HYDRAULIC BRAKE BOOSTER



FEATURES


A hydraulic brake booster system (HBB) has been adopted to improve the braking performance.




CONSTRUCTION DIAGRAM





SYSTEM CONFIGURATION DIAGRAM



FLUID PRESSURE DIAGRAM


The master cylinder hydraulic pressure for the front brakes is assisted by a brake booster which utilises accumulator pressure; the hydraulic pressure generated by the accumulator is led directly to the rear brakes.



ELECTRICAL CIRCUIT DIAGRAM





MAJOR COMPONENTS AND FUNCTION


Name of part
Outline of function
ASTC-ECU
Controls the HBB system.
HBB buzzer and brake warning lamp
Operates and illuminates in response to signals from the ASTC-ECU to notify the driver of any abnormalities in the HBB system.
Pump motor
The pump motor pumps up brake fluid from reservoir and charges high-pressure brake fluid to the accumulator.
Accumulator
The accumulator is pressurised with nitrogen gas and accumulates the hydraulic pressure which is generated by the pump motor.
Switching solenoid valve (built into hydraulic unit)
During the ABS control, the regulator pressure is introduced to the wheel cylinder. During the Active Stability Control/Active Traction Control, the accumulator pressure is introduced to the wheel cylinder, increasing the pressure automatically.
Control solenoid valve (built into hydraulic unit)
Controls (increase, maintain, or decrease) the wheel cylinder hydraulic pressure when ABS operates and when Active Stability Control/Active Traction Control operate.
Pressure switch (built into hydraulic unit)
Two switches are included: a switch for pump control and a switch for low-pressure warning control. The former switch outputs a pump motor drive signal when the accumulator hydraulic pressure drops below the specified hydraulic pressure. If the accumulator pressure drops further, the low-pressure warning control switch turns off so that a warning lamp illumination signal and an HBB buzzer operating signal are output.
Relief valve (built into hydraulic unit)
This is a mechanical valve which is used to prevent excessively high pressure from building up in the power supply system. If the pump motor keeps operating continuously because of a problem such as a pressure switch malfunction, this valve allows excess brake fluid to be returned to the reservoir.
Master cylinder
The master cylinder is a single-piston type which generates hydraulic pressure which is transmitted to the front brakes.
Brake booster
Converts the accumulator high pressure into a pressure which corresponds to the brake pedal depression force, and transmits this to the rear brakes, and also to the booster chamber to generate the brake assist force.
Reservoir
Stores the brake fluid for the master cylinder and the power supply.
Brake fluid level warning switch (built into reservoir)
When the level of brake fluid inside the reservoir drops, this switch turns on and causes the brake warning lamp to illuminate.

PUMP MOTOR, ACCUMULATOR




An electric pump motor (piston type) has been adopted. The accumulator is pressurised with nitrogen gas and collects the hydraulic pressure which is generated by the pump motor.
When the accumulator hydraulic pressure drops to approximately 16 MPa or below, the pressure switch (for pump control) turns off, and a pump motor drive signal is output from the ASTC-ECU to make the pump motor operate. When the pump motor operates, it pressurises the brake fluid, which is discharged through the check valve to be stored in the accumulator. The operation described above generates the hydraulic pressure (high pressure) which is required for normal brake operation and also HBB, EBD, ABS and ASC/ATC operation. When the accumulator hydraulic pressure drops to approximately 11 MPa or below, the pressure switch (for low pressure warning) turns off, and this causes the brake warning lamp to illuminate and the HBB buzzer to sound. Then, when the accumulator hydraulic pressure rises to approximately 17 MPa, the pressure switch (for pump control) turns on, and after approximately 1 second, the pump motor stops opening. If there is a malfunction of the pressure switch at this time, the release valve opens when the pressure reaches approximately 24 MPa to prevent excessive pressure from building up.

CONTROL SOLENOID VALVES AND SWITCHING SOLENOID VALVES


This is basically the same as for the previous PAJERO.

PRESSURE SWITCH




There are two types of pressure switch: one for pump control and one for low pressure warning control. They are both built into the hydraulic unit.


MASTER CYLINDER, BRAKE BOOSTER






BOOSTER OPERATION





  1. When the pedal operating force is transmitted in turn by the operating rod, the power piston and the master cylinder piston, the regulator piston and the spool valve move to the left.
  2. When the spool valve opens line A and closes line B, the accumulator pressure is supplied to the rear brakes. This pressure is introduced to the booster chamber and provides power assistance to move the master cylinder piston to the left.
  3. Then, when the pressure in the booster chamber increases, the pressure which is applied to the rear (left side) of the reaction disc also increases, and the reaction disc starts to change shape. The reaction disc contacts the reaction rod, and the reaction rod in turn contacts the spool valve, so that the master cylinder pressure being applied towards the left and the booster pressure being applied to the right are brought into balance by the reaction disc. This allows the power-assist hydraulic pressure generated by the pedal operating force to be kept at an equilibrium.

<When a loss of accumulator pressure occurs>


If a loss of accumulator pressure occurs because of a problem such as a pump motor problem, the power assist force and rear brake pressure cannot increase in response to the input from the brake pedal. Accordingly, the front brakes can be operated directly by depressing the brake pedal, but the rear brakes do not operate at all.

RESERVOIR, BRAKE FLUID LEVEL WARNING SWITCH




The reservoir is equipped with brake fluid supply ports for the pump motor and for the master cylinder and brake booster, and return ports for returning the brake fluid from the booster and the various solenoid valves. In addition, the brake fluid level warning switch which is built into the reservoir detects the brake fluid level, and causes the brake warning lamp to illuminate when the fluid level drops below a specified level.

FAIL-SAFE FUNCTION


When ASTC-ECU detects the HBB system failure, the brake warning lamp illuminates. If the failure which decreases the brake assist force occurs with this state, the lamp illuminates and the HBB buzzer sounds to alert.

DIAGNOSIS FUNCTIONS



ASTC-ECU has the following functions to ease the system inspection.

All of the above-mentioned items can be tested by the M.U.T.-III.

Diagnosis code set


There are eight diagnostic items on the HBB system. Because all the diagnostic results are recorded in volatile memory (EEPROM*), they are stored in the memory even though the battery terminals are disconnected.
note *: EEPROM=Electrical Erasable Programmable ROM (a ROM in which the data is written electronically and erased electronically)

Diagnosis items
Diagnosis code No.
Item
16
Abnormal power-supply voltage.
Low voltage.
High voltage.
31
Abnormal IG2B power-supply voltage drop.
53
Pump motor system problem.
Pump motor lock.
Current detection circuit problem.
54
Motor relay system problem.
Relay open.
Relay short.
Coil problem.
55
Pomp motor long-term operation.
56
Pressure switch problem.
PH open.
PL open or short.
57
Accumulator low pressure.
Pomp problem.
Accumulator problem.
58
ASTC-ECU problem.

DIAGNOSIS CODE READING PROCEDURE


The diagnosis codes can be read using the M.U.T.-III or by the flashing of the ABS warning lamp or brake warning lamp (Refer to Workshop Manual).

DIAGNOSIS CODE MEMORY ERASING PROCEDURE


Diagnosis codes can be cleared from memory by using the M.U.T.-III (Refer to Workshop Manual).

ACTUATOR TEST


The following actuators can be forcibly operated using M.U.T.-III.
note
  • When the brake warning lamp or the ABS warning lamp illuminates by the system malfunction, the actuator test is not available.
  • The actuator test can be performed only when the vehicle is stationary. When the vehicle speed reaches approx. 10 km/h, the forcible actuator operation is disabled.

ACTUATOR TEST SPECIFICATIONS
Item No.
Inspection item
Drive contents
11
HBB pump motor
Operate the pump motor for one second.
12
HBB buzzer
Sound the buzzer for three seconds.
23
Brake warning lamp
Make the brake warning lamp flash twice.