General System Information

General
Anti-lock Braking System (ABS) brakes have been developed to provide optimum braking, with no loss of directional stability, under widely varying conditions. The stopping distance of a car is influenced by a variety of factors, including weather conditions, road surface, traffic conditions and the brake pressure applied.

The ABS system provides modern braking systems with a control function that enables the maximum braking effect to be applied in critical situations, regardless of the road conditions.

The braking system is divided into two separate diagonal brake circuits. One circuit comprises the right-hand front wheel and left-hand rear wheel (primary circuit), while the other comprises the left hand front wheel and right-hand rear wheel (secondary circuit).

This means that if one of the brake circuits fails, e.g. as a result of a leak, 50%of the total braking effect is always retained.

The main advantages of the ABS system are:
- Reduced tire wear.
- No loss of directional stability on braking.
- Steering control retained even during heavy braking.
- Shortest possible stopping distance.
- Normal power-assisted braking if the ABS is in operative.





Legend
1 Direction Of Travel
2 Braking Force
3 Lateral Force

Illustration Note: All the forces taken up by the tire are either lateral forces or braking forces.

To understand how the ABS system provides optimum braking without any loss of directional stability, we need to examine the forces acting on a wheel during braking.





Legend
1 Braking Force
2 Stable Braking Zone
3 Unstable Braking Zone
4 Slip

On the diagram in the illustration above, curve (1) shows the relationship between braking force (expressed as a coefficient) and tire slip (expressed as a percentage).

The braking force is equivalent to the coefficient of adhesion, i.e. the friction between the tire and the road surface. Each application of a braking force gives rise to a certain degree of slip, ranging from 0% when the wheel is rolling freely to 100% when the wheel is locked.

When the brake is first applied, the braking force increases sharply, but the degree of slip only gradually, up to a certain limit. Beyond that point, the braking force decreases with increasing slip.





Legend
1 Lateral force
2 Stable braking zone
3 Unstable braking zone
4 Slip

The maximum braking force (coefficient of adhesion) is reached at a point known as the limit of optimum slip.

The section of the curve between 0%slip and the limit of optimum slip is known as the stable braking zone. The section of curve between the limit of optimum slip and 100%slip is known as the unstable braking zone, as stable braking cannot be achieved within this zone. This is because the wheel quickly becomes locked after the limit of optimum slip has been reached, unless the braking force is immediately reduced. Slip also occurs when lateral force is applied on the tire (e.g. on cornering).

Curve II on the diagram shows lateral force as a function of slip. As can be seen, lateral force falls away sharply with increasing slip. At 100%slip, i.e. when the wheels have locked up, no lateral force remains for steering and the driver will no longer be able to control the vehicle.





Legend
1 Braking/lateral force
2 Stable braking zone
3 Unstable braking zone
4 ABS operating range
5 Limit of optimum slip
6 Slip

Both curves are shown on this next chart, which also shows the range within which the ABS system is operative. During braking, the system allows the braking force to increase to a point just before the limit of optimum slip and then prevents it from increasing further. The system modulates the hydraulic pressure to keep the braking force as close as possible to the optimum value (the limit of optimum slip) regardless of the pressure applied to the brake pedal.

Because the ABS system prevents the degree of slip from exceeding the limit of optimum slip, the car never enters the unstable zone. At the same time, some lateral force is preserved to ensure that steering control can be retained (curve II).





The relationship between braking force and slip is influenced by a variety of factors including the road surface and the tire type, tread pattern and pressures. Variations in these factors will affect the braking force and stopping distance in curves I and 11 but not the principle of operation.

Principles Of Operation





The ABS system on the Saab 9000 has three principal components:
- A set of four wheel sensors
- Hydraulic unit (including valve block assembly)
- Electronic control unit (Electronic Control Unit (ECU))

The ABS Mark IV system is a dual-circuit, four-port braking system.

The ABS Mark IV system is fitted to all model year 1993 and later Saab 9000 models except manual turbo models with the Traction Control System. These cars are equipped as before with the ABS Mark 11 system.

Signals from the four wheel sensors are sent to the ECU, which continuously monitors the speed, acceleration and deceleration of the wheels, the road speed and tire slip. If a wheel is about to lock up, the ECU sends signals to the solenoid valves for the wheel concerned, thereby modulating the pressure in the brake circuit for the wheel to provide optimum braking effect and friction between tire and road.

Hydraulic Unit





The hydraulic unit consists of a valve block, master cylinder, pump and motor in one unit. The hydraulic unit is secured in a vacuum-operated servo (brake pressure booster). The brake fluid reservoir is separate from the hydraulic unit.

ECU





The ECU processes the signals from the wheel sensors and, on detecting any lock-up tendency in one or more of the wheels, sends signals to the solenoid valves in the valve block. The pump starts and stops as required (the warning lamp lights up if faults develop in the ABS control). The ECU is located in the engine compartment on the side of the battery tray.

Brake Warning And ABS (Anti-Lock) Warning Lights





The brake warning light will come on if the level in the fluid reservoir falls below the MIN mark.

The ABS warning light will also come on:
- in the event of a malfunction in the ECU
- in the event of a break in circuit continuity
- in the event of weak signals being received from the wheel sensors

The ABS is always inoperative when the ABS warning light is on, and the car will then have ordinary power-assisted braking without ABS.

Front Wheel Sensors And Sensor Wheels





The front wheel sensors are mounted radially relative to the trigger wheel and operate on the same principle as a generator. Each time a tooth on the rotating sensor wheel passes the sensor, it distorts a magnetic field, causing a signal to be sent to the ECU, which processes the signals to produce the control information it requires, such as wheel speed, retardation and slip.

Rear Wheel Sensors And Sensor Wheels





The rear wheel sensors are mounted axially relative to the trigger wheel: the trigger wheels are therefore of a different design to those for the front wheels, although they operate in exactly the same way.

Valve Block





The valve block, which is incorporated in the hydraulic unit, modulates the pressure to the brake calipers when the ABS system is operative. The valve block contains eight solenoid valves: four inlet valves and four outlet valves. Each brake circuit has one inlet and one outlet valve per wheel.

Overview Of Operating Principles
The ABS Mark IV is based on a vacuum brake servo unit with a flange-mounted tandem master cylinder.
- The central component of the ABS system is the hydraulic unit incorporating the brake servo unit and master cylinder.
- The brakes in the primary circuit (right-hand front wheel and left-hand rear wheel) are activated by the primary piston in the master cylinder.
- The brakes in the secondary circuit (left-hand front wheel and right-hand rear wheel) are activated by the secondary piston in the master cylinder.
- The ABS system is supervised by an electronic control unit (ECU) that detects any tendency for a wheel to lock up during braking.
- A sensor at each wheel continuously senses the speed of rotation of the wheel.
- An a.c. voltage, the frequency of which is proportional to wheel speed, is generated in the sensor. These signals are transmitted continuously in the form of an a.c. voltage sinusoidal wave to the ECU, which processes the signals from the four sensors and computes a value known as the reference speed. The individual signals from each sensor (individual wheel rpm) are then compared with this reference speed. In the event of a tendency being detected for any of the wheels to lock up, the speed indicated by the signal from the sensor for that wheel will differ noticeably from the reference speed.
- The hydraulic pressure in the individual brake circuits is modulated by the solenoid valves in the valve block, which in turn are controlled by signals from the ECU.
- Up to 12 brake-pressure modulation cycles are possible per second and wheel.

Safety Overview





The logic system controlling the solenoid valves in the ABS system is such that the valves will assume their rest (de- energized) positions under no-voltage conditions. ABS system will cease to operate the moment the ECU senses any departure from normal operating conditions; for instance:
- open circuits in the valve block weak signals from the wheel sensors, break in continuity in any electrical circuit
- malfunctions in the ECU. Any of these events will cause the ABS warning light to come on. If the ABS system should become inoperative for any reason, all the solenoid valves will be deenergized and the braking system will operate in the same way as a conventional system.

Function





When the brake pedal is depressed, pedal force is boosted by the brake servo unit and pressure builds up in the master cylinder. The pressure on the pistons in the master cylinder is propagated in the brake system and acts on the brake piston in each caliper. The brake piston presses the brake pads against the brake disc.

When the brake pedal is released, the pistons in the master cylinder return to their original positions and the return passage is opened. Pressure is relieved and the brake pistons are retracted to the rest (brakes off) position by means of the piston sealing ring in the brake cylinder.