speed sensor failure

speed sensor failure usually leads to incorrect operation of the speedometer (the arrow jumps), but other troubles can happen depending on the car. namely, there may be failures in gear shifting if the automatic transmission is installed, and not the mechanics, the odometer does not work, the ABS system or the internal combustion engine traction control system (if any) will be forcibly disabled. In addition, on injection cars, errors with codes p0500 and p0503 often appear along the way.

If the speed sensor fails, it is hardly possible to repair it, so it is simply replaced with a new one. However, what to produce in such a situation is also worth finding out by doing a few checks.

The principle of the sensor

In most cars with manual transmission, the speed sensor is installed in the area of ​​\uXNUMXb\uXNUMXbthe gearbox, if we consider cars with automatic transmission (and not only), it is located closer to the output shaft of the box, and its task is to fix the speed of rotation of the specified shaft.

in order to deal with the problem, and understand why the speed sensor (DS) is faulty, the first thing to do is to understand the principle of its operation. This is best done using the example of the popular domestic car VAZ-2114, since, according to statistics, it is on this car that speed sensors most often break.

Speed ​​sensors based on the Hall effect generate a pulse signal, which is transmitted through the signal wire to the ECU. The faster the car goes, the more impulses are transmitted. On the VAZ 2114, for one kilometer of the way, the number of pulses is 6004. The speed of their formation depends on the speed of rotation of the shaft. There are two types of electronic sensors - with and without shaft contact. However, at present, it is usually non-contact sensors that are used, since their device is simpler and more reliable, therefore they have replaced older modifications of speed sensors everywhere.

To ensure the operation of the DS, it is necessary to place a master (pulse) disk with magnetized sections on a rotating shaft (bridge, gearbox, gearbox). When these sections pass near the sensitive element of the sensor, the corresponding pulses will be generated in the latter, which will be transmitted to the electronic control unit. The sensor itself and the microcircuit with a magnet are stationary.

Most cars equipped with an automatic transmission have two shaft rotation sensors installed on its nodes - primary and secondary. Accordingly, the speed of the car is determined by the speed of rotation of the secondary shaft, so another name for the automatic transmission speed sensor is output shaft sensor. Usually these sensors work according to the same principle, but they have design features, so in most cases their mutual replacement is impossible. The use of two sensors is due to the fact that, based on the difference in the angular speeds of rotation of the shafts, the ECU decides to switch the automatic transmission to one or another gear.

Signs of a broken speed sensor

In the event of problems with the speed sensor, the motorist can indirectly diagnose this by the following signs:

• Speedometer not working properly or completely, as well as an odometer. namely, its indicators either do not correspond to reality or “float”, and chaotically. However, most often the speedometer does not work completely, that is, the arrow points to zero or jumps wildly, freezes. The same goes for the odometer. It incorrectly indicates the distance traveled by the car, that is, it simply does not count the distance traveled by the car.
• For vehicles with an automatic transmission, switching is jerky and at the wrong moment. This happens for the reason that the electronic control unit of the automatic transmission cannot correctly determine the value of the movement of the car and, in fact, random switching occurs. When driving in city mode and on the highway, this is dangerous, because the car can behave unpredictably, that is, switching between speeds can be chaotic and illogical, including very fast.
• Some cars have an electronic control unit ICE (ECU) forcibly disabling the anti-lock braking system (ABS) (the corresponding icon may light up) and / or the engine traction control system. This is done, firstly, to ensure traffic safety, and secondly, to reduce the load on the internal combustion engine elements in emergency mode.
• On some vehicles, the ECU is forcibly limits the maximum speed and / or maximum revolutions of the internal combustion engine. This is also done for the sake of traffic safety, as well as to reduce the load on the internal combustion engine, namely, so that it does not work at low load at high speeds, which is harmful to any motor (idling).
• Activation of the Check Engine warning light on the dashboard. When scanning the memory of the electronic control unit, errors with codes p0500 or p0503 are often found in it. The first indicates the absence of a signal from the sensor, and the second indicates the excess of the value of the specified signal, that is, the excess of its value of the limits allowed by the instruction.
• Increased fuel consumption. This is due to the fact that the ECU selects a non-optimal ICE operation mode, since its decision-making is based on a complex of information from several ICE sensors. According to statistics, the overspending is about two liters of fuel per 100 kilometers (for a VAZ-2114 car). For cars with a more powerful engine, the overrun value will increase accordingly.
• Decrease or "float" idle speed. When the vehicle is braked hard, the RPM also drops sharply. For some cars (namely, for some models of the Chevrolet machine brand), the electronic control unit forcibly turns off the internal combustion engine, respectively, further movement becomes impossible.
• The power and dynamic characteristics of the car are reduced. namely, the car accelerates poorly, does not pull, especially when loaded and when driving uphill. Including if she is towing cargo.
• The popular domestic car VAZ Kalina in a situation where the speed sensor does not work, or there are problems with the signals from it to the ECU, the control unit is forcibly disables electric power steering on the car.
• Cruise control system not workingwhere it is provided. The electronic unit is turned off forcibly for traffic safety on the highway.

It is worth mentioning that the listed signs of breakdown can also be symptoms of problems with other sensors or other components of the car. Accordingly, it is necessary to carry out a comprehensive diagnosis of the car using a diagnostic scanner. It is possible that other errors associated with other vehicle systems have been generated and stored in the memory of the electronic control unit.

Causes of sensor breakage

By itself, the speed sensor based on the Hall effect is a reliable device, so it rarely fails. The most common causes of failure are:

• Overheat. Often, the transmission of a car (both automatic and mechanical, but more often automatic transmission) warms up significantly during its operation. This leads to the fact that not only the sensor housing is damaged, but also its internal mechanisms. Namely, a microcircuit soldered from various electronic elements (resistors, capacitors, and so on). Accordingly, under the influence of high temperature, the capacitor (which is a magnetic field sensor) begins to short-circuit and becomes a conductor of electric current. As a result, the speed sensor will stop working correctly, or completely fail. Repair in this case is quite complicated, because, firstly, you need to have the appropriate skill, and secondly, you need to know what and where to solder, and it is not always possible to find the right capacitor.
• contact oxidation. This happens for natural reasons, often over time. Oxidation can occur due to the fact that when installing the sensor, protective grease was not applied to its contacts, or due to damage to the insulation, a significant amount of moisture got on the contacts. When repairing, it is necessary not only to clean the contacts from traces of corrosion, but also to lubricate them with protective grease in the future, and also to ensure that moisture does not get on the corresponding contacts in the future.
• Violation of the integrity of the wiring. This can happen due to overheating or mechanical damage. As mentioned above, the sensor itself, as a result of the fact that the transmission elements are significantly warmed up, also works at high temperatures. Over time, the insulation loses its elasticity and can simply crumble, especially as a result of mechanical stress. Similarly, the wiring can be damaged in places where the wires are broken, or as a result of careless handling. This usually leads to a short circuit, less often there is a complete break in the wiring, for example, as a result of any mechanical and / or repair work.
• Chip problems. Often, the contacts connecting the speed sensor and the electronic control unit are of poor quality due to problems with their fixation. namely, for this there is a so-called "chip", that is, a plastic retainer that ensures a snug fit of the cases and, accordingly, the contacts. Usually, a mechanical latch (lock) is used for rigid fixation.
• Leads from other wires. Interestingly, other systems can also lead to problems in the operation of the speed sensor. For example, if the insulation of the wires of others located in the highway in close proximity to the wires of the speed sensor is damaged. An example is the Toyota Camry. There are cases when the insulation on the wires was damaged in the system of its parking sensors, which caused interference of the electromagnetic field on the wires of the speed sensor. This naturally led to the fact that incorrect data was sent from it to the electronic control unit.
• Metal shavings on the sensor. On those speed sensors where a permanent magnet is used, sometimes the reason for its incorrect operation is due to the fact that metal chips stick to its sensitive element. This leads to the fact that information about the supposedly zero speed of the vehicle is transmitted to the electronic control unit. Naturally, this leads to incorrect operation of the computer as a whole and the problems described above. in order to get rid of this problem, you need to clean the sensor, and it is advisable to dismantle it first.
• The inside of the sensor is dirty. If the sensor housing is collapsible (that is, the housing is fastened with two or three bolts), then there are cases when dirt (fine debris, dust) gets inside the sensor housing. A typical example is the Toyota RAV4. To fix the problem, you just need to disassemble the sensor housing (it is better to pre-lubricate the bolts with WD-40), and then remove all debris from the sensor. As practice shows, in this way it is possible to restore the work of a seemingly “dead” sensor.

Please note that for some cars the speedometer and / or odometer may not work correctly or not at all due to the failure of the speed sensor, but because the dashboard itself does not work correctly. Often, at the same time, other devices located on it also “buggy”. For example, electronic speedometers may stop working correctly due to the fact that water and / or dirt got into their terminals, or there was a break in the signal (power) wires. To eliminate the corresponding breakdown, it is usually enough to clean the electrical contacts of the speedometer.

Another option is that the motor that drives the speedometer needle is out of order or the arrow is set too deep, which causes a situation where the speedometer needle simply touches the panel and, accordingly, cannot move in its normal operating range. Sometimes, due to the fact that the internal combustion engine cannot move the stuck arrow and makes significant efforts, the fuse may blow. Therefore, it is worth checking its integrity with a multimeter. in order to know which fuse is responsible for the speedometer (ICE arrows), you need to familiarize yourself with the wiring diagram of a particular car.

How to identify a broken speed sensor

The most common speed sensors installed on modern cars work on the basis of the physical Hall effect. So, you can check this type of speed sensor in three ways, both with and without its dismantling. However, be that as it may, you will need an electronic multimeter that can measure DC voltage up to 12 volts.

The first thing to do is to check the integrity of the fuse through which the speed sensor is powered. Each car has its own electrical circuit, however, on the VAZ-2114 mentioned car, the specified speed sensor is powered through a 7,5 Amp fuse. The fuse is located on the heater blower relay. On the instrument cluster in the front dashboard, the output plug with the address - "DS" and "control controller DVSm" has one number - "9". Using a multimeter, you need to make sure that the fuse is intact, and the supply current passes through it specifically to the sensor. If the fuse is broken, it must be replaced with a new one.

If you dismantle the sensor from the car, then you need to find out where it has a pulse (signal) contact. One of the multimeter probes is placed on it, and the second is placed on the ground. If the sensor is contact, then you need to rotate its axis. If it is magnetic, then you need to move a metal object near its sensitive element. The faster the movements (rotations) are, the more voltage the multimeter will show, provided that the sensor is working. If this does not happen, then the speed sensor is out of order.

A similar procedure can be carried out with the sensor without dismantling it from its seat. The multimeter in this case is connected in the same way. However, one front wheel (usually the front right) must be jacked up to perform the test. Set the neutral gear and force the wheel to rotate while simultaneously observing the readings of the multimeter (it is inconvenient to do this alone, respectively, an assistant will be needed to perform the check in this case). If the multimeter shows a changing voltage when the wheel is rotated, then the speed sensor is working. If not, the sensor is defective and needs to be replaced.

In the procedure with the wheel hanging out, instead of a multimeter, you can use a 12-volt control light. It is similarly connected to the signal wire and ground. If during the rotation of the wheel the light turns on (even tries to light up) - the sensor is in working condition. Otherwise, it should be replaced with a new one.

If the brand of the car involves the use of special software for diagnosing the sensor (and its other elements), then it is better to use the appropriate software.

The detailed operation of the speed sensor can be checked using an electronic oscilloscope. In this case, you can not only check the very presence of a signal from it, but also look at its shape. The oscilloscope is connected to the impulse wire with the wheels of the car hung out (the sensor is not dismantled, that is, it remains in its seat). then the wheel rotates and the sensor is monitored in dynamics.

Checking the mechanical speed sensor

Many older cars (mostly carbureted) used a mechanical speed sensor. It was installed similarly, on the gearbox shaft, and transmitted the angular speed of rotation of the output shaft with the help of a rotating cable embedded in a protective casing. Please note that for diagnostics it will be necessary to dismantle the dashboard, and since this procedure will be different for each car, you need to clarify this issue further.

Checking the sensor and cable is performed according to the following algorithm:

• Dismantle the dashboard so that there is access to the inside of the dashboard. For some cars, it is possible to dismantle the dashboard not completely.
• Remove the fixing nut from the cable from the speed indicator, then start the internal combustion engine and switch gears to reach the fourth.
• In the process of checking, you need to pay attention to whether the cable rotates in its protective casing or not.
• If the cable rotates, then you need to turn off the internal combustion engine, insert and tighten the tip of the cable.
• then also start the internal combustion engine and turn on the fourth gear.
• If in this case the arrow of the device is at zero, then this means that the speed indicator has failed, respectively, it must be replaced with a similar new one.

If, when the internal combustion engine is running in fourth gear, the cable does not spin in its protective casing, then you need to check its attachment to the gearbox. This is done according to the following algorithm:

• Shut off the engine and remove the cable from the drive located on the gearbox on the driver's side.
• Remove the cable from the engine compartment and check the tips, as well as whether the transverse square shape of the cable is damaged. To do this, you can twist the cable on one side and observe whether it is spinning or not on the other side. Ideally, they should rotate synchronously and without effort, and the edges of their tips should not be licked.
• If everything is in order, and the cable rotates, then the problem lies in the drive gear, respectively, it must be further diagnosed and, if necessary, replaced with a new one. How to do this is indicated in the manual of a particular car, since the procedure differs for different brands of cars.

How to fix the problem

After it was possible to determine the breakdown of the speed sensor, then further actions depend on the reasons that caused this situation. The following troubleshooting options are possible:

• Dismantling the sensor and checking it with a multimeter using the above method. If the sensor is faulty, then most often it is changed to a new one, since it is quite difficult to repair it. Some "craftsmen" are trying to solder the elements of the microcircuit that have flown off manually using a soldering iron. However, this does not always work out, so it is up to the car owner to decide whether to do so or not.
• Check sensor contacts. One of the most popular reasons why a speed sensor does not work is contamination and / or oxidation of its contacts. In this case, it is necessary to revise them, clean them, and also lubricate them with special lubricants in order to prevent corrosion in the future.
• Check the integrity of the sensor circuit. Simply put, "ring" the corresponding wires with a multimeter. There can be two problems - a short circuit and a complete break in the wires. In the first case, this is caused by damage to the insulation. A short circuit can be both between separate pairs of wires, and between one wire and ground. It is necessary to go through all the options in pairs. If the wire breaks, then there will be no contact on it at all. In the event that there is a slight damage to the insulation, it is allowed to use heat-resistant insulating tape to eliminate the breakdown. However, it is still better to replace the damaged wire (or the entire bundle), because often the wires work in high temperatures, so there is a high risk of repeated damage. If the wire is completely torn, then, of course, it must be replaced with a new one (or the entire harness).

Sensor repair

Some auto repairers with electronics repair skills are engaged in self-restoration of the speed sensor. namely, in the case described above, when the capacitor is soldered under the influence of high temperature, and it begins to short and pass current.

Such a procedure consists in disassembling the case of the speed sensor to check the performance of the capacitor, and if necessary, replace it. usually, microcircuits contain Japanese or Chinese capacitors, which can be completely replaced with domestic ones. The main thing is to choose the appropriate parameters - the location of the contacts, as well as its capacity. If the sensor housing is collapsible - everything is simple, you just need to remove the cover in order to get to the condenser. If the case is non-separable, you need to carefully cut it without damaging the internal components. In addition to the requirements listed above for choosing a capacitor, you also need to pay attention to its size, since after soldering to the board, the sensor housing should close again without any problems. You can glue the case with heat-resistant glue.

According to the reviews of the masters who performed such an operation, you can save several thousand rubles this way, since the new sensor is quite expensive.

Hack and predictor Aviator

A speed sensor failure is a non-critical, but rather unpleasant problem. Indeed, not only the readings of the speedometer and odometer depend on its normal operation, but also the fuel consumption increases, and the internal combustion engine does not operate at full capacity. In addition, separate vehicle systems are forcibly switched off, which may affect, among other things, traffic safety, both in urban mode and on the highway. Therefore, when identifying problems with the speed sensor, it is advisable not to delay their elimination.