A speedometer measures and displays the speed of a car, while the odometer measures the distance that the vehicle has traveled. As a car owner, you may be wondering what is wrong when your car’s speedometer stops working, but the odometer still works. You are in luck because we have researched this topic for you, and here is what we found.
A failure in some components can cause the speedometer to stop working and remain at 0 MPH, while the odometer is still working. The following are possible causes of the problem:
- Defective speed sensor.
- Broken gears.
- Damaged cable/wiring.
- Faulty engine control unit.
Poor car maintenance culture can result in these problems. Read on for more information on problems that can cause your car's speedometer to malfunction while the odometer is still working and how to fix them.
How Do You Read A Speedometer?
The speedometer can be found at the right side of the instrument cluster on the dashboard. It has two semicircles, each with a set of numbers; while the larger semi-circle shows the speed in miles per hour (mph), the smaller one shows the speed in kilometers per hour (km/h).
What Could Be Wrong If My Speedometer Doesn't Work But My Odometer Does?
Whenever you notice that your speedometer is not working but your odometer is working, you should check some of your vehicle's components to find the source of the problem. One of the following could be the cause of the problem:
Defective Speed Sensor
The Vehicle Speed Sensor (VSS) is responsible for measuring the output of the transmission or the wheel speed. This information is in turn used by the engine control unit in modifying the unit’s functions, such as the timing of the ignition, the ratio of the air and fuel, transmission shift points, and diagnostic routines initiation.
A faulty speed sensor will result in the inability to measure the tire speed which then affects the information given to the engine control unit such that the speedometer does not function, rather, it stays in place at 0 MPH, while the odometer is working. This problem is caused by poor maintenance of the vehicle.
The speedometer has a gear that connects to the vehicle’s transmission output and turns the needle on the dashboard. It is however important to note that it is the mechanical speedometer that makes use of gear, as the digital speedometer does not use gears.
The gears on the speedometer estimate the speed of the vehicle based on the rotation of the car tires. Speedometer gears are specific to different tire sizes, such that a change in the size of tires should consequently bring about a replacement of the speedometer gear to ensure accurate readings.
When the gear rotates, it determines the rotational speed according to the number of teeth it has. Having defective speedometer drive gear will result in a malfunctioning speedometer and the odometer may still be working.
A flexible cable consisting of twisted wires is part of the components that make up the speedometer, as there is a connection between the shaft that spins the car’s tires and the speedometer that records the rotation of the tires. The cable is long and can bend, yet it functions like a mini driveshaft.
Thus, as one end of the cable rotates, the other end also rotates. The cable feeds back to the speedometer at the top end. When the wiring is damaged, it will produce false reports to the speedometer, resulting in the speedometer not working while the odometer is working.
Faulty Engine Control Unit
A car's electronic control unit (ECU) is responsible for controlling an internal combustion engine in a series of moving engines and control systems to ensure optimum engine performance.
Prior to the inclusion of ECUs in the manufacturing of vehicles, the mixture of air and fuel, the timing of the ignition, and idle speed were all regulated mechanically and controlled by mechanical and pneumatic intermediates.
The ECU works by reading values from diverse sensors in the engine compartment, interpreting the data using a multidimensional performance map, and adjusting the machines in the moving engines and control system. Most engine systems now have a built-in idle speed measurement in the ECU.
The sensor for the crankshaft position is a key component in the timing functions of the injection of fuel, spark events, and valve timing. It monitors the engine rotational speed. A stepper motor with programmable throttle stop or idle air-bypass control is responsible for controlling the idle speed.
Effective control of the idle speed will determine the engine load at idle. However, when the ECU goes bad, it becomes unreliable, thereby causing the speedometer not to work even though the odometer is working.
What Should I Do If My Speedometer Is Not Working But The Odometer Is?
Whenever you notice that your speedometer is not working but your odometer is working, it is recommended to have your car checked immediately. A way of fixing a speedometer that is not working is by replacing the component that is faulty or damaged.
This repair can be performed by someone who's familiar with cars or by a professional.
How Can You Tell If Your Speedometer Is Broken?
A broken speedometer will either stay at 0 MPH, move randomly, or will not read the correct speed. Also, the check engine light coming on and going off, and the overdrive light turning on and off for no reason are signs that you may have a bad speedometer.
One factor that can affect the correct speedometer reading is the change in tire size without a corresponding change in the speedometer drive gear.
You can check the accuracy of your speedometer with a stopwatch. Start the watch as you pass a mile marker while on the highway, and then stop it once you pass the next marker. The second hand of the stopwatch is your speed.
Can I Still Drive My Car If The Speedometer Doesn't Work?
When the speedometer does not work, it is not possible to monitor your instant speed level while driving because it will be stuck at 0 MPH.
You can still drive a car with a bad speedometer. Although this should not cause further damage to the car, you risk being stopped or fined for exceeding the designated speed limit in certain areas. It is recommended that any problems be resolved as soon as possible upon detection.
Does The ABS Sensor Affect The Speedometer?
A vehicle’s anti-lock braking system (ABS) comprises speed sensors, which transmit information about the vehicle’s speed to the car’s ABS computer system. At first, a faulty ABS may only cause the ABS warning light to come on, but in rare cases, a faulty ABS sensor will cause the speedometer to stop registering.
How Do I Know If I Have A Bad Speed Sensor?
Not only does the speed sensor indicate the vehicle speed, but it also helps with the ignition timing, cruise control operation, and regulation of the fuel flow. A bad sensor will result in a speedometer that is not working and disrupt the driving functions of the car as it affects the cruise control.
It is good to note that the check engine light will come on if there is a problem with the sensor. The first step to reaching the root of the problem is by making use of a code scanner to examine the error codes.
If you have a bad speed sensor, locate the sensors and check them along with the wires connecting to them for any visible damage. If you find a defect, immediately replace it with a new unit.
Can A Speed Sensor Be Cleaned?
Since the sensors are magnetic, they attract dust from the road and debris generated by constant sharp braking. To avoid damaging the ABS, the sensors should be cleaned regularly.
However, when the sensors get bad and go out of calibration due to mechanical damage or wear, or fails due to thermal damage, they should be replaced. Although replacement is the best option, you can seek the help of a professional to determine if the sensor can be cleaned.
How Much Does It Cost To Get A Speedometer Sensor Fixed?
The cost of getting a speedometer fixed depends on three main factors: the type of car, the cause of the problem and the auto repair shop you are visiting. However, the estimated cost of getting your speedometer fixed ranges from $144 to $400. This is for either repairing or replacing the damaged speedometer.
To Wrap Up
When a speedometer stays at 0 MPH or acts erratically, it may be going bad. When your speedometer is not working while your odometer is working, you should check for a defective speed sensor, broken gears on the speedometer, damaged cable/wiring, and a faulty engine control unit.
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