Methods for checking 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.

Causes and symptoms of malfunctions

In new generation cars, when the ignition is turned on, automatic self-diagnosis of the anti-lock braking system occurs, during which the performance of all its elements is assessed.

In addition to the display of indicator lights on the dashboard, there are the following signs of a malfunction of the ABS system:

• When applying pressure to the brake pedal, there is no reverse knocking or vibration of the pedal;
• During emergency braking, all wheels are blocked;
• The speedometer needle shows a speed less than the actual speed or does not move at all;
• If more than two gauges fail, the parking brake indicator on the dashboard lights up.

In the event of a malfunction of the anti-lock braking system, a warning lamp lights up on the instrument panel

The reasons for the ineffective operation of the ABS may be:

• Failure of one or more speed sensors;
• Damage to sensor wiring, which results in unstable signal transmission to the control module;
• A voltage drop at the battery terminals below 10.5 V disables the ABS system.

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.

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.

What to do first?

It is necessary to check the ABS sensors located near each wheel hub. Your task is to detect a violation in the connection of the sensors, a broken wire or damage to the ABS sensor housing. In any of these cases, you will one way or another see the corresponding indicator on the panel, well, provided that the system control unit itself is working and not “buggy”.

Checking the ABS sensor - measuring the resistance

1. We jack up the wheel on which you think the inoperative or faulty sensor is located, or each wheel in turn if you don’t know exactly which sensor is faulty.
2. Next, remove the wheel and gain access to the sensor.
3. Remove the housing, as well as the protective control unit and connectors that supply power to the sensors.
4. After that, we insert wires into the circuit of wires with PIN connectors and connect them with the sensor and multimeter.
5. We measure the resistance and compare it with the one that should be by default (you can find it in the manual) or with a representative of the manufacturer of your car.
6. We check the wiring for breaks or short circuits.
7. Rotate the wheel while watching the multimeter readings, the resistance should change.

• Device - leg - 5-26 Ohm.
• Device - "ground" - from 20 kOhm or more.

For more details on how to check the ABS sensor, watch this video:

Checking the ABS sensor using a tester - measuring the voltage

1. Let's jack up the wheel.
2. Turn on the multimeter, set the DC voltage measurement mode.
3. We connect the electrodes of the device to the connectors and check the readings, while rotating the wheel (about 1 rpm).
4. A working ABS sensor will show a voltage on the device of ~0.25-1.2 Volts. If the wheel rotation speed is higher, the readings will increase accordingly.

How to check a sensor with an oscilloscope?

To diagnose the serviceability or malfunction of the ABS sensor, you can even use an oscilloscope or, more simply, a tester. When connected, a graph will be displayed on the device; using amplitude analysis, you can judge the serviceability or malfunction of the sensor.

The problem is that this device is not available at every service station, not to mention the garage in which you are going to conduct all your “experiments”. The device is expensive and quite difficult to understand, so to work with it you need to have certain knowledge and skills.

In modern cars, the ABS system has a self-diagnosis function; using special software, you can read the error code and then decipher it using a special table.

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.

Anti-lock braking system and safety

Many people mistakenly believe that ABS does not bring any benefit and is even an obstacle to safe driving. This is a rather deep-rooted opinion, and to the question “how do you justify this?” very often you can hear something like “on loose snow, ABS increases braking distance.” In fact, the length of the braking distance is not particularly different. But the main goal of this system is not to shorten the braking distance, but to increase traction during braking. It is impossible to skid and at the same time go around an obstacle - the wheels will not obey you. But if your car is equipped with an ABS system, then you don’t have to worry - feel free to press the brake pedal all the way and continue to maneuver.

It is important to understand that your mind adapts to this system, and it is very difficult to get used to the ABS not working. It is for this reason that it is necessary to immediately diagnose and repair the anti-lock braking system at the first sign of a malfunction.

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