Your speedometer is supposed to be an accurate measure of how fast your vehicle is going.
That’s why, if you are going at erratic speeds, it will also follow that your speedometer should also be acting in an erratic way as well.
However, what happens if your speedometer is shaking up and down even if you are driving at steady speeds?
Why is your speedometer shaking or acting that way?
The reason why your speedometer might be shaking or fluctuating can be due to a problem with the cable sending the signals from your sensor to the ECU. Or it could be a problem with the sensor itself such as when it struggles to detect the moving gear or the magnet in the transmission.
Your speedometer should never act in an erratic way such as when it fluctuates rapidly.
That’s why, if your speedometer is seemingly erratic in the sense that it moves up and down in a manner that is far from your vehicle’s actual speed, then there should be a problem when it comes to the speedometer’s sensors or the cable that sends the signal from the sensor all the way to the car’s ECU.
Is it normal that my speedometer goes up and down?
When it comes to your speedometer, one of the things you should know about it is that it should be able to accurately reflect how fast your vehicle is going.
After all, that’s the job of the speedometer. However, even if that’s the basic function of the speedometer, the fact of the matter is that it shouldn’t be moving in such a way that doesn’t reflect the way your car is moving.
So, let’s look at it this way. When you are driving your car, your vehicle won’t go from 45 to 90 in a hurry without even accelerating steadily from 45 until it reaches 90 a few seconds later.
That is basically what the laws of physics are all about as an object that is picking up speed should accelerate at a steady pace before it reaches the desired speed. This should be something that your car’s speedometer should reflect as well.
Basically speaking, when you are accelerating from 0 to 50, the speedometer should also reflect that acceleration in the sense that it starts from zero and steadily reaches 50 or higher than that if you do intend to drive at higher speeds.
So, in a sense, the needle on the speedometer or the digits on the digital speedometer should also incrementally move or increase similar to how your vehicle’s speed is incrementally increasing as well.
All that said, the point here is that your speedometer should not shake so much and fluctuate from one speed to the next such as when it is showing 50 now but would suddenly become 100 just half a second later even though you didn’t even accelerate to 100.
This is not normal behavior for a speedometer because this is not an accurate way of reflecting how fast your vehicle is moving or accelerating.
Why is my speedometer shaking?
So, now that you know that your speedometer should not be shaking because it isn’t normal for the speedometer’s needle to be fluctuating in an erratic manner, you must be wondering why this is happening.
For you to understand why this is happening, you should first understand how modern-day speedometers work.
Today’s speedometers work using sensors that send signals to the car’s computer or the engine control unit or ECU.
The sensors, either magnetic or optical, are located in the car’s transmission.
Meanwhile, a gear or a magnet is connected to the drive shaft such that, when the car is moving, the gear or magnet is moving in accordance with the speed of the vehicle.
Every time the gear or magnet passes by the sensor, the sensor will pick the disruption up and will send the signal in pulses to the ECU using a wire.
As the ECU receives the pulses, it will then calculate how fast the car is moving based on the time difference between each pulse sent.
From there, the speedometer would be able to reflect the car’s accurate speed.
That said, if your speedometer is shaking or fluctuating, there should be a problem with the input part of the speedometer such as when the sensors are not working properly.
And if the sensors are actually working properly, then the problem might be in the wire that is sending the signal over to the ECU.
More on which sensor is used in Speedometer can be found here.
So, if the sensor is the problem such as when it is dirty or if something is blocking it, it will only follow that its ability to pick up the motion of the gear or magnet will be disrupted. As such, the pulses that it will end up sending to the ECU will be shaky or erratic due to the very fact that it wasn’t picking up the motion of the gear or magnet properly.
Meanwhile, there could also be a problem with the wire that sends the signal in the sense that it might have already deteriorated to the point where it can still send signals to the ECU but such signals end up getting cut off midway due to how the wire no longer functions properly.
This means that the signal that the ECU picks up will just become as erratic.
As such, it will follow that the speedometer on your car’s dashboard will begin to shake and fluctuate.
How do I fix my speedometer shaking?
The truth of the matter here is that, when there is a problem with your speedometer such as when the needle is shaking or fluctuating, then there is nothing that you yourself can do unless you are a trained mechanic.
Your best bet here is to take your car to a mechanic to have the speedometer sensor and wire checked.
The mechanic might be able to clean the sensor in the transmission or opt for an overall replacement that includes a new sensor and a new wire connected to the ECU.
However, there are some cases when the sensor or the cable are not damaged but would just require a bit of lubricating.
But this is something that only a qualified mechanic should be able to do, and that is why you shouldn’t do this yourself.
Always take the mechanic route instead of trying to DIY your way through something that you weren’t trained for.
Autoblog: Symptoms of a bad speedometer
Honda-Tech: Speedometer shaking