The article describes a simple generator on a 555 timer, with which you can check the functionality and correctness of the readings electronic speedometers using an electronic Hall sensor as a speed sensor.
Many modern cars, such as GAZelle (GAZ 2705, 33021), Volga, KRAZ and others, use electronic speedometers with a microammeter and a stepper motor. Such speedometers work in conjunction with an electronic Hall sensor mounted on the gearbox. When the vehicle is moving, the sensor is driven into rotation by the gearbox secondary shaft gear. For one revolution of the sensor shaft, six pulses of electric current are generated.
These pulses enter the speedometer circuit. The speed indicator in the speedometer is a microammeter. In addition, amplified pulses from the transmitter are fed to a stepper motor, which rotates the drums of the distance indicators.
According to the technical documentation, which can be found in, to check the serviceability of such a speedometer, it is necessary to apply rectangular pulses of positive polarity with an amplitude of 6...7 V, a duration of 200...250 μs and a frequency of 100... from the signal generator G5-54 to the input of the Hall sensor connected to the speedometer. 200 Hz.
If the user or fleet mechanic is not interested in the high accuracy of checking speedometer readings, but only needs to occasionally check their performance, then the design of a simple rectangular pulse generator proposed by the author can easily cope with this task.
Electrical circuit diagram generator shown in Fig.1. It is assembled on a 555 universal timer chip. The connection circuit is typical. The values of elements C2, R2-R4 are selected in such a way as to obtain a square wave with a frequency of 100...200 Hz at the output. The required pulse frequency of the assembled generator can be adjusted using trimming resistor R3. The circuit is designed for use in cars with an on-board voltage of 12 V. If the voltage of the vehicle's on-board network is 24 V (for example, in KRAZ), then the circuit must be supplemented with an integrated stabilizer DA2, connecting it to the power circuit break as shown in the dotted line in the diagram .
Construction and details
All elements of the circuit are assembled on a printed circuit board made of one-sided foil-coated fiberglass laminate with dimensions of 30x20 mm. The printed circuit board drawing and the arrangement of elements are shown in Fig. 2. For ease of repetition, the drawing is shown from the foil side. The design uses output radio components installed vertically. There are no special requirements for them. Conductors are soldered to the XT 1-KhTZ points, at the other end of which a connector similar to the connector for connecting the Hall sensor is installed. This connector contains all the circuits necessary for the operation of the generator: plus/minus power and the speedometer input. The printed circuit board is mounted in a suitable electrically insulated housing. The author used for this purpose a piece of plastic cable duct with a cross section of 25x16 mm.
Assembly, adjustment and use
A correctly assembled generator does not require adjustment. You should pay attention to the correct connection of the connector pins, since if the supply voltage accidentally reaches the output of the generator, it will fail:;0. There is no need to use radio measuring instruments to configure the device. It is enough to have a known working speedometer. The device is connected instead of the Hall sensor and the trimming resistor R3 is used to achieve the desired speedometer reading, for example 60 km/h. If the control range is not enough, then to increase the limiting frequency of the generator, you should slightly reduce the resistance of resistor R4, and to decrease it, increase it.
The speedometer, as the name suggests, shows the speed of the car. Following the speed limit is important not only to avoid fines, but also to make safe turns and other maneuvers. The higher the speed, the larger the safe turning radius should be. If the radius is less than necessary, there is a high probability of the car skidding and overturning the car. Therefore, the serviceability of the speedometer is as important as the high-quality operation of the steering or braking system.
How does a speedometer work?
There are two main modifications of speedometers:
- mechanical;
- electronic.
The principle of operation of a mechanical speedometer is to transform the shaft rotation speed into energy, which moves the needle. The speedometer drive is located in a manual or automatic gearbox and is connected to the indicator using a flexible cable protected by a metal casing. The tips of both sides of the cable are made in the form of a tetrahedron, due to which they effectively transmit rotation from the drive to the indicator. A mechanical speedometer is always connected to the odometer (vehicle mileage indicator) and forms a single unit with it.
The operating principle of an electronic speedometer consists of a sensor that produces pulses of a certain frequency and duration (depending on the speed of the car). The sensor is connected either to a separate electronic speedometer or to the on-board computer. Both the computer and the speedometer perform the same function - they count the number of pulses per unit of time and convert the value into understandable kilometers or miles per hour.
Speedometer malfunctions
The most common malfunctions are:
- cable break or damage;
- the cable tip jumping off the driven gear;
- malfunction of the mechanical or electronic indicator;
- pulse sensor malfunction;
- poor contact or broken wire that connects the sensor and the indicator or computer.
Video - How to fix a speedometer
Diagnostics and repair of a mechanical speedometer
- For diagnosis you will need:
- 12 Volt motor;
- flat and Phillips screwdrivers;
- flashlight; jacks and stands;
- instructions for repairing or servicing your car.
To check the speedometer, raise the front passenger side of the vehicle using a jack. For information on how to do this safely, read the article (Replacing and restoring shock absorbers). Remove the front panel (dashboard) to access the instrument cluster. On some car models you can do without this operation, so carefully read the repair and operating instructions for your car. Remove the instrument cluster and unscrew the cable fixing nut from the indicator, start the engine and engage 4th gear. Check if the cable is spinning in the protective casing? If yes, turn off the engine, insert and tighten the cable end, then start the engine again, engage 4th gear and look at the indicator readings. If the arrow does not change position, the indicator is faulty and must be replaced.
If the cable does not turn when the engine is running and the gear is engaged, it is necessary to turn off the engine and remove the cable from the drive located on the driver's side of the gearbox. Pull the cable out of the engine compartment and inspect the ends to see if the shape (square) is damaged. Twist the tip on one side of the cable and observe the tip on the other side. If both tips rotate synchronously, without effort, and the edges of the tips are not licked, then the problem is a worn drive gear, so it needs to be replaced. This operation is described in the vehicle repair and operating instructions.
Diagnostics and repair of electronic speedometer
For diagnosis and repair you will need:
- flat and Phillips screwdrivers;
- tester;
- a set of keys;
- scanner for an injection engine (you can use a regular oscilloscope instead).
Run self-diagnosis of the on-board computer (BC). On most fuel-injected cars manufactured after 2000, the BC supports this function. If the BC gives an error, you need to decipher it using a special table, which is located in the instructions for servicing and repairing your car. But, the diagnostic results will show whether the entire speedometer system is working or not. To fix the problem, you will have to look for the damage yourself. To do this, lift the car as described above. Connect the oscilloscope to the middle contact of the speed sensor (installed in place of the speedometer drive) and the positive contact of the battery. Start the engine and engage 1st gear.
A working sensor will produce a pulse signal with a voltage of at least 9 Volts with a frequency of 4 - 6 Hertz. If the sensor is working properly, you need to turn off the gear and use a tester to check the wire that connects the sensor to the electronic control unit (ECU) controller. Or use an oscilloscope to check the sensor signals at the ECU input. If there are signals, you need to check the terminals and wire that connects the ECU and the instrument cluster (speedometer indicator). If you have a special scanner, then it is advisable to check the speedometer indicator, this will allow you to more accurately determine the cause of the malfunction.
Most often, the speedometer stops working due to water and dirt getting into the terminals, as well as due to a break or break in the signal wires. Therefore, in most cases it is enough to dry and clean the contacts. If the test results indicate that the speed sensor is faulty, it will need to be replaced. This procedure, as well as replacing a damaged indicator, is described in detail in the operating and repair instructions for your car.
1. It is necessary to carefully inspect the bottom of the car, as well as the condition of the odometer drive mounting nuts. If you notice that the nut has traces of mechanical impact or is surprisingly clean, despite the fact that all other parts are dirty or dusty, you are looking at a typical representative of “hand-rejuvenated” cars.
2. The second sign that the mechanical speedometer is twisted is that the numbers are unevenly spaced in relation to each other. Such a “dance” of numbers indicates not only that there was interference in the operation of the odometer, but also that the master himself is a “teapot” or simply unkempt.
1. Take a closer look, if the mileage is low, and they have already been changed, it’s worth considering!
2. The general condition can also tell a lot: if the steering wheel is worn to holes, the inscriptions on the buttons are not clear, the door handles do not radiate newness at all, and the pedal pads are no longer original, while the car’s mileage is 100 thousand km. - you are simply being fooled into impudence.
3. You can also tell a lot by the appearance of the car. As a rule, significant mileage is indicated by all sorts of chips, cloudy headlights, etc.
4. If it’s a motor, be sure to look at the condition of the turbine and pipe.
Speedometer is a device designed to measure the speed of a car. In the modern automotive industry, a predominantly electronic type of device is used.
The domestic automotive industry began to use an electronic speedometer since the release of the VAZ-2110, the power system of which was based on an injector.
Therefore, if the speedometer does not work even on relatively old cars, the reason should be sought in the electrical wiring elements.
The speed measurement system in a modern car includes such elements as:
- Speed sensor installed in the gearbox;
- Electronic engine control unit;
- Speedometer display on the instrument panel;
- Wiring.
During operation of the engine and gearbox, the sensor removes information about its rotation frequency from the gearbox output shaft and transmits it to the ECU in the form of electrical impulses. The higher the vehicle speed, the shorter the time interval between sensor signals.
The electronic control unit calculates the speed of the machine based on the frequency of the received pulses. This is the operating principle of an electronic speedometer. In parallel with the correction of engine operating modes, the control unit transmits information about the speed of the vehicle to the speedometer and diagnostic block. 
If there is a trip computer with a “K” output of the DC, speed data can be duplicated on its display.
Causes of speedometer malfunction
If the speedometer stops working, troubleshooting is carried out in several directions. The following failures may be the cause of failure:
- Speed sensor failure;
- Damage to electrical wiring;
- Oxidation of “mass” contacts;
- Malfunction of the speedometer itself;
- ECU malfunction;
- Incorrect installation of the instrument panel after removal.
As a rule, no other causes of the malfunction are detected. Sometimes a device failure is caused by a blown fuse in the electrical circuits responsible for the operation of the dashboard. However, this problem can be classified as a wiring fault.
A diagnostic sign of fuse F19 failure is:
- Failure of the entire instrument panel;
- Diagnostic unit failure;
- Failure of the automatic door locking system;
- Reverse lamp failure.
Diagnostics
Troubleshooting begins by disconnecting the wiring block from the speed sensor harness and checking them using a test light.
To make a control light bulb, you need any car lamp that can operate at a voltage of 12 V, and two wires about 1 meter long each. One of the wires is fixed to the positive terminal, the second - to the negative terminal of the lamp. The resulting device also includes a Krona battery. 
To carry out the test, one wire of the warning lamp is attached to the ground of the body or battery, and the second is made with short, frequent touches to the middle contact of the DC connector. If there are no faults in the connector-speedometer section, the speedometer needle will slightly tremble or rise. If the needle shakes, the answer to the question why the speedometer does not work can be considered found - the speed sensor requires replacement.
In cases where the needle’s response to tapping on the central contact of the block cannot be detected, it is necessary to “test” the speedometer power circuit. The procedure is carried out using a multimeter (multitester), or by using the same light bulb - a control.
The wiring harness is first disconnected not only from the speed sensor block, but also from the speedometer itself. One terminal of the tester or warning lamp is connected to the end of the wire located under the hood, the other to the interior end of the speed meter current supply circuit.
If the tester in the “continuity” mode indicates a violation of the integrity of the circuit, further troubleshooting is carried out in this direction. It is necessary to check the fuses, the connection points of the wires, and their integrity inside the insulating braid.
The search area can be reduced by gradually “ringing” individual sections of the circuit. On model 2114 and other VAZ products, the cause of speedometer failure is often oxidation of the “mass” contacts attached to the car body.
In cases where the speedometer needle does not work, but there is no evidence of malfunctions in the electrical supply circuit, a logical conclusion is drawn about the malfunction of the device itself. Additional testing can be done by temporarily installing a known good instrument panel. 
Repair
Repair of the speed measurement system directly depends on the identified malfunction:
Speed sensor
- Clean from dirt;
- Clean the pad contacts from corrosion and oxides;
- If the above measures do not help, the sensor is replaced.
Wiring
- Check and clean “mass” contacts;
- Solder or secure with twists the places where the wires are broken, due to which the speedometer stopped working;
- Cover areas where the braid is damaged with insulating tape;
- Replace failed fuses;
- Clean the pad contacts from oxides and corrosion.
Speedometer
If the speedometer stops working, it must be replaced. On domestic cars assembled using an electronic type of speed meter, the speedometer changes along with the instrument panel. You can carry out this operation yourself. To do this, you only need a Phillips screwdriver and pliers.
It is impossible to restore the functionality of the device with your own hands. A master electronics engineer can do this. However, given the fairly low prices for spare parts for Russian-made cars, contacting a specialist is not economically feasible.
Repairing an old speedometer can be much more expensive than completely replacing the old instrument cluster with a new one.
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Every third report from the Autocode service shows that the car’s mileage is incorrect. On average, each car travels about 20 thousand km per year. However, on sale you can find many 5-7 year old vehicles with mileage of 50-60 thousand km, or even less. Owners of such cars may claim that they only used the car “on major holidays.” But, most likely, the actual mileage is much higher than that indicated on the dashboard. Let's figure out what methods fraudsters use and how to determine the twisted mileage.
Why do they twist the mileage?
Most often, the odometer readings are changed in order to sell the car at a higher price. However, there are a number of other reasons why sellers resort to this procedure. Twisting of readings may be caused by the need to:
- avoid expensive maintenance (the on-board computer of some foreign cars contains information about the time of maintenance; if this schedule is violated, it begins to issue alarm messages);
- hide the fact of replacing the dashboard (after an accident or for other reasons);
- keep silent about malfunctions of components that may affect the correct operation of the speedometer (for example, generator, battery, etc.).
You may be faced with the fact that the mileage of used cars imported into Russia from those countries where the amount of transport tax is calculated depending on the kilometers traveled by the car over a certain period of time is increased.
For example, such a system has been in operation for several years in the Netherlands, where a car’s mileage is tracked using GPS. In some US states, car owners have to pay $0.012 per mile.
By the way, under American law, mileage abuse is a criminal offense. Serious liability for such actions is provided for in Germany and France (up to 1 year and up to 2 years of imprisonment, respectively). Russian legislation does not provide for punishment for mileage inaccuracy.
Ways to twist mileage
Fraudsters have several techniques in their arsenal to deceive gullible customers. The choice of one method or another primarily depends on the type of device installed on the machine, which is responsible for calculating the distance traveled.
Here it is necessary to clarify and talk about the fact that many inexperienced car owners mistakenly associate the increase in mileage with adjusting the speedometer readings. In fact, it shows the speed of movement, and the number of kilometers traveled by the vehicle is recorded by another device - odometer.
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The device works in close connection with the speedometer. And the panels that display the readings of these two devices are usually located next to each other. Apparently this is where some confusion in concepts arose. In order not to confuse the reader even more, we agree that the use of both definitions is acceptable.
A car can be equipped with one of three types of odometers:
- mechanical;
- electromechanical;
- electronic.
Mechanical and electromechanical devices were used in the automotive industry until the end of the last century. They are distinguished by a rather primitive device: the speed of the gearbox gearbox is transmitted through a special cable to a meter, the readings of which are displayed on the dashboard. It is easiest to check the mileage of such a device.
Method number 1. The odometer is disassembled, and the necessary readings are set on the meter manually.
Method number 2. To implement it, you will need to disassemble the dashboard and attach any power tool with high rotation speed (screwdriver, drill, etc.) to the speedometer cable using a special attachment. After this, the readings are twisted to the desired value. Of course, this can be done manually, but using a power tool speeds up the process many times over.
For an electromechanical odometer, twisting is carried out in a similar way. The only difference is that if, when taking readings from a mechanical device, the on-board power of the car is turned off (the terminals from the battery are removed), then when carrying out manipulations with an electromechanical device, the power cannot be turned off (otherwise the meter wheels will not rotate). Therefore, there is a high probability of a short circuit occurring.
The cost of the work is quite affordable and ranges from 1 to 1.5 thousand rubles. It’s quite easy to find advertisements in newspapers or on the Internet for companies willing to provide such a service. They are usually hidden under signs like this: “Speedometer adjustment and repair.”
Quite a few home-grown “Kulibins” in their own garages make a living by turning mileage. Usually people find out about them through word of mouth.
Rolling up the electronic odometer
The operation of devices of this type is based on reading the readings of special sensors (they can be optical or magnetic), which are installed on the gearbox shaft or directly on the vehicle wheel. The readings go into the on-board computer, which records them and transmits them to the electronic display.
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On expensive car models (Toyota, Audi, etc.), mileage data can be stored in several memory blocks at once. According to experts, it is most difficult to change the mileage traveled on a BMW (the car can have up to 10 backup storage points). However, experts unanimously claim that if you have the appropriate equipment, you can check the mileage of absolutely any vehicle.
Several tricks are used to manipulate electronic odometers.
Method number 1. Designed for fraud with budget cars. To implement it, it is enough to remove the dashboard and connect the car’s on-board computer to a laptop on which the appropriate software is installed, or to a special device - a programmer. After this, the actual readings change.
Method number 2. Used for fraud with expensive cars that have several backup data storage units. In its principle it is almost identical to the first. However, it is very important for the fraudster to detect all information storages, otherwise during further operation the car computer can restore data from the backup storage, then the real mileage will again be displayed on the display.
The cost of the service depends on the complexity of the work and can vary from 2.5 to 10-12 thousand rubles.
How to check if the speedometer on a car is twisted
In order not to overpay for a supposedly “almost new car,” you need to remember how you can find out about the fact of fraud.
Unfortunately, there is currently no technical means of checking whether the mileage has been adjusted or not on a vehicle with a mechanical or electromechanical odometer.
Here you will have to be guided by the data of the external examination. The fact that the speedometer is twisted can be determined by the presence of traces of removing the instrument panel, the degree of wear of tires, brake discs, etc.
How to find out if the mileage on a car with an electronic odometer is wrong
To establish the fact of fraud, you will need to conduct computer diagnostics. If you have the appropriate software, equipment and knowledge, you can check the car for twisted mileage yourself. But it is better to contact a trustworthy service center.
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The ideas of many car enthusiasts about the wonders of computer diagnostics are too exaggerated. Those who think that there is a special item in the memory of the on-board computer, by looking into which you can check the real mileage, are mistaken. Most often, you can find out about the fact of interference in the electronic filling of a vehicle only by indirect signs.
Usually this is a discrepancy in data, for example, about the time of an event recorded by the on-board computer. For example, if during a check the car’s odometer shows 75 thousand km, and the memory contains information about an error recorded after 150 thousand km. Or the owner swears that his “iron horse” has run no more than 50 thousand km, but when dividing the kilometers traveled by the number of engine hours, the average speed of movement is 4-5 km/h.
The more such oddities revealed by checking the car’s speedometer, the more reason the buyer has to wonder whether he needs such a “dark horse”.
How to find out real mileage online
You can check whether the mileage is twisted on the website. To do this, just enter state in the search bar. vehicle number. After this, within a few minutes you will receive a full report about the desired vehicle.
In addition to data on actual mileage, using the service you can obtain information about the number of accidents in which the vehicle was involved, former owners, the presence of fines, check for restrictions and find out a lot of other information from the car’s history.
