Brakes (Description): Description and operation

Description

The ABS2E system is an ECU controlled system deriving its input from four speed sensors mounted at the wheels which allow it to control the hydraulic modulator preventing the wheels locking during braking.

Each wheel is individually sensed with the front wheels being controlled by separate solenoid valves and the rear wheels being controlled by a single solenoid valve and a copy valve. The copy valve is used to ensure balanced operation of the rear brakes when an ABS sequence is triggered. This feature is known as 'Select Low' and its purpose is threefold; namely to keep the tyre to road lateral grip high, to maintain rotation and to help reduce oversteer.

The operation of the ABS system is completely dependent upon electronic signals. To prevent the system responding to any inaccurate signals, a built in safety circuit monitors all electric and electronic signals, including battery voltage. If an inaccurate signal or low battery voltage is detected, the ABS system is shut down immediately and the warning light on the fascia is illuminated to inform the driver that the brake system will respond to pedal pressure in the same manner as a non ABS system.

Operation

Servo

Inlet manifold vacuum is transmitted through a hose and non-return valve to the servo. Inside the servo, this vacuum is felt on both sides of the diaphragm. When the brake pedal is pressed, the servo push rod opens a valve and allows atmospheric pressure to be drawn through the filter into the pedal side of the diaphragm. The pressure differential, acting on the diaphragm, increases the pressure being applied at the brake pedal and transmits it to the master cylinder through a push rod.

Master cylinder

When the foot brake is applied the primary plunger moves up the bore of the cylinder and the pressure created acts in conjunction with the primary spring to overcome the secondary spring, thus moving the secondary plunger up the bore of the cylinder simultaneously. Initial movement of both plungers causes them to be pushed off their stop pins, thus closing both primary and secondary centre valves. Further movement of the plungers pressurises the fluid which is directed into the two separate hydraulic circuits connected to the hydraulic modulator.

The primary circuit operates the R.H. front and L.H. rear brakes and the secondary circuit operates the L.H. front and R.H. rear brakes.

The fluid in the chambers behind the plungers is unaffected by any movement of the plungers and can flow unrestricted between chamber and reservoir both before and during brake application.

When the brake pedal is released, the primary and secondary springs force their respective pistons back down the bore of the cylinder. As the plungers contact the stop pins the primary and secondary centre valves are opened allowing fluid to circulate unrestricted between the two hydraulic circuits and the fluid reservoir.

The movement of fluid during brake application/release is compensated for by fluid from the separate reservoirs within the supply tank moving through the feed holes in the cylinder. Conversely the final movement of the plungers causes any surplus fluid to move through the cut-off holes in the fluid reservoirs.

Should a failure occur in one system, the remaining system will still operate effectively, although the brake pedal travel will increase.

Calipers

Pressure at the caliper forces the caliper piston against the inner brake pad and, in turn, against the disc. The caliper body reacts and slides on the guide pins to bring the outer pad into contact with the disc.