You are cruising down the road on the way to work when all of a sudden you hear a siren. You pull over and the officer tells you that you were going ten miles over the speed limit. You explain that the speedometer told a different story, but the cop won’t buy it and they write you a ticket.
If this has happened to you, it may be due to a faulty speed sensor. Unless you like paying tickets, you will want to have it replaced.
How does a speed sensor work?
A vehicle speed sensor, which is also known as a transmission speed sensor, is a tachometer that measures the revolution of the wheels. It does this by sensing the revolutions of a toothed wheel that is on a shaft inside the transmission.
The component is activated when the transmission shaft is turned. there is a sprocket that is attached to the output shaft of the transmission or the engine’s crankshaft.
Once the shaft rotates, the sprocket that is connected to it turns as well. The wheel has open space on some of its teeth. The sensor detects the signals from each tooth of the wheel as it passes by the sensor.
The sensor uses the open teeth to determine when a full rotation has been made. Once that rotation has been made, the data is sent to the engine control unit to calculate the speed. The data is then displayed on the speedometer.
The speed sensor was first used to replace the link from the road wheels to the speedometer, it made cable breakage a thing of the past and simplified the way the gage worked. Where old fashioned speedometers had many moving parts, speed sensors only have a needle spring component.
Most speed sensors are made up of a steel toned wheel and a sensor. The sensors can be active or passive. A passive speed sensor makes an Alternating Current (AC) signal that adjusts its frequency with the speed of the wheel. The wheel must revolve at a rate that is fast enough to make the AC signal. Hence, they are not able to read a signal that is under 2 MPH.
As the passive wheel sensors have trouble reading signals at low speeds, many auto manufactures opt for an active speed sensor. An active speed sensor takes its electromotive force from the ABS module. The module ranges from 8 volts to 10 volts and it changes them by 0.6 to 1.2 volts.
Problems Caused by a Bad Sensor
A speed sensor that is not working can have an impact on more than the speedometer reading. The engine and transmission control systems are both dependent on the data provided by the sensor to make the modifications your automobile needs to run smoothly. Systems such as cruise control may be dependent upon the speed sensor as well.
When a vehicle is on a lift, a mechanic can actually see speed sensor data. This can be quite helpful in determining what is wrong with the sensor. Many accidents are caused by cars going too slow or too fast. Hence, it is important for a driver to know when they might have a bad speed sensor.
Fluctuating Speedometer Readings
The most obvious problem that can arise with a bad speed sensor is a false read on a speedometer. There are some situations in which there will be no reading whatsoever on the speedometer.
When you are driving down the highway, you are expected to keep up with the general flow of traffic. If you are driving in heavy traffic, it will be fairly easy to observe if you are traveling at the same speed as the other people on the road.
If you find yourself alone or with only a few other cars on a stretch of highway, it is hard to gauge how fast you are going without a working speedometer. This can be very dangerous as you may end up driving considerably over the speed limit.
Not only can driving with a nonworking speed sensor be dangerous, but it can also be very expensive. If you are pulled over, you may not only get a ticket, you may be charged with operating an unsafe vehicle.
Issues with the Transmission
The transmission control module of a car uses information from the VSS to determine shift timing. A non-working speed sensor will have an adversary effect on your transmission.
If you have an automatic transmission and you are having problems with it, you should know that there is a really good chance that the problem is being caused by a faulty speed sensor. When the VSS is not working correctly, the transmission may show signs such as hard shifts, and limited gear operation, and lagging shifts.
You Cannot Use Cruise Control
If your sensor is not working and does not send information to the cruise control, the cruise control will be automatically disabled. Cruise control systems require the data from the VSS to maintain a consistent speed.
The Check Engine Light Illuminates
Whenever anything is wrong with your engine, the check engine light will generally come on. The speed sensor is no exception to this rule. When this happens, You should connect your vehicle to an onboard diagnostic scan tool to determine if the error code is indicating a speed sensor malfunction.
The OBD scan tool should be plugged into a diagnostic connector. You should find this near the steering column under the instrument panel.
There are also other lights that may come on when something is wrong with the speed sensor. You may see the automatic brake system light or the traction control system light come on.
The torque converter clutch application won’t work
A torque converter functions as the clutch in automatic vehicles. It is a large donut-shaped component that is one of the most complex parts of your car. The engine of a manual vehicle is attached to the transmission via a clutch. Automatic vehicles have a torque converter instead. An automatic vehicle uses fluid to transfer power into the transmission.
The speed at which the torque converter will be activated is preprogrammed into its system, Therefore if the speed sensor doesn’t work, the torque converter won’t be able to apply the clutch. The engine will not engage with the transmission and this can cause the transmission to get overheated. You may also notice that your gas mileage is not as good as it used to be.
It is never a good idea to drive when any of the car’s parts are not working. You can probably drive your car to the mechanic with a bad sensor, but it would be a very bad idea to drive it any further than that. A bad sensor can wreak havoc with multiple parts of your car. You should be very careful when you shift if you decide to drive with a bad sensor.
Causes of Speed Sensor Failure
As with all engine problems, poor maintenance is one of the major things that can cause a speed sensor not to work. If the component becomes dirty, it will not be able to get the correct readings. Old transmission fluid is especially damaging to a speed sensor.
Cracks in the speed sensor itself may also cause it to become damaged. Some of the sensors are plastic and plastic does not hold up very well to the elements. Things such as extremely hot or cold temperatures may damage the outside of the sensor.
The sensor can also experience electrical problems. There is a lot of electrical wiring in the sensor and that can prevent the sensor from working or cause it to work erratically.
You can employ a voltmeter to see if your engine is working properly. There are a few simple steps you can follow to use the meter.
- Raise the car with two jacks and get under it.
- Disconnect the wire from the sensor and from where it attaches to the frame.
- Turn the ignition on, but do not start the engine.
- Put the probes of your voltmeter into the reference wires that you will find in the connector of the sensor. If there is no voltage, the sensor may not be working right.
Cost of Replacing a Speed Sensor
A speed sensor itself is not expensive at all. You can obtain one for as little as $7.92 or as much as $100. However, you may spend anywhere from $100 to $250 on labor. If you know a fair amount about cars, you can attempt to replace the component yourself. If you are not comfortable working on your own car, you will have a few options for having a professional work on it.
There are plenty of folks who fix cars in their spare time out of their garage. Although a speed sensor is relatively easy to replace, you may want to think twice before taking your vehicle to such a mechanic. If they install the device improperly, you may have a hard time getting them to correct their work. It also may take a long time to get the work done if you are relying on someone who has another part-time job.
Auto body Shops
A mechanic in an auto body shop will normally be a trained professional who looks upon their job as a career. You will have a chance to talk to them personally before they begin work on your car. They are much less expensive than the dealership’s mechanic.
Before you decide to take your car to the autobody shop, you should ask how long they have been in business. You should ask if they have ever worked on a car like yours before. You should also check out AAA’s website before selecting an auto body shop. They offer a full list of approved mechanics.
You can take your car to the dealership for repairs, but you should be aware that this is the most expensive option. A dealership may charge plenty for their services, however, you should know that their mechanics will definitely be trained in working on your make and model of car. They will charge by the job and not by the hour, which ensures that you will not be shocked by the final price.
Finding a New Car and Selling the Old One
A speed sensor should last a long time and when one fails, you just may be ready for a new car. If you decide to get a new vehicle you will have a few options for unloading your old ride.
You can try to sell your car online. There are more than a few websites out there that will let you list your car for a monthly fee. You will be able to post several pictures of the vehicle along with its specs.
It can take a long time to get an interested buyer, You will then have to show them the car. They may decide not to buy and you will have to continue to try to sell the vehicle.
You can use your old clunker as a trade-in. You should know that a dealer is unlikely to give you a very big allowance. They tend to think they have you where they want you when you come in for a new car. They may rely on you taking less money for your trade in to avoid the hassle of selling your car elsewhere.
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