Operation
General
The hydraulic circuit of the ABS VI is divided into three areas, which, independently of each other, control the brake pressures of the caliper pistons on the front left, front right and rear of the vehicle. The pressure in these circuits is modulated by electrically controlled pistons which receive information from the corresponding wheel sensors.
The modulator for the rear brakes comprises two separate modulation pistons which are controlled from one common electric motor. The ABS control unit determines the electric motor's current requirements and controls the rear left and rear right�hand solenoid valves. If only one of the rear wheel sensors has detected a locked wheel, the pressure of both rear brake units is reduced.
The front brakes are each controlled by one modulator. The front modulators work independently of each other and are controlled by information from the corresponding wheel sensor.
The solenoid valves ensure that brake pedal pulsing is reduced during the active phase. If the ABS VI is operating because of a locked wheel (is active), the solenoid valve closes. The brake pressure is controlled by the position of the modulation piston. Fluctuations in pressure are not felt at the brake pedal, since the connection is shut off when the solenoid valve is closed.
The solenoid valves also ensure that if the ABS VI should fail, the braking system will continue working safely. If the modulation motor or the solenoid valve should fail during the active phase, the solenoid valve will open immediately, thus enabling the brake pressure to be controlled directly by the driver. The ABS telltale also lights up.
9.5.2 Operation
Schematic Diagram of the ABS VI.
| 1 | Brake fluid reservoir | 7 + 13 | Front piston |
|---|---|---|---|
| 2 | Brake master cylinder | 8 | Rear tandem pistons |
| 3 + 4 | Rear solenoid valves | 9 + 12 | Front brake |
| 5 | Front right�hand ball valve | 10 + 11 | Rear brake |
| 6 + 14 | Front solenoid valve |
9.5.2 Operation
Illustration of the operation of the ABS VI on the front right�hand brake circuit
| I | Normal braking (without ABS cutting in) |
|---|---|
| � | Solenoid valve (2) open (not loaded) |
| � | Electric motor (5) moves the piston (3) into the upper position. |
| � | Piston is held in the upper position by spring (4) |
| � | Piston opens the ball valve (1) |
The corresponding brake circuit is fully open, the brake force is controlled by the driver.
| II | Pressure hold phase (wheel on the verge of locking) |
|---|---|
| � | ABS control unit immediately closes the solenoid valve |
| � | Electric motor moves the piston down |
| � | Spring force closes the ball valve |
| � | Brake circuit is �isolated" |
The brake pressure is first reduced, then held, thus preventing the wheel from locking. There is no longer any connection between brake caliper and brake pedal.
9.5.2 Operation
Illustration of the operation of the ABS VI on the front right�hand brake circuit (continued).
| III | Pressure reducing phase (activated ABS) |
|---|---|
| � | Solenoid valve (2) remains shut |
| � | Piston (3) is held in the lower position against the spring (4) by the electric motor (5) |
| � | Ball valve (1) is closed |
The brake pressure is dependent on the position of the piston. If a further reduction in pressure is required, the electric motor will move the piston to its �bottom dead centre position".
| IV | Pressure build phase |
|---|---|
| � | Solenoid valve remains closed |
| � | Electric motor moves the piston to the upper position |
| � | Piston opens the ball valve |