Brakes (Description): Description and operation

Description

The ABS6 braking system is an ECU controlled system deriving its input from four speed sensors mounted at the wheels. These signals enable the ECU to calculate the vehicles reference speed and whether it is accelerating or decelerating. From this the ECU can detect a potential wheel lock situation (i.e. when one wheel is decelerating faster than the others and is therefore likely to lock) and send a signal to the corresponding modulator solenoid valve to reduce fluid pressure to the appropriate caliper.

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. This common control of the the rear wheels is known as 'Select Low' and means whichever rear wheel locks first determines the anti lock brake system activation for both rear wheels. The solenoid valves control fluid pressure to the modulator pistons reducing caliper pressure if a wheel locks. The two rear wheel modulators have different diameter pistons and slide pistons. This provides a proportioning control valve, which relieves pressure in the rear calipers during an emergency stop, to prevent the rear wheels locking.

The ABS6 braking system is completely dependent upon electronic signals. To prevent the system responding to any inaccurate signals, the ECU completes a self checking sequence each time the engine is started, illuminating the ABS warning light each time the ignition is switched on. If no fault is found in the system, the warning light will be extinguished when the engine is started. If the warning light remains lit, or if it should come on when driving, it means a fault has been detected by the ECU and the ABS6 braking system has been shut down. The brake system will now respond to pedal pressure in the same manner as a non ABS6 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.

Hydraulic modulator

When the engine is started the brake system ECU sends a signal to pump relay switching on the pump. Brake fluid is pumped through a relief valve into the accumulator where it is pressurised. When the pressure of the fluid in the accumulator reaches 230 kg/cm² the pressure switch sends a signal to the ECU. The ECU removes its signal to the pump relay switching off the pump. With the vehicle in motion and the ABS system operating, the pump is switched on to maintain pressure in the accumulator.