General description of the system
ABS/TCS/ESP systems include several functional blocks and provide the volumetric pressure flow required to vary the braking torque in response to wheel slip information received from the wheel speed sensors. Traction braking system (TCS) is designed to create a moment of stability on the driven wheel that rotates with the greatest slip or to reduce slip on both driven wheels, thus optimizing the traction of the other driven wheel or driven wheels. Electronic Stability Program (ESP) ensures the stability of the car when the car deviates from the specified angle of rotation. This system is designed to bring the vehicle's yaw movement in line with the driver's intentions. In addition to the signals generated by the wheel speed sensors used to control the ABS/TCS systems, signals from the lateral acceleration sensor, yaw rate sensor, steering angle sensor and pressure sensor are also required as input parameters to control the ESP system. In addition, these systems may include an electronic brake force distribution system (EBD) to adjust the effective braking force on the rear wheels instead of the traditional proportional valve.
MK25E is a housing with solenoid valves, built-in pump and ECU controller. Each brake circuit consists of a pair of valves (normally open intake valve / normally closed exhaust valve), providing a change in the brake pressure on the front wheels (individual wheel control) and on rear wheels (it is necessary to select a low level of control or stability control using the ABS plus system) in response to appropriate electrical signals.
The TCS ICU system consists of MK25E ABS with additional elements in the valve block to intervene in braking when the TCS system is operating and active yaw control (clause 3.1). Shut-off valves with built-in relief valve in the TCS valve block provide an increase in brake pressure to control the TCS / ESP systems with a pump, bypassing the actuation block. With electric selective valves (ESV) pump inlet is switched from low pressure accumulators to TMC system. The intake chokes optimize pump inlet performance and also reduce the impact of switching at the end of the TCS control cycle.
Required basic knowledge
Before you begin this section, you should have the necessary knowledge in the following areas. Without this basic knowledge, the use of the diagnostic procedures described in this section will be difficult.
- Basic knowledge of electrical circuits: You must know the basic theory of electricity and understand the meaning of terms such as voltage (volts), current (amps) and resistance (ohms). You need to know what happens in an electrical circuit when a wire breaks or shorts. You must be able to read electrical schematics.
- Using Electrical Circuit Testers: You must be able to use a digital voltmeter correctly. You should be able to measure voltage, resistance and current. You must know how to use the jumper wire as a bypass jumper when testing electrical circuits.
ABS system components
The MK 25E ABS anti-lock braking system consists of a conventional hydraulic braking system and anti-lock components. A typical brake system includes a vacuum booster, a master cylinder, front disc brakes, rear drum brakes, brake connecting pipes and hoses, a brake fluid level sensor, and a brake warning light.
ABS includes hydraulic unit, HECU (hydraulic block controller), system fuse, four wheel speed sensors (one for each wheel), connecting wires, ABS indicator, DDRP indicator (system of separate dynamic distribution of brake forces on the rear wheels), connected to the parking brake warning light, and the rear drum brakes. Cm. "Location of ABS components" in this section.
The hydraulic block, together with the EBCM brake controller attached to it, is located between the expansion tank and the bulkhead separating the engine compartment from the passenger compartment on the left side of the car.
The hydraulic block is mainly composed of stop valves, two solenoid valves for each wheel, hydraulic pump, two accumulators. To prevent the wheels from locking up, the hydraulic unit regulates the fluid pressure supplied to the front and rear wheel brakes.
Location of function blocks
Valve distribution
Only shut-off valve and electric selective valve are activated (ESV) for the controlled wheel. When checking both wheels on one side of the vehicle, both shut-off valves / ESVs are activated. Shut-off valves must be permanently switched on. Details of the valve distribution during the stages of increase, hold and decrease in pressure are indicated in the table.
|
|
normally open valve
|
Normally closed valve
|
Shut-off valve
|
ESV valve
|
|
Pressure holding phase
|
Closed
|
Closed
|
Closed
|
Closed
|
|
Pressure build-up phase
|
cliff
|
Closed
|
Closed
|
cliff (1)
|
|
Decrease phase
|
Closed
|
cliff
|
Closed
|
Closed
|
(1) The controlled wheel ESV must open at the same time as the normally open valve, but remain open 14ms longer than the normally open valve.
HECU (hydraulic block controller)
The HECU controller performs the following main functions:
in order to ensure effective braking and vehicle stability. In the DDRP system, the rear pressure maintaining solenoid is powered by the ignition system. The red brake warning lamp lights up when the following faults occur.
- Controls wheel speed sensor inputs.
- Detects the approach of wheel slip.
- Controls the braking system in anti-lock mode.
- Controls the health of the electrical elements of the system.
HECU continuously senses the speed of each of the wheels to determine the start of slippage. If a wheel slip tendency is detected, the HECU commands the appropriate valves to change the brake fluid pressure in some or all of the hydraulic circuits, thus eliminating wheel slip and providing optimal braking. The HECU continues to monitor the pressure in the hydraulic circuits until the tendency to slip has disappeared. The HECU also continuously receives a signal from the steering angle sensor for optimal ABS performance. That's why "steering angle sensor centering" should be performed after replacing the HECU. If an error is detected, the HECU can disable the ABS and turn on the ABS warning lamp on the instrument cluster. The HECU also monitors the display of DTCs when operating in diagnostic mode.
Solenoid valve relay
The solenoid valve relay supplies power to the pump motor and to the solenoid valves. The contact present in the relay is normally open but closes on command given during initialization. This relay contact will remain closed for the remainder of the trip until any DTC is stored in memory that requires this contact to open. If a DTC is stored in memory that requires a command to turn off the relay, then the supply of voltage from the battery to the pump motor and solenoid valves will stop for the rest of the trip and the ABS will not be able to function. The relay is an integral part of the HECU and is not serviced separately.
Wheel speed sensors and gear rings
There is a wheel speed sensor on each wheel. The sensors provide wheel speed information to the HECU using low voltage AC signals. The signal is transmitted to the HECU via an interface that can cause the HECU to receive false signals from the speed sensors, caused, in particular, by the influence of electromagnetic interference.
ABS warning lamp
The ABS warning lamp is located on the instrument cluster and illuminates when the HECU detects an ABS malfunction. The ABS warning lamp informs the driver about the presence of a malfunction that caused the ABS to turn off. If only the ABS warning light is on, then braking can be performed normally, including the use of the booster.
The ABS warning lamp lights up in the following cases.
- ABS fault detected. As already noted, the ABS warning lamp lights up when a malfunction is detected in the ABS.
- Checking the incandescent lamp of the instrument cluster. When the ignition is switched on, the ABS warning lamp comes on for approximately three seconds and then goes out.
Brake warning lamp
The red brake warning light is located on the instrument cluster and illuminates to alert the driver to a brake system problem that could cause poor brake performance. The lamp will be on when the parking brake is applied or partially released, or if the brake fluid level sensor closes (if the brake fluid level in the master cylinder reservoir is too low). When the brake fluid level sensor is closed (too low level) The brake warning light will stay on until the cause of the fire is removed. In addition, with some system malfunctions, the light will also come on, letting the driver know that the DDRP system (system of separate dynamic distribution of brake forces on the rear wheels) disabled
Electronic Stability Program (ESP)
When this system is in operation, braking forces are applied to the corresponding wheels of the vehicle, which create a moment that counteracts the yaw movement. This means that a certain pressure is applied to one or both wheels on the left side of the vehicle if the yaw movement is clockwise, and to the contours of the wheels on the right side of the vehicle if the yaw movement is counterclockwise.
When the brakes are applied during ESP operation, the pressure in the wheel circuits, in which normally open valves are closed to maintain the absence of pressure on the wheels, rises to a level corresponding to the pressure in the brake master cylinder (TMC) using the EBCM system, which opens the corresponding intake (normally open) valves (electrical brake control). The level of pressure on the controlled wheels also rises within the physical limits to maintain yaw momentum, despite the increase in pressure in the brake system.
The brake pressure in the TMC cylinder causes a corresponding increase in the opening pressure of the shut-off valves (TMC cylinder pressure works to close shut-off valves where the pressure is not balanced). As a result, the pump raises the system pressure by the amount of the TMC cylinder pressure, which can lead to an unacceptably high level of pressure in the hydraulic control unit (HCU) until appropriate countermeasures are taken.
For this reason, the shut-off valves are opened by the EBCM when the TMC cylinder pressure reaches 100 bar in order to limit the pressure. A further increase in pressure in the system using the pump becomes impossible. Thus, the pressure for the operation of the ESP system is supplied only by the TMC cylinder. As with ABS/TCS operation, the pressure in the wheel circuits is varied by means of the intake and exhaust valves. During the depressurization phase of the duty cycle, the ESVs close so that the diverted brake fluid can be returned to the brake system from the low pressure accumulators.
EBD (electronic brake force distribution)
The EBD system complements the effectiveness of the ABS system by controlling rear wheel slip in the braking range, thus optimizing the braking performance. The braking force approaches the maximum braking force of the rear axle and is electronically controlled.
When entering the EBD system, the pressure supply to the rear axle is shut off through the normally open valves when the slip of at least one rear wheel exceeds a predetermined limit. Depending on the actual slippage of the wheels, this may be followed by additional signals to increase the pressure. Exhaust valves are only used to reduce pressure when a blockage is detected on the rear axle. As a rule, the control algorithm is aimed at achieving maximum use of braking force with minimum valve action (noise, pedal response). Since the discharged volumes of liquid are generally very small, they can be stored in a low pressure accumulator. The HCU pump is not required for the EBD system to work. At the end of the EBD system control, the brake fluid is discharged from the low-pressure accumulators into the brake fluid reservoir by briefly switching on the pump.
HBA (auxiliary hydraulic brake)
In the event of a very rapid increase in pressure in the master cylinder of the brake system, an emergency braking mode is recognized and the auxiliary hydraulic brake function is activated (HBA). The hydraulic pump and shut-off valves of both hydraulic circuits are switched on. After that, the pressure on the wheels becomes higher than the pressure of the master cylinder. The pressure compensation feature of this HBA brake design provides a certain increase in braking force by applying additional pressure when the HBA is operating instead of constantly orienting the system to the ABS. This compensation depends on the pressure gradient achieved during the operation of the HBA, subject to the following rule: the faster the driver applies the brake, the more support, i.e. he is compensated.
If the tire pressure reaches the lock pressure, then the normal ABS function controls the stability of the wheels by switching (normally open) intake and (normally closed) exhaust valves. Thus, actuation of the HBA has the same effect as if the driver applied the brakes with more pedal force, sufficient to achieve the lockout pressure.
If pedal effort is significantly reduced, then the HBA function categorizes this as the driver's intention to complete full braking. Therefore, at the end of HBA operation, pressure equalization occurs between the wheel brakes and the master cylinder. This is achieved by intermittent opening of shut-off valves.
DSC (hill descent control system)
The DSC system is a speed control loop for off-road vehicles and is typically used on steep slopes. The system can only work in first or reverse gear and with the gas pedal fully released. The programmed control speed is fixed and cannot be changed by the driver. Pressure is controlled on all four wheels by actuating ESVs, shut-off valves, and a pump.