(a) the basic contents of diagnosis and inspection
Specific diagnosis and inspection can find the fault of ABS system in time, which is a very important part of maintenance. For different models, even the same series of models produced in different years, the inspection methods and procedures will be different, which can be known by comparing the corresponding maintenance manuals. However, the contents of the basic diagnosis and inspection methods of ABS system are unchanged, and generally include the following four steps:
(1) Preliminary inspection
(2) Fault self-diagnosis
(3) Quick inspection
(4) Fault indicator lamp diagnosis
In general, as long as the diagnosis and inspection are carried out according to the above four steps, the fault point of ABS system can be found soon. Fault self-diagnosis is a fast and automatic fault diagnosis method provided to maintenance personnel after installing electronic control unit in automobile, which plays an extremely important role in the whole diagnosis and inspection. The self-diagnosis method will be introduced in detail later.
(2) The basic contents of repair
After diagnosis and inspection, once the fault position in ABS system is accurately determined, it can be adjusted, repaired or replaced until the fault is eliminated. The repair steps are usually as follows.
(1) Release the pressure in the ABS system.
(2) Adjust, disassemble, repair or replace the faulty parts, and finally install them. All this should be done in accordance with the corresponding regulations.
(3) deflate according to the specified steps.
If the wheel speed sensor or electronic control unit is faulty, the first and third steps can be omitted. Just adjust and replace the sensor according to regulations, and the ABS electronic control unit can only be replaced if it is damaged.
(III) Precautions for ABS maintenance
(1)ABS system is inseparable from ordinary braking system. When there is a problem with the ordinary braking system, the ABS system will not work normally. Therefore, we should maintain them as a whole, instead of just focusing on sensors, electronic control units and hydraulic regulators.
(2)ABS electronic control unit is very sensitive to overvoltage and static electricity. A little carelessness will damage the chip in the electronic control unit, leading to the paralysis of the whole ABS. Therefore, when the ignition switch is turned on, do not plug and unplug the connector on the electronic control unit; Before welding in the car, wear an antistatic device (one end of the wire can also be wrapped around the wrist and the other end around the car body), and unplug the connector on the electronic control unit before welding; When the battery is specially charged, the battery should be removed from the vehicle or the battery cable should be removed before charging.
(3) Be very careful when repairing the wheel speed sensor. When unloading, be careful not to collide with the sensor head, and do not use the sensor gear ring as a pry surface to avoid damage. Anti-rust oil should be coated before installation, and knocking or brute force is not allowed during installation. Generally speaking, the air gap of the sensor is adjustable (or not). Non-magnetic plugs, such as plastic or copper plugs, can be used for adjustment, and of course paper can also be used.
(4) When repairing the ABS hydraulic control device, remember to release the pressure first, and then repair it according to regulations. For example, the pressure accumulator of integrated ABS, brake master cylinder and hydraulic regulator are designed together, and the pressure stored in the pressure accumulator is as high as 18000kPa, which should be completely drained before maintenance to prevent high-pressure oil from spraying out and injuring people.
(5) Replace the brake fluid at least once every two years, preferably once a year. This is because DOT3 ethylene glycol brake fluid has strong hygroscopicity. The brake fluid containing water not only corrodes the interior of the braking system, but also significantly reduces the braking effect and affects the normal work of ABS. Be careful not to use DOT5 silicone brake fluid. Replacement and storage of brake fluid and appliances should be kept clean. Do not let dirt and dust enter the hydraulic control device. Do not touch the ABS electronic control unit and wires. Finally, it should be deflated in a prescribed way (different from ordinary braking system).
Second, the diagnosis and inspection of ABS system
(1) Preliminary inspection
Preliminary inspection is the first inspection method when the ABS system can't work normally due to obvious faults, such as when the ABS fault indicator light is on and the system can't work. The inspection method is as follows:
(1) Check whether the parking brake (handbrake) is completely released.
(2) Check whether the brake fluid level is within the specified range.
(3) Check whether the wire plug and socket of ABS electronic control unit are well connected and whether the connector and wire are damaged.
(4) Check whether the following wire connectors (plugs and sockets) are in good contact with wires:
① connector of solenoid valve body on hydraulic regulator;
② Main control valve connector on hydraulic regulator;
③ Connectors connecting the pressure alarm switch and the pressure control switch;
④ Brake fluid level indicator switch connector;
⑤ Four-wheel speed sensor connector;
⑥ Electric pump connector.
(5) Check whether all relays and fuses are in good condition and plugged firmly.
(6) Check whether the battery capacity (measuring the specific gravity of electrolyte) and voltage are within the specified range; Check whether the positive and negative leads of the battery are firmly connected and whether the connectors are clean.
(7) Check whether the grounding (grounding) terminals of ABS electronic control unit and hydraulic control device are in good contact.
(8) Check whether the groove depth of wheel tread meets the requirements.
If the above method cannot determine the fault location, you can switch to fault self-diagnosis.
(B) ABS system fault symptom simulation test method
In the fault detection and diagnosis of ABS system, if only the components are faulty, the circuit detection method can be used for diagnosis. If it is intermittent failure or related mechanical problems, simulation test and dynamic test are needed.
1, simulation test method
(1) jack up the car so that all four wheels are suspended.
(2) Start the engine.
(3) Turn the shift control handle to the forward position (D) and observe whether the ABS fault indicator light on the instrument panel is lit. If the ABS fault indicator is on, it indicates that the speed sensor of the rear wheel differential is faulty.
(4) If the ABS fault indicator is not lit, turn the left front wheel. At this time, if the ABS fault indicator is on, it means that the left front wheel speed sensor is normal; On the other hand, if the ABS fault indicator does not light up, it means that the left front wheel speed sensor is faulty.
(5) The test method of the right front wheel speed sensor is the same as that of the left front wheel speed sensor.
According to the speed signal difference of logic circuit in ABS ECU and the characteristics of alarm circuit, simulation test is set up to facilitate the detection of speed sensor fault.
2. Dynamic test method
(1) Drive the car on the road for at least 12km.
(2) Test whether the ABS fault indicator lights up when the vehicle turns (turns left or right). If the ABS fault indicator light in a certain direction is on, it means that the tire pressure in that direction is insufficient, or it may be due to poor bearing, worn steering rod ball head, poor shock absorber or poor pulse gear of speed sensor.
(3) Drive the car back, and indirectly connect the test line and multimeter (within the voltage range) to the terminals of "ABS power supply" and "solenoid valve relay" on the ABS ECU side.
(4) When driving on the road again, pay attention to the voltage between the "ABS power supply" terminal and the ground during braking, which should be between11.7 ~13.5 V; The voltage between the terminal of solenoid valve relay and the ground should also be above10.8V. The former mainly observes the supply of battery power, while the latter mainly observes the contact quality of solenoid valve relay.
(3) ABS system fault diagnosis table
When fault detection and diagnosis of ABS system are carried out, the fault phenomena and characteristics should be analyzed according to the working characteristics of ABS system, and after confirming the fault symptoms, the detection and diagnosis should be carried out purposefully according to the description of maintenance data. In order to detect, diagnose and find the fault of ABS system, it is necessary to know the installation position of the main components of ABS system on the vehicle first.
1, ABS system failure phenomenon
According to the working principle of ABS system, in the working process of ABS system, there will be some deviations from traditional experience, some of which are normal reactions of ABS system, not fault phenomena, and should be distinguished, such as:
① After the engine is started, stepping on the brake pedal may bounce, indicating that the ABS system has played a role; On the contrary, when the engine is turned off and the brake pedal is pressed, the pedal will sink slightly, which means that the ABS system stops working, which is normal.
② When you turn the steering wheel while pressing the brake pedal, you can feel a slight vibration, which is not a fault. Because when the vehicle turns, the working cycle of ABS system begins, which will bring slight vibration to the wheels and then transfer it to the steering wheel, forming a sense of vibration.
(3) When the car is braking, the brake pedal will sink slightly from time to time, which is a normal phenomenon caused by the change of road adhesion coefficient, not a fault.
(4) When driving at a high speed, if you make a sharp turn or drive on an icy road, sometimes the ABS fault indicator light will light up, indicating that the wheels slip under the above working conditions, and the ABS system has a protective effect, which is also not a fault phenomenon.
The possible faults of ABS system include: wheel locking during emergency braking; When driving, or when the hand brake is released, the ABS operation fault operation indicator light is on; Poor braking effect, or abnormal ABS operation, etc.