Antilock Brakes / Traction Control Systems: Description and Operation

The new XJ-S brake system combines normal system operation, power boost, and anti-lock braking. The previous vacuum servo has been eliminated completely and replaced with an independent electrically driven motor pump unit. Besides providing the obvious benefit of anti-lock braking, this new system provides a greater number of successive fully assisted stops. Also, hydraulic system recharge time is faster.

ABS Components:

A common operating fluid, DOT 4 BRAKE FLUID, is used for both power boost and brake application.

Anti-lock System:

In the XJ-S system, three hydraulic circuits are used-one to each front wheel and one to the rear wheels. Depending on the mode of operation, the circuits operate differently:

Normal Operation
The two front hydraulic circuits operate conventionally via the master cylinder assisted by the hydraulic booster. The rear hydraulic circuit is operated continuously by the controlled pressure in the booster via the pressure reducing valve. Simply stated, during normal operation, the front hydraulic circuits are "static" and the rear circuit is "dynamic".

Anti-Lock Operation
When anti-lock control is required, the front hydraulic circuits also become "dynamic" as boost pressure replaces master cylinder pressure via the main valve. The four wheel speed sensors input to the ECU, which processes the information and modulates the three sets of solenoid valves in the valve block to control the pressure in the three circuits and prevent wheel lock. Because both rear wheels share a common circuit, a tendency for one wheel to lock will result in control of both wheels according to the need of the "locking" wheel.

The state of the anti-lock system is continuously monitored by the ECU. If a failure is identified, it switches off the ABS system and lights the warning. Full boosted braking to all wheels is maintained if the ECU switches off the ABS system. If a failure of the hydraulic boost portion of the system occurs, the brake system will function on the front wheels only without boost pressure.


HYDRAULIC ACTUATION UNIT

The hydraulic actuation unit contains the reservoir and the components used for pressure application, boost application, and anti-lock valving.

Actuating Piston
The actuating piston transmits the motion from the brake pedal to the control valve and the boost piston.

Control Valve
The control valve opens and closes the high pressure line from the accumulator to the actuator hydraulic booster.

Hydraulic Booster
The boost piston is independent from the actuating piston and applies boost pressure on the master cylinder piston. Boost pressure is used to directly operate the rear brakes.

Master Cylinder
The master cylinder operates the front brakes only

Main Valve
The solenoid-activated main valve is opened under anti-lock conditions to apply boost pressure directly to the master cylinder and the front brake circuits.

Central Valve
The central valve opens under anti-lock conditions to allow boost pressure to be applied directly to the front brakes.

Positioning Sleeve
The positioning sleeve is used during anti-lock operation to prevent brake pedal pulsations.

Solenoid Valves
The six solenoid valves direct hydraulic pressure and hydraulic return in the three brake circuits during anti-lock conditions. During normal operation, the valves are at rest, allowing conventional application.

MOTOR/PUMP UNIT

The motor pump unit supplies the system operating hydraulic pressure.

Pump
The pump is driven by an electric motor and is switched on and off to maintain a range of operating pressure.

Accumulator
The accumulator stores the pump pressure and provides reserve for normal, anti-lock, and pump failure operations. A check valve prevents pressure loss.

Pressure Control Valve
The pressure control valve limits the maximum system pressure.

Combined Pressure Switch
Two sets of contacts in the pressure operated switch signal the ECU for activation of warnings and anti-lock shutdown. The other contacts switch the pump on and off.

PRESSURE REDUCING VALVE
Since the rear brakes operate directly off boost pressure, a pressure limiting device is necessary The pressure reducing valve is located in the rear brake circuit and limits the pressure applied to the rear brakes.

ABS Wiring Diagram:

ELECTRICAL CIRCUIT

The ABS electrical circuit is protected by two 30-amp fuses, which receive direct battery feeds. The system is switched on by the ignition switch via the ECU.

Electronic Control Unit
The ECU processes information from the wheel speed sensors and the various switches for system operation, control, and warnings.

Main Relay
The main relay provides the power for system operation. The control circuit is switched by the ECU from an ignition ON signal.

Pump Relay
The pump relay provides remote pump switching. The control circuit is ignition fed (via the load relay) and is switched by the pump contacts in the combined pressure switch.

Reservoir
The reservoir contains three fluid level operated reed switches. Two "normally open" switches light the BRAKE FAILURE warning if the level drops too low. The third "normally closed" switch signals the ECU if the level drops below a minimum level for full ABS operation. In this case, the ECU switches off the front wheel ABS and lights the ABS warning.

Combined Pressure Switch
The combined pressure switch contains three pressure-operated switches. The two "normally closed" switches open when normal operating pressure is reached, switching off the pump and the BRAKE FAILURE warning. The third "normally open" switch closes if the system pressure falls too low and signals the ECU, which lights the ABS warning.

Main Switch
The main switch activates the main valve on signal from the ECU.

Diagnostic Socket
The diagnostic socket is provided for conducting system self tests.

Brakes Not Applied:

SYSTEM OPERATION: BRAKES NOT APPLIED

Control Valve
The control valve is open to the reservoir and closed to accumulator pressure from the motor pump unit.

Pump
The pump is switched on or off as determined by system pressure.

Accumulator
The accumulator stores boost pressure for use as soon as the brakes are applied.

Boost Piston
The boost piston is retracted.

Positioning Sleeve
The positioning sleeve is at rest.

Master Cylinder
The master cylinder is at rest.

Main Valve
The main valve is switched off.

Equalized Hydraulic Pressure
The hydraulic pressure is equalized throughout the system (except in the accumulator to control valve line).

Brakes Applied - Normal Operation:

SYSTEM OPERATION: BRAKES APPLIED - NORMAL OPERATION

Control Valve
As the driver applies force to the brake pedal, the actuating piston moves forward. The lever mechanism moves the control valve forward, opening the port from the accumulator and closing the return port to the reservoir. Boost pressure is applied to the boost piston and the rear brakes.

Boost Piston
The boost piston increases the pedal force acting on the master cylinder piston.

Main Valve
The main valve remains at rest, connecting the master cylinder to the reservoir.

Central Valve
As the master cylinder piston moves forward, the central valve closes and pressure is built up in the front brake circuits.

Positioning Sleeve
The positioning sleeve moves forward with the boost piston. The displaced fluid returns to the reservoir through the main valve, which is at rest.

Pressure Reducing Valve
Boost pressure is applied to the rear brakes through the pressure reducing valve.

Solenoid Valves
All solenoid valves are at rest, allowing pressure application and preventing return to the reservoir.

Boost Pressure Control
As the boost pressure increases, it acts between the boost piston and the actuating piston, moving the actuating piston back, which moves the control valve to close the port from the accumulator. The control valve closes the accumulator pressure port when the pedal force and boost force acting on the actuating piston equalize. The return port to the reservoir remains closed. The resulting pressure in the booster is proportional to the pedal force; low pedal force=low pressure; high pedal force=high pressure.

"Dynamic" , "Static"
During normal operation, boost pressure is applied directly to the rear brake circuit, thus the description "dynamic." The front brake master cylinder is supplied with reservoir gravity pressure only, thus the description "static."

Brakes Applied - ABS Operation:

SYSTEM OPERATION: BRAKES APPLIED - ANTI-LOCK OPERATION

Main Valve
When the ECU senses the need for ABS control it activates the main switch, which moves the main valve, closing the reservoir feed line and applying boost pressure to the master cylinder circuit.


Central Valve
The central valve is held open by the pressure allowing boost pressure to be applied directly to the front brake circuits.

Positioning Sleeve
The positioning sleeve is gradually moved to its rest position by the boost pressure. This action pushes back the boost piston, the actuating piston, and the brake pedal, preventing ABS pulsations from being transmitted to the brake pedal.

Solenoid Valves
The solenoid valves are modulated as necessary by the ECU to prevent wheel lock.

"Dynamic"
During ABS operation both the front and rear brake circuits are "dynamic" as the front brakes are also operated directly by boost pressure.

SOLENOID VALVE OPERATION

Solenoid Valve Operation - Phase One:

Phase One

Solenoid Valve Operation - Phase Two & Three:

Phase Two