HYDRAULIC BRAKE BOOSTER

M23500000073ENG0000010000

FEATURES

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

With having the independent power source (pump motor), the system has the following features.

Has its own power source, so that a stable power supply can be maintained at all times regardless of the engine condition.

The need for a negative pressure line from the engine has been eliminated.

With the hydraulic assist, the system has the following features.

Quick response to emergency braking.

Compared to the vacuum brake booster which requires large capacity, it is smaller in size.

The master cylinder, the reservoir and the hydraulic unit are integrated as one unit for fewer pipes.

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)	Switches the brake hydraulic circuit when ABS operates and Active Stability Control/Active Traction Control operate.
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 16.1 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 10.8 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 18.3 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.

The pressure switch for pump control outputs a low pressure signal (switch: OFF) to the ASTC-ECU to cause the pump motor to operate when the hydraulic pressure drops to approximately 16.1 MPa or lower. In addition, when the hydraulic pressure is high (approximately 18.3 MPa or higher), the pressure switch outputs a high pressure signal (switch: ON) to the ASTC-ECU to cause the pump motor to stop operating.
The pressure switch for low pressure warning control outputs a low pressure signal (switch: OFF) to the ASTC-ECU to cause the brake warning lamp to illuminate and the HBB buzzer to sound when the hydraulic pressure drops to approximately 10.8 MPa or lower. In addition, when high hydraulic pressure is being restored (approximately 12 MPa or higher), the pressure switch outputs a high pressure signal (switch: ON) to the ASTC-ECU to so that normal hydraulic pressure is transmitted.

MASTER CYLINDER, BRAKE BOOSTER

The master cylinder piston is a single-type piston with a center valve, and it controls only the front brakes.
The brake booster has an integrated structure which envelopes the master cylinder at the front and rear. The front section consists of a regulator piston, a return spring, a spool valve for switching the brake fluid line, a reaction rod and a reaction disc which changes shape in response to the pressure in that order, and the rear section consists of an operating rod, a power piston and a booster chamber in that order.
The operating rod and the power piston are directly linked, and the regulator piston which transmits the pedal operating force and the spool valve are also directly linked.
The force from the master cylinder is applied to the regulator piston in the forward direction (to the left in the illustration), and the power assist force from the booster is applied to the regulator piston in the reverse direction (to the right in the illustration). This produces a balance between the two via the reaction disc, so that a hydraulic pressure is generated which is proportional to the pedal depression force.
The regulator piston is equipped with a return spring to maintain the spool return force when there is no power assist force being applied by the booster.

BOOSTER OPERATION

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.
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.
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.

Diagnosis code output
Actuator test

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 Programable 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
01	HBB pump motor	Operate the pump motor for one second.
02	HBB buzzer	Sound the buzzer for three seconds.