What is a speed sensor and how to check it

Speed sensor: features of work, malfunction and replacement

Speed sensor is one of the many automotive sensors responsible for measurement information signal generation, its transmission, further conversion and processing by the electronic control unit and some other devices. If there is no ECU in your car, there is no less need for sensors. The output of even the least important of sensors affects at least how comfortable it will be to operate the car. Since the speed sensor is quite an important component, Avto.pro will help you to understand the peculiarities of its device, will analyze the main faults, and will tell you when it can be replaced even by an inexperienced car enthusiast.

Briefly about speed sensors operation

In general, automotive speed sensors are divided into only two types: – Electronic; – Mechanical.

It is worth starting with the mechanical one, because chronologically it is the one that preceded the more advanced electronic device. At the heart of the mechanical sensor is a group of coupled gears and a small cable. These sensors were installed directly on the speedometer drive mechanisms not far from the gearbox. Mechanical devices are very simple and have a solid service life, but do not give accurate readings in the whole range of speeds and still depend on a number of external conditions. The same cannot be said for electronic speed sensors.

Modern sensors are based on the Hall effect. If you say that they do not work on the frequency-pulse signal, it does not get easier, so we will try to explain everything. So, the sensor forms a so-called pulse signal, the pulse frequency of which has a dependence on the rotation speed of the shaft. The car moves faster – the shaft rotates faster – the sensor produces pulses of higher frequency – the speedometer displays reliable information about the speed change to the higher side. Of course, a person cannot understand the “usual” signal for electronics. For this reason, a controller is introduced into the system which counts the frequency of pulses coming from the speed sensor per unit of time, and then translates this value into human-understandable km/h or mph.

Electronic sensors have been implemented in two ways: – With contact from the shaft; – Without contact.

The first sensors are simply called contact sensors. They use drive gears and a flexible cable (sometimes a short length of rigid shaft). The cable or shaft is used to transmit torque from such a vehicle axle, gearbox shaft, or transfer case. The angular rotation is translated into electrical impulses, which are transmitted further through the system and translated into human-understandable values. These are the sensors that have found the most widespread use in the automotive industry. There are two reasons: they are reliable, and they can be used instead of a mechanical speedometer drive without expensive modifications to the latter.

The increasingly popular non-contact sensors are based on the same Hall effect, but are technically implemented differently than contact sensors. They use one of the auxiliary devices: a rotor or a reference disc. Non-contact sensors make full use of the Hall effect, while less complex contact sensors use the same effect, the magnetoresistive effect or the operation of optrons (optronic pairs).

Read more about contact and non-contact sensors

Non-contact speed sensors are based on the Hall effect already mentioned, so they have no moving parts. The point of the Hall effect is that on a flat wire, through which a direct current is passed on opposite sides, when it is in a magnetic field, a voltage is created on a pair of other opposite sides. In order for the sensors to work, it is necessary to place a pulse disc or special disc on the shaft of the unit, which can be an axle, gearbox or gearbox. These elements have magnetized areas. The pulse signal is generated by the rotor, away from the sensitive part of the car sensor with a Hall chip, begins to rotate. The signal then travels to the controller.

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The Hall-effect contact speed sensors and their magnet chip of the same name are permanently stationary, and the magnetic field is changed by the rotation of a special slotted ring, otherwise known as the shutter. The ring itself is connected to a shaft or flexible drive cable, through which the rotation is transmitted.

Magnetoresistive and optoelectronic sensors

Auto speed sensors can be based on the magnetoresistive effect. In many ways it resembles the Hall effect, but only at first glance. The point is that some materials can quickly change their electrical resistance if they are placed in a magnetic field. What catches your eye when studying such sensors is the chip in which the magnetoresistive element is integrated. It is composed of semiconductor elements. In addition, such a sensor is equipped with a direct drive and a multi-pole magnet.

Optoelectronic contact speed sensors are very simple, but this is their only advantage. The fact is that they are less sensitive (to deviations of the main parameter) and more inertial (have a large lag in measurement) than the above-described sensors. The sensor is operated by an optocoupler, which is a phototransistor and an LED separated by a slotted disk. The latter is fixed on the drive shaft. By rotating the disk and interrupting the light flux between the pair of elements and generates a pulse signal.

How the speed sensor affects the operation of the engine

A special controller is introduced into the speed detection system, which perceives the pulse signal from the sensor. It is the controller that transmits the signal to the electronic control unit, which calculates the amount of fuel needed for optimum engine operation. For example, if the speed of the vehicle is reduced, the amount of fuel to be supplied to the engine is also reduced. This saves fuel considerably and allows the engine to be operated in the most sparing mode.

If the speed sensor is defective, the control unit will not receive a signal corresponding to the real speed of the vehicle. Fuel will be supplied evenly no matter how hard the driver presses the accelerator pedal. In practice, it turns out that the fuel is consumed in excess and the power unit sometimes works with jerks. The statistics shows that the good sensor allows saving about 2 liters of fuel per 100 km of run. One more peculiarity of modern cars with Hall-effect speed sensors: malfunction of electric power steering with a faulty sensor (error P-0501).

Why speed sensor fails

Like almost all electronic components of a car, the sensor suffers first of all from oxidation of contacts and violation of wiring integrity. Often the problem can be detected by visual inspection, though to be sure you should use a tester. Each of the contacts should be disconnected and checked. Oxidized and dirty contacts should be cleaned and coated with a special lubricant. Another weak point of the most widespread systems of speedometer determination is a cable of speedometer. With time it is torn in several places at once that as a result affects the correctness of car’s speed detection. The old cable should be replaced, and before installation it should be coated with machine oil just in case.

Oddly enough, but the most widespread and seemingly reliable speed detection systems do not tolerate driving with sudden speed increases. Because of this, the integrity of the plastic tail in the sensor is quickly broken. The cable also begins to delaminate and crack. As mentioned above, the cable needs to be lubricated. The tightness of the plastic tail should be checked to see if it is tight enough to prevent the socket from coming loose later.

Speed Sensor Failure

If you suspect that the speed sensor installed in the car is not working properly, do not rush to conclusions. First you need to be sure of some of the following: – There are problems at idle (the car periodically stalls, including when coasting); – The speedometer does not work; – The speedometer does not work properly; – Fuel consumption has increased seriously; – The engine loses power during rapid acceleration.

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As mentioned above, the sensor needs to be inspected. The speed sensor is located differently on different cars, but, in general, all automakers try to place it closer to the gearbox. In most cases, to access the sensor, you need to remove the air filter housing – the device is approximately in line with it. It is often located right above the main gear . Usually the sensor is secured with a nut. When the sensor is removed, you need to do the following: 1. Decide on the terminals: grounding, pulse signal and voltage; 2. Connect the contact of voltmeter to the pulse signal terminal, and the second contact to ground the car body or some metal part of the internal engine; 3. Rotate the sensor axis with a piece of plastic. The readings of frequency and rotation on the voltmeter should increase in the course of increasing the speed of rotation of the sensor axis.

This test is only relevant for Hall effect sensors. Something similar can be done without removing the sensor. To do this, you need to suspend the drive wheel on a jack, connect a voltmeter or other measuring device in the same way, then turn the suspended wheel and watch the voltage and frequency readings.

Selecting a new sensor

Replacing a new sensor is not too difficult, but it is important to choose the model that is compatible with your vehicle. For example, sensors may have different lengths depending on what gearbox the car has. For this reason, you can be guided by something from the following: – VIN code; – Car data: model, year of manufacture, type of engine and gearbox; – Code of a suitable device.

In today’s online stores you can search both by code and by car data. Often the search algorithm looks like this: enter vehicle data, get the code and continue searching in other places. We advise car owners to save the codes of suitable devices and their analogs.

In a car with a mechanical sensor, you can put an electronic model of the device. To do this, it is enough to determine the model and year of manufacture of the vehicle device and pass this information to a specialist. Then he will pick up and install a suitable sensor, socket pads, cables and tachograph. It is worth noting that sometimes to install a more modern sensor you need to use a special adapter.

Review by Manufacturers

The diversity of automotive electronics contributes to the emergence of a large number of companies-packers in the market of auto parts. They sell, under their own brand names, sensors from large and small manufacturers. Practice shows that packers prefer to sell inexpensive low-quality products in countries where the purchasing power of vehicle owners is low. For example, JP Group, ERA and SWAG offer inexpensive speed sensors which quality is not always satisfactory. We recommend to pay attention to more expensive variants of the following firms: – Valeo; – Pierburg; – Behr-Hella.

These are the most famous manufacturers, whose sensors are included into original equipment of the overwhelming majority of cars. In fact, a motorist can buy almost a full copy of the original, but at a lower price. When choosing a sensor, it is especially important to be guided by what company produced it. Here you should pay attention first of all to the well-known European, Korean, Japanese and American manufacturers. The so-called first-tier packers can also pleasantly surprise you, but there are not many such packers on the market – their names will be easy to remember.

Installing the new sensor

Given the rather large number of different types of sensors, the removal and installation can vary somewhat depending on the model and year of the car. In some cases, the devices have a cable drive, due to which the cable must also be disconnected, while in others the device looks very simple: a smooth plastic housing with a hole for the fixing element . In general, replacing the speed sensor looks like this: – Turn off the ignition or disconnect the battery terminals; – Find the old sensor and remove it. You will need either a pair of screwdrivers (slotted and Phillips) with pliers, or a 10 or 22 wrench with the same pair of screwdrivers. To unscrew the contact sensor cable, it is better to use pliers; – By prying the unscrewed sensor, remove it. It is recommended to continue cleaning the space around the device. A brake cleaner is especially useful here; – Install the new sensor.

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In fact, the more modern your car is, the easier it will be to find the sensor and the easier it will be to replace it. If you have to work with an old sensor, all actions should be as delicate as possible – the device has something to break.

Conclusion The speed sensor is a very important component of all modern and obsolete cars. If it fails, the vehicle becomes less fuel efficient, and its owner risks not getting real information about how fast he or she is going. Of course, all this is fraught with unpleasant consequences. If you notice that the speedometer does not work properly, it is worth checking both it and the corresponding sensor as early as possible. It is not difficult to find a new device and its replacement can be done in a matter of minutes.

High consumption and irregular revs: what will happen if the speed sensor breaks, and how to fix it?

There is one sensor in a car, breakage of which in some cases is dangerous as a trifle like a broken speedometer, but in other cases leads to really unpleasant consequences. We’re talking about the speed sensor. Not the one that is on the hub and ensures operation of ABS (it is often called ABS sensor), but the one that is somewhere in the gearbox, transfer case or reducer. Why in some cases, its failure is not too dangerous, but in other cases it provides a serious headache for the car owner? It all depends on what type of DSA (car speed sensor) is used and what it is used for in a particular car.

There is contact, there is no contact

All speed sensors can be roughly divided into two large groups. The first group includes rather primitive mechanical devices which are almost obsolete now. Such sensors are just a set of gears, which are usually located somewhere on a gearbox. There is only one part attached to it – a speedometer cable. The cable goes directly to the speedometer. In this case, the DSA is just twisted by the box shaft, the cable is connected to the speedometer mechanism, which shows the speed of the car depending on the speed of the gearbox shaft. It’s very simple and not too accurate. But it is quite reliable.

Such a sensor seldom broke. More often the cable of the speedometer drive failed, not the sensor, but the result of both failures was always the same: the speedometer pointer was helplessly zeroed and did not react to the speed of the car. It was sad, sad, not very convenient, but this breakage had no effect on the parameters of all other systems of the car. As it drove with the sensor, so it drove without it. Only it is not clear at what speed.

You won’t find such a device on new cars. Nowadays electronic sensors of the second group have become popular. In most cases, they do not transmit mechanical rotation from the transmission or gearbox, but a signal about their rotation speed to the electronic control unit (ECU). The ECU counts speed according to its algorithm, sends it to the speedometer and the latter shows it to the driver with an arrow. I say “in most cases” because sometimes the signal can go directly to the speedometer controller, which sends the data to the ECU. But it does not change the essence of the matter. The main thing is that the electronic sensor is able to change the engine operation parameters. And this is why a failure of such a PSA will definitely affect not only the speedometer operation, but also the engine operation. Let’s dwell on it in a little more detail below, but for now, let’s see how modern electronic speed sensors are arranged.

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They can also be divided into two groups: contact and contactless. The first ones are similar to simple mechanical DSAs in the principle of operation. They have a mechanical connection of the gearbox (or transfer case, or gearbox) with the sensor itself. The connection is usually realized by the same simple cable or shaft. But this is where the resemblance ends, because it is not so much the mechanics as the electricity that begins to work next. More precisely, the Hall effect (let me remind you: this is a physical phenomenon based on the fact that when a conductor with a direct current is placed in a magnetic field, an electric voltage arises). As a result, a signal is formed, which can either go straight to the speedometer or first to the ECU. Sensors of this type are so called – contact sensors based on the Hall effect. And this is the most popular design.

In addition to contact sensors based on the Hall effect, there are also non-contact Hall sensors. In this case, the reference (setting) disk can stand on any shaft whose rotational speed is directly proportional to the speed of the car (for example, on the same secondary shaft of the transmission).

At the same time, in addition to the Hall effect, sensors can use other effects. There are sensors based on the magnetoresistive effect or on the optoelectronic effect. In the first case, the sensor has a magnetoresistive element that is able to change its resistance in a magnetic field. There are some common features with the Hall sensor, but the effect used is still different. Optoelectronic sensors are one of the simplest. In them the signal is formed by a pair – LED and phototransistor. Between them on a shaft (or with an associated drive) rotates a disk with slots, the phototransistor “counts” flashes from the LEDs and generates a signal with data about the speed of the shaft for its controller.

Many people have probably already noticed that, from a technical point of view, speed sensors are very similar to, for example, crankshaft position sensors. They use the same effects and essentially solve a similar task: to record rotation and transmit data about its speed to the control unit. Only the DPKV “monitors” the engine, and the DSA monitors the transmission.

What if it breaks?

Now let’s talk about what the speed sensor is used for. Its first function is obvious: the speedometer can’t work without it. But, as I’ve already said, modern sensors are much smarter than old gizmos with a cable. Therefore, they have much more tasks.

Using the signal from the DSA, the control unit can adjust the fuel mixture composition and the ignition advance angle. And it does this not only on the basis of speed data, but also taking into account the acceleration and deceleration of the car. In just one kilometer of road DCA can transmit up to 25,000 signals to the ECU (although usually the number of pulses is more modest – six thousand), so this sensor perfectly notices all changes in speed, not only the speed at a particular time. And this data is constantly used by the ECU for mixture and ignition correction. It is necessary not so much for dynamics, as for fuel economy (though for dynamics too). Therefore, if the signal from DCA is lost, the car can sometimes react to it only with an increase in fuel consumption. But other manifestations of sensor’s death are also possible: uneven idling or sudden jerking or slackening when speeding up. And nobody cancelled the Check Engine lamp, which can be illuminated due to an ACS error.

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Another function of this sensor is to ensure the operation of some systems related to active safety and comfort. Some dynamic stabilization systems use speed data to work adequately, so their errors can also be caused by incorrect or missing data from the DSA. Well, and a little less common are steering faults. In this case, the speed sensor helps to change the force on the steering wheel depending on the speed of the car (at high speed the force on the steering wheel should increase, and on the spot the steering wheel should twist easier, for example for parking). So, the faulty speed sensor can provide the whole bunch of “glitches” of unknown origin.

Of course, there is a question: how to check speed sensor?

A little bit of diagnostics and sadness

Very well, if there is an error which can be read by a scanner. It seems that the culprit of “stupidity” of the car can be found at once. But you should not immediately rush to change this sensor. It fails seldom, but its wire can let you down at any moment. Therefore, you must check it first, especially all connectors (they are the first to get oxidized).

Unfortunately, without experience and an oscilloscope, you can check more or less accurately only one sensor – the one, which is based on the Hall effect. The principle of checking is quite simple: one probe of the voltmeter must be pressed to the “ground” of the car, the second probe to the terminal of the pulse signal. And with the ignition on, make the sensor spin. This can be done in different ways. Some people are more convenient to remove the sensor and twist it with something suitable, others prefer to jack up the wheel and twist it. It’s all up to your desire, how you want it. The main thing is to find the right contacts. If the sensor is working, when you spin it, there will be noticeable voltage spikes. If the voltmeter doesn’t respond to rotation, the sensor is dead. Or you connected it incorrectly to the multimeter, which is also possible.

All other sensors, unfortunately, cannot be checked this way. I will not tell you how to check them with an oscilloscope. If you have an oscilloscope, you already know it, and if you don’t have one, you won’t buy it just to check the DSA. And this is right. So you’ll have to accept it and have it checked at the service.

I’m not going to give you advice on replacing the speed sensor. They are very different, and it all depends on the car. Somewhere it’s easy to remove, and somewhere you’ll have to struggle. So in any case, you have to study the manual of your own car. And there’s only one piece of advice: don’t try to look for similar sensors. You need to use the one that comes from the factory. Even if the sensor is replaced by a native one as it should be and the car doesn’t mind the “kolhozny” DSA, because of some other characteristics, it may show absolutely different things than the factory sensor should show. And the car will hardly go better after such replacement. Of course, to find a good analogue from a well known manufacturer is a sacred thing, but you should not just set any random device. It will cost you more.

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