System Control

ABS Control: Description Of Operation
The brake system can be in one of four conditions:
1 Brakes off
2 Brakes applied without Antilock Brake System (ABS) modulation
3 Brakes applied with ABS modulation
4 ABS operation with one brake circuit inoperative


Brakes Off
In the brakes off position the diaphragm of the vacuum- operated servo and the valve piston are pressed against the rear end position by the return spring. The same vacuum exists on both sides of the diaphragm because an overflow valve in the diaphragm is open. The return spring of the master cylinder is forced against the rear end position. In this position both central valves are open and the brake system is kept at atmospheric pressure. A stop pin prevents the pistons from moving backwards. The complete brake system is held at atmospheric pressure.

The inlet and outlet valves in the valve block are in the rest (de-energized) position. The central valves are open.


Brakes Applied Without ABS Modulation





When the brake pedal is depressed, the vacuum passage is closed and the air passage is constantly open. The greatest possible pressure difference acts on the diaphragm, resulting in maximum braking pressure.

Full power assistance is obtained in the brake system

The central valve of the primary piston in the master cylinder is closed and the pressure in front of the primary piston rises. The pressure also acts on the secondary piston which is pushed forwards, with the result that the central valve of the secondary piston is also closed.

The hydraulic pressure in the two brake circuits rises, and as the surface areas of the pistons are identical, the pressure through the brake system is the same in both brake circuits.

The pressure is propagated through the brake system and acts on the brake piston in each brake housing. The brake pistons force the brake pads against the brake discs. When the brake pedal is released, the pistons in the master cylinder are returned and the central valves open. The pressure drops and the brake piston in each brake caliper stays in the brakes off position by means of the piston sealing ring.


Brakes Applied With ABS Modulation





ABS modulation is initiated when the system detects a tendency for a wheel to lock. The Electronic Control Unit (ECU) receives information on wheel rpm from a sensor on each wheel. If there is a tendency for the wheels to lock, the ECU modulates the braking force with the aid of electrical control valves. The braking force is modulated on each wheel separately.

The hydraulic unit only operates in the ABS position. The pressure applied to the inlet and outlet valves is modulated with the aid of the hydraulic pump. The pressure of the brake fluid supplied is determined by the brake pressure in the master cylinder, which is in turn proportional to the force applied to the brake pedal.

The pressure in the respective circuits is dependent on the retardation in wheel speed sensed by the sensors. The ECU controls the inlet and outlet valves so that maximum braking force (coefficient of adhesion) is maintained between the wheel and road surface (occurring at approx. 20% slip).

ABS-modulated braking will continue until:
- The car has been brought to a standstill

- The force applied by the driver to the brake pedal has been reduced to the level at which there is no danger of the wheels locking (determined by the relationship between braking force and the coefficient of adhesion).

- The brake pressure is modulated by the position of the brake pedal. The hydraulic pump starts up and braking takes place through the pressure of the hydraulic pump. The pump pressure also returns the brake pedal to Us original position.


Brakes Applied With ABS Modulation (One Circuit Inoperative)





When the brake pedal is depressed and a leak has occurred for example in the primary circuit, the primary piston is pressed forwards by the pushrod until it acts mechanically on the secondary piston. The secondary piston closes the central valve and hydraulic pressure is built up in the secondary circuit.

In the event of leaks in the secondary circuit, the secondary piston is pressed forwards to the end of its travel in the master cylinder.

In both events a longer pedal movement is required so that the desired braking effect is obtained.

ABS braking effect is then only obtained on the circuit which is operative, and the brake fluid level warning light will come on.

In the event of braking without ABS with one circuit inoperative, the brake system will provide power assistance to the operative circuit.

If the pedal position transmitter reaches its last position (position seven), the hydraulic pump will start up regardless of the type of brake.