Master Cylinder Assembly
The master cylinder assembly produces hydraulic pressure to operate the brakes when the brake pedal is pressed.
The assembly is attached to the front of the brake servo assembly, and comprises a cylinder containing two pistons in tandem. The rear piston produces pressure for the primary circuit and the front piston produces pressure for the secondary circuit. A brake fluid reservoir is installed on top of the cylinder. The reservoir is internally divided to provide an independent supply of fluid to each brake circuit, and so prevent a single fluid leak from disabling both primary and secondary brake circuits. Should a failure occur in one circuit, the remaining circuit will still operate effectively, although brake pedal travel and vehicle braking distances will increase. If the fluid level in the reservoir is too low, a float operated switch in the reservoir filler cap connects an earth to the instrument pack, which illuminates the brake warning lamp.
Brakes applied
When the brake pedal is pressed, the output rod in the brake servo assembly pushes the primary piston along the cylinder bore. This produces pressure in the primary pressure chamber which, in conjunction with the primary spring, overcomes the secondary spring and simultaneously moves the secondary piston along the cylinder bore. The initial movement of the pistons, away from the piston stops, closes the primary and secondary center valves. Further movement of the pistons then pressurizes the fluid in the primary and secondary pressure chambers, and thus the brake circuits. The fluid in the chambers behind the pistons is unaffected by movement of the pistons and can flow unrestricted through the feed holes between the chambers and the reservoir.
Brakes released
When the brake pedal is released, the primary and secondary springs push the pistons back down the bore of the cylinder. The rapid movement of the pistons cause partial vacuums to form in the pressure chambers, which opens the center valves and allows fluid to circulate unrestricted between the two hydraulic circuits and the reservoir. When the pistons reach the brakes off position, the center valves are held open by the piston stops.
ABS modulator schematic
The ABS modulator is a 4 channel unit that controls the supply of hydraulic pressure to the brakes in response to inputs from the SLABS ECU. The modulator is attached by three mounting bushes to a bracket on the LH inner front wing, and connected to the primary and secondary hydraulic circuits downstream of the master cylinder assembly. Three electrical connectors link the ABS modulator to the vehicle wiring.
Passages within the ABS modulator, separated into primary and secondary circuits, connect to the various internal components that control the supply of hydraulic pressure to the brakes:
• Shuttle valves and non return valves control the flow through the internal circuits.
• Shuttle valve switches, connected in series to the SLABS ECU, provide a brakes on/off signal.
• A damper chamber and restrictor are included in each circuit to refine system operation.
• Inlet and outlet solenoid valves control the flow to the individual brakes.
• An expansion chamber is connected to each circuit to absorb pressure.
• A return pump is connected to both circuits to provide a pressure source. The ABS modulator has three operating modes: Normal braking, ABS braking and active braking.
Normal braking mode
When the brake pedal is pressed, pressurized fluid from the master cylinder assembly moves the shuttle valves to open lines 'A' and close the shuttle valve switches. Pressurized fluid then flows through the open inlet solenoid valves to operate the brakes. The closed shuttle valve switches supply a brakes on signal to the SLABS ECU. If the SLABS ECU determines that EBD is necessary, it energizes the inlet solenoid valves for the brakes of one axle. The inlet solenoid valves close to isolate the brakes from any further increase in hydraulic pressure.
ABS braking mode
When in the normal braking mode, if the SLABS ECU determines that ABS braking is necessary, it energizes the inlet and outlet solenoid valves of the related brake and starts the return pump. The inlet solenoid valve closes to isolate the brake from pressurized fluid; the outlet solenoid valve opens to release pressure from the brake into the expansion chamber and the return pump circuit. The brake releases and the wheel begins to accelerate. The SLABS ECU then operates the inlet and outlet solenoid valves to control the supply of hydraulic pressure to the brake and apply the maximum braking effort (for the available traction) without locking the wheel.
Active braking mode
When ETC or HDC are enabled, and the SLABS ECU determines that active braking is necessary, it starts the return pump. Hydraulic fluid, drawn from the reservoirs through the master cylinder, shuttle valves and lines 'B', is pressurized by the return pump and supplied to lines 'A'. The SLABS ECU then operates the inlet and outlet solenoid valves to control the supply of hydraulic pressure to the individual brakes and slow the wheel(s).
SLABS ECU
The SLABS ECU is attached to a bracket behind the front passenger glovebox. Brake related inputs are processed by the SLABS ECU, which then outputs control signals to the ABS modulator. Five electrical connectors interface the SLABS ECU with the vehicle wiring.
The SLABS ECU continually calculates vehicle speed using the wheel speed inputs from all four ABS sensors. The calculated vehicle speed is then used as a reference against which individual wheel speeds are monitored for unacceptable acceleration or deceleration. The ABS sensor inputs are also used by the SLABS ECU to detect vehicle deceleration rate, vehicle cornering rate and rough terrain.
The engaged forward gear and (on manual gearbox models) the clutch status are computed from the engine data input, the engine speed input and vehicle speed. Reverse gear status is provided by an input from the reverse lamp switch (manual gearbox models) or the BCU (automatic gearbox models). On automatic models, the BCU also provides the neutral selected input.
In addition to controlling the brake related functions, the SLABS ECU:
• Controls the operation of the self leveling suspension (SLS) system (where fitted). REAR SUSPENSION, DESCRIPTION AND OPERATION, Description.
• On V8 models, outputs a rough road signal to the ECM when traversing rough terrain.
• Outputs a vehicle speed signal.
The vehicle speed signal is output to the following systems (where fitted):
• Active Cornering Enhancement. FRONT SUSPENSION, DESCRIPTION AND OPERATION, Description - ACE.
• Air conditioning. AIR CONDITIONING, DESCRIPTION AND OPERATION, Description.
• Cruise control. ENGINE MANAGEMENT SYSTEM - Td5, DESCRIPTION AND OPERATION, Description.
ENGINE MANAGEMENT SYSTEM - V8, DESCRIPTION AND OPERATION, Description - engine management.
• Engine management. ENGINE MANAGEMENT SYSTEM - Td5, DESCRIPTION AND OPERATION, Description.
ENGINE MANAGEMENT SYSTEM - V8, DESCRIPTION AND OPERATION, Description - engine management.
• In-car entertainment. IN CAR ENTERTAINMENT, DESCRIPTION AND OPERATION, Description.
• Instrument pack. Dam INSTRUMENTS, DESCRIPTION AND OPERATION, Description.
ABS sensors
The ABS sensors supply the SLABS ECU with a sinusoidal speed signal from each wheel. An inductive sensor, installed in the hub bearing of each wheel, senses off a 60 tooth exciter ring integrated into the inner race of the hub bearing. Each ABS sensor has a fly-lead connecting it to the vehicle wiring.
HDC switch
The HDC switch is a latching push switch installed on the fascia, in the switch pack inboard of the steering wheel. When pushed in, the switch connects an ignition supply to the SLABS ECU to initiate HDC.
Warning indications
The SLABS ECU operates audible and visual warnings to convey brake system status.
Audible warning
A repetitive chime, at a frequency of 2 Hz, draws attention to warning lamp indications. The chime is produced on the speaker in the instrument pack.