How does a speed sensor work

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How does a speed sensor work

This page is for personal, non-commercial use. Transmission speed sensors are used to calculate the actual gear ratio of the transmission while in use. There are generally two speed sensors that work in conjunction to provide accurate transmission data to the vehicle's powertrain control module. The first is known as the input shaft speed ISS sensor. As described, this sensor is used to monitor the speed of the transmission' input shaft.

The other sensor is the output shaft speed OSS sensor. If either of these two sensors falls out of alignment or experience electrical issues, it impacts the operation of the entire transmission. After registering data, the two transmission speed sensors, also commonly referred to as vehicle speed sensors VSSsend data to the powertrain control module PCMwhich compares these two inputs and calculates which gear the transmission should engage for efficient driving.

The actual gear ratio is then compared to the desired gear ratio. Should either or both of these speed sensors fail, you may notice one or more of the following issues:.

Without a valid speed signal from these sensors, the PCM will not be able to correctly control the shifting of gears within the transmission. This may cause the transmission to shift roughly or more quickly than normal. It's also common that a problem with these sensors can impact the shift timing, extending the interval between transmission shifts.

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An automatic transmission is hydraulically controlled and designed to shift smoothly. When the transmission shifts hard, it can damage internal components including valve bodies, hydraulic lines, and in some cases mechanical gears. If you notice that your transmission is shifting harshly or roughly, you should contact a mechanic as soon as possible. Since the transmission speed sensors monitor the input and output shaft speed, it also plays a part in monitoring the cruise control.

When the sensors fail to deliver accurate data to the onboard computer on your car, truck, or SUV, the powertrain control module PCM will send an error code to the vehicle's ECU. As a safety precaution, the ECU will shut down the cruise control and render it inactive. If you notice that your cruise control does not engage when you press the button, contact a mechanic so they can inspect the vehicle and determine why the cruise control is not working.

It may be due to faulty transmission speed sensors. It also can indicate that there is an increase in tailpipe emissions that surpasses acceptable limits for environmental contaminants from motor vehicles. In any case, if you notice that the Check Engine Light comes on, you should contact a mechanic to scan the error codes and determine why the Check Engine Light is illuminated. Once the problem has been fixed, the mechanic will reset the error codes.

If the problem rests with the speed sensors, depending on your particular transmission, a mechanic may be able to replace the sensor. Some speed sensors are internal to the transmission and will require the transmission to be removed from the vehicle before the sensors can be replaced.

This article originally appeared on YourMechanic. Autoblog is partnering with YourMechanic to bring many of the repair and maintenance services you need right to you. Get service at your home or office 7 days a week with fair and transparent pricing. We get it. Ads can be annoying. But ads are also how we keep the garage doors open and the lights on here at Autoblog - and keep our stories free for you and for everyone.

And free is good, right?New cars contain an engine control unit and lots of different electronic sensors which communicate with it. The transmission speed sensor also referred to as the engine speed sensor is one of these sensors and its job is to calculate the speed at which the wheels are rotating. This ultimately determines the speed at which the vehicle is traveling. Related: Symptoms of a Faulty Speed Sensor. When we look at the speedometer on the dashboard, the speed sensor is what helps determine the current miles per hour.

Meanwhile, the speed sensor sends this information to the engine control unit so that it can regulate other functions of the vehicle.

Some of these regulated functions include ignition timing, transmission shift points, and the air to fuel ratio in the internal combustion chamber. Engine speed sensors are commonly referred to as transmission speed sensors because they are connected to the transmission.

When the transmission is being used, the speed sensors will detect its gear ratio. You will actually find two speed sensors in the transmission of each new vehicle out today. One speed sensor is called the input shaft speed sensor and the other is the output shaft speed sensor. These two sensors work together to relay information back to the engine control unit. The clutch connects the input shaft to the engine which allows it to turn at the same speed.

This is where the output shaft comes into play because it is linked to the drive wheels and changes speed based on the gear you select. The output shaft speed sensor detects the speed of the drive wheels and sends this information back to the engine control unit.

Together these two shafts allow the wheels to rotate at a speed that is manageable while the engine continues to run at a speed that makes it perform efficiently. This would not be possible without functional input and output shaft speed sensors. Once the engine control unit receives this information, it compares the two different speeds of the shafts and then automatically sets the appropriate gear that is needed to sustain a smooth driving experience.

For this reason, automatic transmission vehicles do not require the driver to change gears because that process is done for them. This makes driving so much easier in comparison to manual transmission vehicles where the driver changes the gears themselves.

Share Pin Tweet 3 shares. Your best bet is to check the factory service manual for your specific vehicle. Facebook Pinterest Twitter.A vehicle speed sensor is often not something people consider when thinking about their cars.

They aren't something that a lot of car owners even know about. Below is a snapshot of what types of vehicle speed sensors there are, and what they do. This speed sensor indicates the speed at which the car is actually traveling.

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It is used to control certain handling systems, as well as certain braking systems. It is controlled by a signal generator that is spun, and makes an electrical pulse that is then sent to the vehicle's computer.

Often times, it will be used to control transmission shift points in addition to the systems mentioned above. There are four sensors that are at the wheel locations.

The axle shaft will have a notched wheel attached to it that will turn the inside of the wheel sensor. It then gives it a signal that goes to the ABS computer telling the speed of each wheel separately. That information will then in turn engage and even disengage the brakes as necessary to prevent any skidding, etc. We welcome your comments and suggestions. All information is provided "AS IS.

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How a Vehicle Speed Sensor Works. Written by Teresa Agresto. To ensure our content is always up-to-date with current information, best practices, and professional advice, articles are routinely reviewed by industry experts with years of hands-on experience. Vehicle Repairs. My Chevy Suburban K has the push button switching for 2wd, auto-4,A wheel speed sensor or vehicle speed sensor VSS is a type of tachometer.

It is a sender device used for reading the speed of a vehicle's wheel rotation. It usually consists of a toothed ring and pickup. The wheel speed sensor was initially used to replace the mechanical linkage from the wheels to the speedometereliminating cable breakage and simplifying the gauge construction by eliminating moving parts. These sensors also produce data that allows automated driving aids like ABS to function. The most common wheel speed sensor system consists of a ferromagnetic toothed reluctor ring tone wheel and a sensor which can be passive or active.

The tone wheel is typically made of steel and may be an open-air design, or sealed as in the case of unitized bearing assemblies. Greater numbers of teeth will require more machining operations and in the case of passive sensors produce a higher frequency output signal which may not be as easily interpreted at the receiving end, but give a better resolution and higher signal update rate.

Symptoms of a Bad or Failing Transmission Speed Sensor

In more advanced systems, the teeth can be asymmetrically shaped to allow the sensor to distinguish between forward and reverse rotation of the wheel. A passive sensor typically consists of a ferromagnetic rod which is oriented to project radially from the tone wheel with a permanent magnet at the opposite end.

how does a speed sensor work

The rod is wound with fine wire which experiences an induced alternating voltage as the tone wheel rotates, as the teeth interfere with the magnetic field. Passive sensors output a sinusoidal signal which grows in magnitude and frequency with wheel speed.

A variation of the passive sensor does not have a magnet backing it, but rather a tone wheel which consists of alternating magnetic poles produce the alternating voltage. The output of this sensor tends to resemble a square waverather than a sinusoid, but still increases in magnitude as wheels speed increases. An active sensor is a passive sensor with signal conditioning circuitry built into the device. This signal conditioning may be amplifying the signal's magnitude; changing the signal's form to PWMsquare wave, or others; or encoding the value into a communication protocol such as CAN before transmission.

The vehicle speed sensor VSS may be, but is not always, a true wheel speed sensor. For example, in the Ford AOD transmission, the VSS is mounted to the tailshaft extension housing and is a self-contained tone ring and sensor. Though this does not give wheel speed as each wheel in an axle with a differential is able to turn at differing speeds, and neither is solely dependent on the driveshaft for its final speedunder typical driving conditions this is close enough to provide the speedometer signal, and was used for the rear wheel ABS systems on and newer Ford F-Seriesthe first pickups with ABS.

Wheel speed sensors are in anti-lock braking systems. Many of the subsystems in a rail vehicle, such as a locomotive or multiple unitdepend on a reliable and precise rotary speed signal, in some cases as a measure of the speed or changes in the speed.

This applies in particular to traction controlbut also to wheel slide protectionregistration, train control, door control and so on. These tasks are performed by a number of rotary speed sensors that may be found in various parts of the vehicle.

Speed sensor failures are frequent, and are mainly due to the extremely harsh operating conditions encountered in rail vehicles.

how does a speed sensor work

Although rail vehicles occasionally do use drives without sensors, most need a rotary speed sensor for their regulator system. The most common type is a two-channel sensor that scans a toothed wheel on the motor shaft or gearbox which may be dedicated to this purpose or may be already present in the drive system.

The form of the teeth is of secondary importance; target wheels with involute or rectangular toothing can be scanned. Depending on the diameter and teeth of the wheel it is possible to get between 60 and pulses per revolution, which is sufficient for drives of lower and medium traction performance.

This type of sensor normally consists of two hall effect sensorsa rare earth magnet and appropriate evaluation electronics. The field of the magnet is modulated by the passing target teeth.

This modulation is registered by the Hall sensors, converted by a comparator stage to a square wave signal and amplified in a driver stage. Unfortunately, the Hall effect varies greatly with temperature.

This also very much reduces the maximum permissible air gap between the sensor and the target wheel. In this case the maximum possible air gap is only 0.The transmission speed sensors are used to calculate the actual gear ratio of the transmission while in use.

How Does an ABS Speed Sensor Work?

The first is known as the input shaft speed ISS sensor. The other sensor is the output shaft speed OSS sensor.

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If either of these two sensors falls out of alignment or experiences electrical issues, it impacts the operation of the entire transmission. After registering data, the two transmission speed sensors, also commonly referred to as a vehicle speed sensors VSSsend data to the powertrain control module PCMwhich compares these two inputs and calculates which gear the transmission should engage for efficient driving.

The actual gear ratio is then compared to the desired gear ratio. Should either or both of these speed sensors failyou may notice one or more of the following 3 issues:. Without a valid speed signal from these sensors, the PCM will not be able to correctly control the shifting of gears within the transmission. This may cause the transmission to shift roughly or more quickly than normal.

It's also common that a problem with these sensors can impact the shift timing, extending the interval between transmission shifts. An automatic transmission is hydraulically controlled and designed to shift smoothly. When the transmission shifts hard, it can damage internal components including valve bodies, hydraulic lines, and in some cases mechanical gears. If you notice your transmission is shifting harshly or roughlyyou should contact a local ASE certified mechanic as soon as possible.

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Since the transmission speed sensors monitor the input and output shaft speed, it also plays a part in monitoring the cruise control.

As a safety precaution, the ECU will shut down the cruise control and render it inactive. If you notice that your cruise control does not engage when you press the button, contact a mechanic so they can inspect the vehicle and determine why the cruise control is not working. It may be due to faulty transmission speed sensors. It also can indicate an increase in tailpipe emissions surpassing acceptable limits for environmental contaminants from motor vehicles. In any case, if you notice that the Check Engine Light comes onyou should contact a local mechanic to scan the error codes and determine why the Check Engine Light is illuminated.

Once the problem has been fixed, the mechanic will reset the error codes. If the problem rests with the speed sensors, depending on your particular transmission, the professional ASE certified mechanics may be able to replace the sensor. Some speed sensors are internal to the transmission and will require the transmission to be removed from the vehicle before the sensors can be replaced. The most popular service booked by readers of this article is Vehicle Drivetrain Inspection.

how does a speed sensor work

Our certified mobile mechanics perform over services, including diagnostics, brakes, oil changes, scheduled mileage maintenances, and will come to you with all necessary parts and tools. Our certified mobile mechanics make house calls in over 2, U. Fast, free online quotes for your car repair.The sensor works using a iron cored coil with a magnet attached to one end.

When a peice of ferrous metal is moved towards the end of the sensor it changes the shape of the magnetic field in the coil, this changing magnetic field then induces current to flow in the windings of the coil resulting in a small amount of electricity being generated. This sensor only detects movement of ferrus objects near the sensor so is typically used for speed sensing for example a wheel sensor in an ABS system usually in combination with a toothed steel wheel.

They are very rugged sensors and are not affected by dirt and have a very high signal output making them less sensitive to noise ideal for automotive applications For example the sensor I have made will generate a sine wave of 50Vp-p into a 1Kohm load that is a peak of 50mV.

Wheel speed sensor

A large nut and bolt 2. A relay I used a miniature 12V relay, but others should work 3. A broken mobile phone 4. A bit of 2 core wire twisted pair will help with noise immunity, but is not necessary.

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Rip apart the mobile phone and recover the earpiece, rip this apart and you will be left with a small but strong magnet. Now dissasemble the relay very carfully it is very easy to break the coil wires until you are left with just the coil and it's iron core.

Place the magnet on the end of the core of the coil, there is usually a 'head' to the core a bit like a nail - stick the magnet on this end. Make sure the magnet is at the same end as the connections for the coil. Now drill the centre out of the bolt and fit the sensor inside using plenty of hotmelt glue or epoxy. You should end up with the very tip of the iron core of the coil showing at one end as in the photo. Make sure the connections inside don't touch the case, prehaps a peice of plastic tube might be usefull as an insulator.

Here I have a test rig lashed up on my bench - a gear from my lathe mounted in the chuck of an electric drill with a 'G' clamp on the trigger. The sensor is then connected to my scope. Here you see the sensor working, the closer you can get the sensor to the moving teeth, the larger the output.

Here the scope is set to 5V per division so it is outputting about 40Vp-p Although this is with no load as I was in too much of a rush this morning and forgot to put on a load resistor - If I had used a 1K resistor the output would have probably dropped to about 5V If you can be very patient I will post some circuits in the future to use this in real life, I.

Question 7 months ago. How similar is this to the pickup coil in an ATV ignition system? It looks like the same principle I thought I'd seen this technology used in drag racing. They used a sensor like that to read driveshaft speed which in turn helped ID tire spin. It's traction control when wired to retard engine timing for fractions of a second. I haven't heard much recently or seen it up close. Anything interesting there for you?

Sadly, my hotrod doesn't have the problem of removing power to reduce out of control tire spin. Not yet anyhow. What sort of landy you have? Mine is an old range rover with a faulty speedo, and I'd be very glad to install an alternative to know what's my current speed. I'd be glad to see continuation of your post - have you installed your sensor? Reply 8 years ago on Introduction. Sadly I had to part with my Landrover before I got any further with the project. My first child turned up - now just turning 4 on Monday!

You will have to take on the mantle and post an instructable showing the sensor in action. Reply 4 years ago.Anti-lock braking systems ABS are common on newer model cars. ABS controls the braking ability of the car under select driving conditions that result in a loss of traction. This system features a controller, valves, and speed sensor that work in conjunction to determine the safe braking ability of your car.

ABS speed sensors monitor the rotation of each tire, making sure that each wheel is rotating properly. Any slippage or difference between the wheels triggers the ABS system. ABS begins with sensors deciding if the wheels are losing traction. Once this decision is made, the controller like a computer engages a valve system in the vehicle that controls available brake fluids in the lines.

Since brakes work based on pressure generated by fluid compressed in the brake lines, a pump engages in the braking system to rapidly apply the brakes. This application occurs faster than any human can apply the brakes. Many drivers feel a pulsing feeling in the brake pedal or hear a grinding noise when the ABS engages on their vehicle. ABS speed sensors are located at the wheel hubs of the vehicle. These sensors constantly monitor the rotation of each wheel to evaluate whether the ABS system needs to override manual braking of the vehicle in slippery conditions.

The speed sensor calculates revolutions as well as evaluates continuity between all wheels. Any detection of differences in rotation can result in ABS engaging to control braking.

Speed sensors are comprised of a magnet wrapped in a coil and a toothed sensor ring mounted around the CV joint hub. An electrical field given off by the contact between the magnet and the toothed ring creates a signal.

The composition of the signal includes measurements of the number of pulses per second created by the electrical field existing between the magnet coil and sensor ring. This signal converts to a digital signal and is transmitted to the ABS controller. Speed sensors convert the pulses created by the contact between the magnet and sensor ring through a coil attached to the sensor. This voltage is passed along to the controller. The controller counts the number of pulses to determine wheel speed and evaluates whether the ABS system should intercede to control braking.

Evaluating Speed ABS speed sensors are located at the wheel hubs of the vehicle. Generating the Speed Sensor Signal Speed sensors are comprised of a magnet wrapped in a coil and a toothed sensor ring mounted around the CV joint hub. Sending Messages to the ABS Controller Speed sensors convert the pulses created by the contact between the magnet and sensor ring through a coil attached to the sensor.


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