How to check the ABS sensor

Author: Ivan Baranov

The braking system of modern cars is equipped with an anti-lock braking system, which reduces the braking distance and allows you to drive the car in a sharp deceleration mode. To check the ABS sensor, you can use a tester, an oscilloscope, or perform diagnostics without instruments.

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Symptoms of a problem

To monitor the operation of the system, there is a yellow or orange indicator light. In normal conditions, when the ignition is turned on, a lamp on the instrument cluster is activated, then self-diagnosis of system elements is carried out and the icon automatically goes out.

Signs of breakdown of ABS system elements:

• turning on the system warning lamp while driving;
• stable wheel locking during heavy braking;
• absence of sounds indicating ABS operation (brake pedal vibration);
• the appearance in the memory of the electronic system of error codes related to the ABS.

If signs of ABS malfunction appear, operation of the vehicle is prohibited. The brakes, designed to work together with the anti-lock braking system, do not work correctly when it is turned off.

Sensor types

There are two types of sensors on cars:

• passive sensor built on the basis of a coil;
• active sensor that uses the Hall effect.

The passive sensor turns on after the start of movement and reads data from the toothed pulse ring. The passage of a tooth past the device causes the generation of a current pulse, which is read by the control unit. The sensors start working at speeds above 5 km/h and do not respond to contamination.

The active sensor consists of electronic components and a permanent magnet, which is mounted on the hub. When the magnet rotates in the device, a potential difference arises, which is formed into a microcircuit control signal. The information is then sent to the block. Sensors of this design are rare and cannot be repaired.

Operating principle of the ABS sensor

Any diagnostic measure will not be effective if the driver does not have an idea of ​​the operating principles of the unit or element of the system being examined. Therefore, before the stage involving surgical intervention in the operation of this device, first of all, you should study the principle of its operation.

What is an ABS sensor?

Let's start with the fact that this simple device can be found on each of the 4 hubs of the car. The solenoid is located in its sealed plastic housing.

Another important element of the sensor is the so-called pulse ring. The inner side of the ring is made in the form of gear cutting. It is mounted on the back of the brake disc and rotates with the car wheel. There is a sensor at the end of the solenoid core.

The fundamental features of the operation of this system are based on reading the electrical signal coming from the throttle directly to the reading device of the control unit. So, as soon as a certain torque is transmitted to the wheel, a magnetic field begins to arise inside the electromagnet, the value of which increases in proportion to the increase in the rotation speed of the pulse ring.

As soon as the rotation of the wheel reaches the minimum number of revolutions, the pulse signal from the presented sensor begins to enter the processor device. The pulse nature of the signal occurs due to the toothed rim of the pulse ring.

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The subsequent operation of the valve body of the ABS system depends on the frequency of the signal registered in the receiving device. The executive elements of the hydraulic brake force distributor are solenoids, a hydraulic pump and valve mechanisms.

Depending on the intensity of the signal entering the valve body, valve mechanisms controlled by solenoids come into operation. In the event that a wheel locks, the hydraulic unit, taking into account the corresponding signal, reduces the pressure in this brake circuit.

At this moment, the hydraulic pump comes into operation, pumping the brake fluid back into the GTZ reservoir through the open bypass valve. As soon as the driver reduces the force on the pedal, the bypass valve closes, and the pump, in turn, stops working.

At this moment, the main valve opens and the pressure in this brake circuit returns to normal.

The presented modification of the ABS peripheral element is the most common and is used on most domestic and foreign cars.

Due to the relative simplicity of this design, the system elements are highly resistant to mechanical wear and have good performance.

If a part fails, it is not so expensive to carry out the manipulations described below. It's easier to buy and replace the sensor with a new one.

Checking ABS sensors with a tester

The car owner can check the condition of the sensors independently. This will require a few minutes of time and a simple tester. In order to check the sensor with a multimeter, you need to know the resistance value of a working coil. The measurement is made by removing the device or by disconnecting the plug connector.

Self-testing is carried out according to the following algorithm:

1. Place the car on a level surface or a lift. Ensure reliable fixation against accidental movement.
2. To make access to the sensor easier, it is recommended to remove the wheel.
3. The wire is laid along the wheel arch and connected to the general wiring using a plug, which must be carefully disconnected. If the plug is covered with a layer of dirt, then it must be removed and the connector pads should be wiped with a clean rag. If the rear wheel sensors are checked, the connecting plugs are located in the vehicle interior. To access the plug, you need to fold back the seat cushion and peel back the carpet. On some cars, you will need to peel back the soundproofing mats located under the carpet.
4. Switch the multimeter to ohmmeter operating mode.
5. Connect the sensor leads to the multimeter probes and measure the resistance. Compare the value with the standard values ​​available in the operating instructions. If such information is not available, then the readings are taken from reference literature. Usually a resistance of 0.5-2 kOhm is considered normal.
6. Ring the sensor wiring harness to check for a short circuit.
7. Check the operation of the sensor. To do this, rotate the wheel and look at the ohmmeter readings. Depending on the rotation speed, the resistance will change.
8. Switch the measuring device to the “voltmeter” operating mode.
9. Rotate the wheel at a frequency of about 1 rpm, while checking the voltage value. According to standards, it should be in the range of 0.25-0.5 Volts. As the speed increases, the voltage increases.
10. Check all sensors using the procedure described above. It must be remembered that the devices on the front and rear axles differ in design and have different resistance.

Measuring resistance with a multimeter
Based on a certain resistance value, you can draw a conclusion about the state of the sensor:

• the resistance is below the permissible level - sensor malfunction;
• the resistance is close to or equal to zero—turn-to-turn short circuit in the sensor coil;
• variable resistance that changes when the position of the wiring harness changes - breaks in the wires inside the cable;
• the resistance value tends to infinity - wire or coil breaks.

If it was not possible to find out the resistance value of a working ABS sensor, you need to measure the remaining devices. A device with parameters noticeably different from the others is most likely the cause of the ABS malfunction.

Additionally, you should use a tester to check the integrity of the wiring harness from the sensor plug to the block on the control unit:

1. Disconnect the connectors from the sensors and control unit.
2. Find information about the pinout of the control unit plug. The pinout can be found by the valve body model, which is printed on the housing identification plate.
3. Ring the wiring harnesses.

Checking the voltage on the sensor winding

Another way to check the performance of ABS sensors is to measure the output voltage that appears on the winding when the wheel rotates. To do this we do the following:

• We hang the wheel using a jack and remove it.
• We find the ABS sensor and disconnect its connector from the wiring leading to the control unit.
• We switch the multimeter to AC voltage measurement mode.
• We connect the probes to the contacts of the sensor connector.
• Rotate the wheel hub (at a speed of 1÷2 rpm).
• If there is a voltage of 200÷1700 mV (its value varies depending on the specific sensor model), it can be stated that the sensor is working. There's no reason to worry. If you were unable to find the technical specifications of the sensor, it is worth checking all ABS sensors installed on the car in a similar way. If, as a result of testing, a low voltage is detected on one of them (compared to the others), this indicates that it is not operating correctly.

On a note! If, when measuring the resistance of all sensors, the ohmmeter readings indicate their performance, but when the wheel rotates, a low voltage (or absence) appears on the sensor windings, then this may be a consequence of wear or breakage of the gear wheel.

How to check using an oscilloscope and adapter

Testing with an oscilloscope allows you to more accurately test the sensor for functionality. The oscilloscope shows graphs of changes in the signal generated by the device. The sensor can supply pulses that comply with the standards, but the amplitude of the signal will be incorrect and will cause an ABS operation error. The test is performed on the car, without removing the sensor.

In order to check the ABS sensor with an oscilloscope, you must:

1. Disconnect the sensor plug by analogy with the procedure used when measuring parameters with a tester.
2. Connect an oscilloscope or oscilloscope to the sensor pins.
3. Start rotating the hub through the wheel or other method at a frequency of 2-3 rpm.
4. Record the signal oscillation curve.
5. Carry out a similar procedure with the sensor located on the other side of the axis.

The serviceability of the sensor is indicated by:

• identical amplitude of oscillations of signals from sensors located on the left and right sides of the same axis;
• uniformity of the curve, without noticeable lateral deviations;
• amplitude stability in height, which should not exceed 0.5 volts.

Another way to measure parameters is to connect a laptop equipped with a special adapter. The device connects to a USB port and transmits signals to the corresponding software.

Sample USB Oscilloscope

The oscilloscope and adapter are highly specialized devices and are rarely used for personal use. The equipment is used in service stations.

Ways to check functionality

To determine the condition of a part, we will perform a series of steps to diagnose it, moving from simple to complex:

1. Let's check the fuses by opening the unit (inside the passenger compartment or in the engine compartment) and inspecting the corresponding elements (indicated in the repair/operation instructions). If a burnt component is found, we will replace it with a new one.
2. Let's inspect and check:
   integrity of connectors;
3. wiring for abrasions that increase the risk of a short circuit;
4. contamination of the part, possible external mechanical damage;
5. fixation and connection to ground of the sensor itself.

If the listed measures do not help to identify a device malfunction, it will have to be checked using instruments - a tester (multimeter) or an oscilloscope.

Tester (multimeter)

This method of diagnosing the sensor will require a tester (multimeter), instructions for operating and repairing the car, as well as PIN - wiring with special connectors.

The device combines the functions of an ohmmeter, ammeter and voltmeter

Tester (multimeter) is a device for measuring electric current parameters, combining the functions of a voltmeter, ammeter and ohmmeter. There are analog and digital device models.

To obtain complete information about the performance of the ABS sensor, you need to measure the resistance in the device circuit:

1. We lift the car with a jack or hang it on a lift.
2. Remove the wheel if it prevents access to the device.
3. Remove the cover of the system control unit and disconnect the controller connectors.
4. We connect the PIN to the multimeter and the contact socket of the sensor (the connectors for the rear wheel sensors are located inside the cabin, under the seats).

   
   We connect the PIN to the tester and the contact socket of the sensor

5. We measure the resistance (tester in ohmmeter mode) at the contacts of the device. We compare the readings of the device with the car’s operating manual, where the required parameters should be indicated.
6. We check the electrical circuit by ringing the sensor wiring for a possible short circuit.
7. We spin the wheel manually and at the same time measure the resistance - the tester readings should change depending on the rotation speed.
8. We switch the tester to the “voltmeter” mode and measure the voltage on the sensor - we turn the wheel at a frequency of 1 rev/sec, monitoring the readings of the device. Voltage parameters from 0.25 to 1.2 V are considered optimal, but it should be taken into account that increasing the wheel speed necessarily increases them.

The device readings must correspond to the data specified in the repair and operation manual for a particular vehicle. If the device resistance:

• below the minimum threshold - the sensor is faulty;
• approaches zero - short circuit;
• unstable (jumping) at the moment of twitching of the wire - a violation of contact inside the wiring;
• infinity or no readings - wire break.

Attention! The resistance of the ABS sensors on the front and rear axles is different. The operating parameters of the devices are 1–1.3 kOhm in the first case and 1.8–2.3 kOhm in the second.

Video “Diagnostics of the ABS sensor”

How to check using an oscilloscope (with connection diagram)

In addition to self-diagnosis of the sensor with a tester (multimeter), it can be checked using a more complex device - an oscilloscope.

The device examines the amplitude and time parameters of the sensor signal

An oscilloscope is a device that studies the amplitude and time parameters of a signal, which is intended for accurate diagnosis of pulse processes in electronic circuits. This device detects problems with connectors, broken connections to ground, and broken conductors. The test is performed by visually observing the vibrations on the device display.

To diagnose the ABS sensor with an oscilloscope, you must:

1. Fully charge the battery so that during the measurement you can observe voltage drops (jumps) on the connectors or conductors.
2. Find the touch sensor and disconnect the upper connector of the part.
3. Connect an oscilloscope to the contact socket.

   
   Connecting the device to the ABS sensor connector (1 - toothed disc-rotor; 2 - sensor)

4. Rotate the suspended wheel (by lifting the car on a jack or lift) at a constant frequency of 2-3 revolutions per second.
5. Detect the amplitude of signal oscillations on the device display.
6. Rotate the second wheel of the axle and similarly detect vibrations.

The serviceability of the ABS sensor is indicated by:

• equal amplitude of signal fluctuations when the wheels of one axle rotate;
• absence of amplitude beats when diagnosing with a sinusoid signal of lower frequency;
• maintaining a stable, even amplitude of signal fluctuations not exceeding 0.5 V when the wheel rotates at a frequency of 2 rps.

Note that an oscilloscope is a rather complex and expensive device. Modern computer technologies make it possible to replace this device with a special program downloaded from the Internet and installed on a regular laptop.

Video “Laptop instead of an oscilloscope”

Checking a part without instruments

The easiest way to diagnose a device without instruments is to check the magnetic valve on the induction sensor. Any metal product (screwdriver, wrench) is applied to the part inside which the magnet is installed. If the sensor does not attract it, it is faulty.

Most anti-lock brake systems of modern cars have a self-diagnosis function with errors displayed (in alphanumeric encoding) on ​​the on-board computer screen. You can decipher these symbols using the Internet or the machine’s operating instructions.

Checking the sensor without instruments

The most crude way to check the health of the sensor is to test the magnetic field created during operation of the device. To do this, a steel object is applied to the sensor, which should be attracted when the ignition is turned on.

It is possible to visually inspect the device for cracks in the housing or noticeable breaks and oxidation in the wiring. It is recommended to inspect the plug and the condition of the contacts in it; oxidation is the cause of deterioration in signal conductivity.

How to fix problems

After checking with instruments and identifying the faulty unit, you can begin repairs. Some owners repair the sensors by replacing the wiring harness or rewinding the coil.

Sensor failure

A faulty passive type sensor can be repaired yourself:

1. Remove the sensor from the hub. The fastening bolt often becomes sour, so you should unscrew it carefully. For removal it is allowed to use WD40 type fluid.
2. Remove the protective coil housing. Removal is done with a file. The cut should be done extremely carefully so as not to damage the housing and winding.
3. Remove the protective film from the winding by prying it off with a sharp knife.
4. Carefully unwind the wire from the spool. During the removal process, the version of a conductor break is confirmed. As a result, you will be left with an empty ferrite core, resembling a spool of thread in shape.
5. Wind a new winding. Copper wire from the coils of common relays of the RES-8 type can be used as a conductor. Winding can be done using a drill with smooth speed control. Be careful as breaking the wire will return you to the start. It is recommended to wind the conductor to the top level of the coil.
6. Check resistance. Most coils have a value in the range of 0.9-1.2 kOhm. To clarify, it is recommended to measure the parameter on a known-good sensor located on the opposite side of the axis. The resistance is adjusted by unwinding the excess wire. If the reading is low, you will need to use another wire or re-wind. Secure the wire from unraveling with tape or other adhesive tape.
7. Solder the wires to the coil terminals that serve as a connection between the winding and the harness. For outputs, it is recommended to use multi-core insulated cable, which has increased strength.
8. Install the coil into the old housing. If it received significant damage when disassembling the device, then the coil is filled with epoxy resin. To do this, the part is located in a metal container of a suitable size, for example, a capacitor housing. The air gap between the coil and the glass is carefully filled with resin. When pouring, it is advisable to avoid large air voids. After the resin has completely hardened, the body is removed.
9. Reinstall the sensor mount, securing it with epoxy resin. Conduct a visual inspection of the product for cracks and voids in the insulation. Detected defects are filled with resin.
10. Place the repaired sensor in its original place and check the functionality of the ABS system. When installing the device, it may be necessary to modify the resulting body, which is done with a file and sandpaper. The field installed sensor should have a gap between the coil and the toothed ring within 0.9-1.1 mm. When reducing the gap, it is recommended to bring it up to standard by installing gaskets.

Cut body

Repaired sensor assembly

Sensor with holder removed

Making a resin body
You need to drive the car for a while, checking the brakes at different speeds. There are cases when the ABS spontaneously activates at certain wheel speeds - usually just before stopping. Then you will need to search for the gap, adjusting it with shims or sharpening the sensor body.

Another repair option is to install a modified crankshaft position sensor from domestic cars:

1. Remove the “original” sensor and modify the body of the “donor” part. Most often, this role is played by the DPKV from the ZMZ-406 engine, which has a resistance within 800 Ohms. When modifying, you should strive to ensure a parallel arrangement of the core with the wound coil and the toothed ring mounted on the axis. The gap between the sensor and the ring should be within 0.2-0.3 mm.
2. Test the operation of the device. On some Japanese-made cars, the ABS lamp may turn on periodically. The situation is corrected by changing the connection of the harness contacts.

Both options for repairing the sensor require perseverance and the ability to work with various tools from the owner. If the car user doubts his abilities, it is recommended to purchase a new device or find the product at a car dismantling site.

Wiring problem

If the problem of sensor loss of functionality lies in the wiring, then it can be replaced:

1. Unscrew the sensor mount to the wheel hub.
2. Disconnect the wire plug.
3. Remove the sensor along with the wire. This will require removing the mounting brackets installed on the wiring.
4. Measure the installation distances of the brackets. It is recommended to draw a diagram and photograph the factory location of the fasteners.
5. Cut the sensor from the wire, leaving some extra length for soldering.
6. Check the integrity of the remaining cable on the sensor. If the section is intact, then you can begin installing a new wiring segment.
7. Remove all protective covers and fastenings from the old cable.
8. Select a wire with a suitable outer diameter and cross-section.
9. Install the previously removed protection and fastening elements onto the new harness. To facilitate assembly, it is recommended to use a soap solution.
10. Solder the sensor and plug into place.
11. Carefully isolate the joint. The accuracy of operation and service life of the repaired part depend on the tightness of the connection.
12. Reinstall the sensor, check the functionality of the ABS system, and make sure there are no errors during operation.

Dismantled sensor with harness

There is a section of wire in front of the sensor that needs to be left

Removed fasteners and covers

Repaired wire

Using a laptop as an oscilloscope

From a technical point of view, professionals consider using an oscilloscope to be the most correct method for checking ABS sensors. However, the device is expensive, and it makes no sense to purchase it just to test sensors. Well, if you have a laptop, then using it as an oscilloscope is quite simple. First, we make a measuring cord to connect the sensor connector to the microphone input. To do this, you will need a 3.5 mm mini jack plug, two resistors and a couple of pieces of wire of a suitable length. The cash costs for purchasing the necessary components will not exceed 50 rubles. We solder the wires to the contacts of the mini-jack, and install a resistive divider in the gap (to protect the sound card) according to the diagram below.

Then everything is simple:

• We install a special Oscilloscope program on the laptop (for example, Winscope or Avangard).
• We connect the wires to the ABS sensor connector, and the plug to the microphone input.
• Rotate the wheel hub.
• If the sensor is working properly, we see a clear sinusoidal signal on the screen.

On a note! If you don't have an assistant, it will be very difficult to rotate the wheel and watch the laptop screen at the same time. For convenience, we recommend using the recording function. By turning on the playback mode (after performing the test described above), you will be able to see the test results.

ABS sensors play an important role in the operation of the car's braking system - the braking efficiency and uninterrupted operation of the unit as a whole depend on them. The sensor elements send data about the degree of rotation of the wheels to the control unit, and it analyzes the incoming information, building the desired algorithm of actions. But what to do if you have doubts about the serviceability of the devices?