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ABS Description and Operation

This vehicle is equipped with a TRW ABS/SCB/TC/VSC module.

This module provides the following vehicle performance enhancement systems:

The following components are involved in the operation of the above systems:

The slip control boost system is a hydraulic braking system which is capable of supplying boosted braking, slip control functions, and regenerative brake blending.

The system is made up of two major assemblies, the master cylinder assemble (no vacuum booster is used), and the electro-hydraulic control unit. Also included are various internal and external sensors used for system control and fault isolation.

The master cylinder assembly is made up of a reservoir and a master cylinder with primary and secondary pistons.

The electro-hydraulic control unit is used to electronically control pressure at the wheels. A pump and accumulator supply hydraulic brake fluid pressure to perform braking applications. A proportional supply valve controls the flow and pressure of fluid from the accumulator to the brakes. Standard digital slip control valves and, in some cases, proportional versions of them, are used to control pressures at the brakes independently for slip control. The regenerative axle isolation valves along with the proportional relief valve are used in regenerative brake blending situations. A simulator, consisting of a plunger, spring, orifice, and check valve provide the force and travel feedback to the driver through the master cylinder. There are four internal pressure transducers, master cylinder pressure, high pressure accumulator, boost pressure, and regenerative axle circuit pressure sensors.

General Operation

The driver applies the pedal and thereby builds pressure in the primary chamber of the master cylinder with the primary piston. The fluid from the primary chamber is transmitted from the master cylinder by the brake tube to the electro-hydraulic control unit through the orifice and check valve and into the simulator chamber. With the slip control boost system active and working normally, the fluid from the primary master cylinder chamber does not get applied to the wheel brakes but to the simulator. The pedal simulator spring, the orifice, and the check valve combine to affect a force/travel characteristic that feels smooth, normal, and controllable to the driver. A pressure transducer on the primary master cylinder chamber/pedal simulator (Master Cylinder Pressure), a pedal travel sensor on the input piston assembly, and the brake switch are inputs into the electronic control unit. These inputs are used to determine the driver's braking intent which is used to control the torque applied to the wheels.

Base Brakes (Friction Braking)

The braking torque applied to the wheels in proportion to the driver's braking intent is achieved with a combination of regenerative braking from the powertrain and hydraulically applied friction braking controlled by the electro-hydraulic control unit. The friction braking is achieved by the electro-hydraulic control unit controlling a combination of valves in a way that allows accumulator pressure to be used to actuate the wheel brakes. The accumulator is maintained at pressure during normal operation by the pump and motor, independent of the brake being applied. The pressure built in the braking circuits by the electro-hydraulic control unit by means of the boost valve is applied directly to the rear brakes. The front circuit pressure, however, are applied to the master cylinder secondary pistons, and they in turn apply pressure to the front while brakes. The slip control boost system is designed and used with its electro-hydraulic control unit and master cylinder primary and secondary pistons configured as they are for fail-safe purposes.

Fail-Safe System Operation (Four or Two Wheel Push Through)

With the system active, and working normally, the normally open and normally closed valves become active when the driver steps on the pedal, and friction braking is desired. This opens up the simulator to the fluid input from the master cylinder, and allows the boost valve to build pressure in the boost circuit that will actuate the wheel brakes. If the brake is applied with no power, or if the system detects a critical failure, these valves are not energised, resulting in a fail-safe system operation (push through) that allows the driver to push primary chamber fluid directly into the braking/boost circuits of all four wheels (four wheel push through). If a hydraulic failure occurs in the boost circuit a further pedal input will result in the primary piston of the master cylinder directly contracting the secondary pistons and applying pressure to the front hydraulic circuits (two wheel push through). Slip control functions are done by controlling the four isolation valves and the four dump valves in a manner common to how slip control functions are done in standard systems and is not described here.

Regenerative Brake Blending

Regenerative brake blending is done by using the driver braking intent information to request a powertrain braking torque, by receiving feedback about how much powertrain torque is applied, and controlling wheel brake pressures to fulfill the braking desired along with the powertrain regenerative pressure applied. To maximise the regenerative powertrain energy recovery during braking (while still maintaining suitable brake balance), the rear brake pressures may be reduced as compared proportionally to the fronts brakes. This pressure reduction is achieved by controlling the rear isolation valves in conjunction with the pressure reducing valve, which both controlled proportionally. Feedback from a pressure transducer (regenerative axle pressure) is used for this control. The boost valve itself controls the pressure applied to the front wheel brakes during regenerative brake blending.

Accumulator and Pump Motor Operations

The accumulator is charged independently of the brakes being applied, meaning that the motor may run at any time when the vehicle is powered. The pressure transducer (High Pressure Accumulator) is used by the EBCM to determine when the accumulator pressure is low. The EBCM controls the motor such that it runs from the time when the accumulator pressure is seen to be low until the high pressure accumulator is fully charged. A dual pump arrangement is used and is driven by an electronically commutator motor.

EBCM Controls

The EBCM controls the current to the boost valve so that pressure can be applied to the wheel brakes. The boost valve is a spool type valve which is referenced to the controlled pressure so that for a given current applied, a specific pressure is expected. The boost valve controls the pressure apply from the accumulator through the rear isolation valves directly to the rear wheel brakes, and through the front isolation valves, to the individual front pressure chambers behind the secondary pistons in the master cylinder, which in turn apply pressure to the front wheel brakes.

Base Brake Valves (Normal Open)

The normally open base brake valve is used to keep boost pressure from feeding back to the master cylinder which would increase the force back to the pedal. It closes off a passage which is used to apply pressure to the wheel brakes from the master cylinder in the case of a system failure. In this case the system failure referred to is one which results in a control mode known as push through. In normal operation the normally open base brake valve is on (closed) during a boost pressure apply event, and is left off (open) during failed system operation.

Base Brake Valves (Normal Closed)

The normally closed base brake valve is used to close off a passage which allows fluid from the back side (spring side) of the simulator and from the tank port of the boost valve to exit to the reservoir. In normal braking, turning this valve on (opening) allows the simulator to compress the spring and function normally. During failed system operation (push-through) the valve is left off (closed) keeping fluid from exiting the back side of the simulator, which in turn keeps master cylinder fluid from entering the front side of the simulator. In this way, fluid from the master cylinder during failed system, and push through operation is applied only to the wheel brakes, and not lost in the simulator. In normal operation the valve is turned on (opened).

Stability control provides added stability during aggressive manoeuvres. Yaw rate is the rate of rotation about the vehicle's vertical axis. The stability control is activated when the EBCM determines that the desired yaw rate does not match the actual yaw rate as measured by the yaw rate sensor.

The desired yaw rate is calculated by the EBCM using the following inputs:

The difference between the desired yaw rate and the actual yaw rate is the yaw rate error, which is a measurement of oversteer or understeer. When a yaw rate error is detected, the EBCM attempts to correct the vehicle's yaw motion by applying brake pressure to one or more of the wheels. The amount of applied brake pressure varies, depending on the correction required.

The electronic brake control module (EBCM) is able to detect many malfunctions whenever the ignition is ON. However, certain failures cannot be detected unless active diagnostic tests are performed on the components. Shorted solenoid coil or motor windings, for example, cannot be detected until the components are commanded ON by the EBCM. Therefore, a power-up self-test is required at the beginning of each ignition cycle to verify correct operation of components before the various control systems can be enabled. The EBCM performs the power-up self-test when the ignition is first turned ON. The system relay, solenoids and the antilock brake system (ABS) pump motor are commanded ON and OFF to verify proper operation and the EBCM verifies the ability to return the system to base braking in the event of a failure. The power-up self-test may be heard by the driver, depending on how soon the engine is cranked and started after turning ON the ignition.

The electronic brake control module (EBCM) illuminates the antilock brake system (ABS) indicator when a malfunction which disables ABS is detected. Usually, the ABS indicator is turned OFF during the following ignition cycle unless the fault is detected during that ignition cycle. However, the setting of a wheel speed sensor related DTC may cause the ABS indicator to remain illuminated during the following ignition cycle until the vehicle is operated at a speed greater than 13 km/h (8 mph) or, occasionally, 64 km/h (40 mph), depending on which DTC sets. This allows the EBCM to verify that no malfunction exists, before turning OFF the ABS indicator. It is important to verify that United Nations Economic Commission for Europe regulation number 13 (ECE 13) is not the cause of an ABS indicator which is illuminated when no DTCs are set, before attempting to diagnose other possible causes.

The following indicators are used to inform the driver of several different factors.

Brake Warning Indicator

The instrument panel cluster illuminates the brake warning indicator when the following occurs.

ABS Indicator

The instrument panel cluster illuminates the ABS indicator when the following occurs:

Traction Control/Stability Control Indicator

The instrument cluster turns the traction control/stability control indicator ON when the following occurs: