Anti-lock braking system (ABS) control

When the brake pedal is depressed brake fluid is pushed out into the system and the brakes are applied.

If for example the right front wheel displays a tendency to lock the following occurs:

• the pump is started and is then running while ABS control is in operation.
• inlet valve (C4) closes. This means that the hydraulic pressure on the brake cannot increase even if the pedal is depressed further. This phase of the control is called pressure retention  .
• If the wheel locks anyway the outlet valve (D4) opens and the brake fluid is allowed back to the pump, via the receiver (A2). The hydraulic pressure is eased and the wheel accelerates. This phase of the control is called pressure reduction  .
• When the control module senses that the wheel accelerates the outlet valve shuts (D4) and the inlet valve (C4) opens. The hydraulic pressure increases and the wheel is braked. This phase of the control is called pressure increase  .
• The control process with the three phases is repeated until the risk of the wheels locking is over or braking stops.

In the graphics the front and rear right wheels are in the pressure retention  phase, the left front wheel is in the pressure increase  phase and the left rear wheel in the pressure reduction  phase.

When the pump is operating the brake fluid is returned to the lines between the master cylinder and the inlet valves. If one of the outlet valves opens at the same time as the inlet valves are closed the brake fluid will travel only to the master cylinder. This means that the brake pedal is pushed upwards. If one or more of the intake valves opens part of the brake fluid goes to the wheel brakes and the pedal is pushed a little way upwards, or sinks slightly. When the pressure is controlled it causes a pedal movement that signals to the driver that the ABS is operating.

In the master cylinder pistons there are central valves. The valves are normally open and connect the space in front of the piston with the brake fluid reservoir via a channel. When the pistons are affected at braking the valves shut so that pressure can build up in front of pistons.

The receiver compensates for the volume/pressure on pump intake side. After finishing the control the pump can continue for approximately 1 second, this is to ensure that the receivers are empty.