
- A faulty or dead battery cannot distribute power evenly to all the electric components. Similarly, it may interrupt the speed signal from the sensor to the speedometer. You will then get the wrong readings.
Final Words
“Why is my speedometer going crazy?” – I guess you have the answer now. Indeed, I have added multiple causes of a fluctuating speedometer.
Mostly, the lousy speedometer cable or wheel sensors, malfunctioning ECU, and wheel sizes make your speedo jump high and low. You should check these things in your car to see if they are OK. Alternatively, take specific measures to sort out the issue.
I hope you have understood everything I have added to this article. I would be happy to answer you. Please, leave any valuable comments or queries regarding the speedometer in the comment section.
Thank you!