Brake Control System - Design and Function: Other Volvo S80 I

Hydraulic system

Also new is that the rear wheels are also individually controlled which has meant that 2 valves per wheel have been introduced, 8 in total. If the system is supplied with Traction control or STC, 2 Hydraulic valves, 2 solenoid valves and 2 by-pass valve are installed. The control module is screwed into the ABS hydraulic modulator. The control module receives the wheel speed sensor signals which are put onto the network.

Wheel speed sensor

The sensors are active, which means that they have a built in electronic circuit. The control module transmits a voltage of U bat , the sensor sends back a digital power signal of 7 or 14 mA, depending on position. This solution gives a more robust signal, compared to the voltage signal. The current flow over the connections is higher which minimizes the chance of corrosion.

Pulse Wheel

S/V 70 have 48 toothed magnetic pulse wheels on all four wheels.

Control Module

There are three versions of the control module available

Scheme 39

1. ABS
2. ABS+TC
3. ABS+STC (the control modules are not programmable).

Anti-Lock Braking System (ABS)

Is activated by the likelihood of the wheels locking during braking and ensures that the maximum braking effect is retained without the wheels locking. This is to retain the cars driveability and steering capacity.

The function comes into operation at speeds in excess of 7 km/h (4 mph). At lower speeds the wheels can lock, but this has no practical effect on the brake or steering capacity.

The ABS system is a four channel system. This means that each wheel is controlled individually.

There is an inlet valve (C1, C2, C3 and C4) and outlet valve (D1, D2, D3 and D4) for each channel in the ABS hydraulic modulator. Furthermore there are two receivers (A1 and A2), one for the primary (LF+RR) and one for the secondary (RF+LR) circuits.

In start position all the valves are in the resting position, that is to say that the inlet valves are open and the outlet valves are shut and the receivers have no pressure.

There is a return valves for each inlet valve. The valve is opened by the high brake pressure at the wheel when the pedal is released. This makes the brakes release quickly and avoids "delayed release".

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

1. the pump is started and is then running while ABS control is in operation.
2. 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 .
3. 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 .
4. 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 .
5. 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.

Electronic Brakeforce Distribution (EBD)

Is activated during normal braking (without ABS control) and divides the braking force on the rear wheels.

Normally there is a variation in the front and rear wheels slippage dependent on how hard braking is and how heavily the car is loaded. The Electronic Brakeforce Distribution (EBD) function ensures that the rear wheel speed is 0-2% higher than the front wheel speed. The function is, in principle, load sensing.

The function is controlled by the control module and controls only the valves to the rear wheels. The pump does not operate, the brake fluid is taken up by the receivers.

Scheme 40

Traction Control function

Monitors and controls wheel spin in the driving wheels at lower speeds in order to increase forward movement. If a wheel starts to spin that wheel is automatically braked so that the drive power is transferred to the other wheel. Activated only at speeds up to approximately 40 km/h (25 mph). When the traction control has been activated it can be used up to 80 km/h (50 mph). Traction control can be likened to an electronic limited slip differential.

Traction control is always connected after starting but can be deactivated using a switch. If the driver depresses the brake when the system is operating the function is deactivated.

The control module monitors how hard the traction control is working and calculates theoretical brake temperature. If there is a risk of the brakes overheating the function is deactivated. At the same time the warning lamp and a diagnostic trouble code is stored. When the brakes have cooled the warning lamp goes out and the function is reactivated, but the diagnostic trouble code (DTC) is still stored in the memory.

Note. This diagnostic trouble code (DTC) can also be stored if the brakes have had to work hard.

In cars with Traction control the ABS hydraulic modulator contains the following components: hydraulic valve (1), solenoid valve (2), and a by-pass valve (3), 2 of each, 1 per circuit.

There is a central valve in the hydraulic cylinder primary cylinder. The valve is normally open and ensures that the pump receives sufficient brake fluid during traction control (TC) operation.

The function is similar to the TRACS in the X70.

Traction control

If for example the front wheels have a tendency to spin when the car is driven off the following occurs

1. The solenoid valves (2) close and block the connection between the pump pressure side and the master cylinder
2. The inlet valve (C1) for the left front wheel closes so that wheel cannot brake.
3. The pump starts and then runs constantly during operation.
4. The pump takes brake fluid from the master cylinder via the hydraulic valve (1) and pumps the brake fluid to the right front wheel.
5. The wheel is braked to the same speed as the left front wheel and drive is split between the wheels.
6. The control module then controls the wheel spin (braking power) by closing/opening the inlet valves (C4) and the outlet valve (D4).

When the control is stopped the pump is switched off, the solenoid valves (2) opens and the in- and outlet valves resume the position for normal braking.

Control is stopped if

1. Wheelspin stops, for example because of increased friction between the road surface and the tires.
2. There is a risk of the brake discs overheating.
3. Braking takes place (the brake pedal is depressed, signal from the stop (brake) lamp switch). In addition the pressure from the master cylinder hydraulic valve (1) is shut off and the pump can no longer receive brake fluid from the master cylinder.

The pump motor is controlled with a signal which has a fixed frequency but a variable pulse width. This means that the engine speed (RPM) and sound vary during traction control.

Because the pump supplies more brake fluid than is normally required for control there are two by-pass valves (3) which open at a certain pressure. When the valves open the brake fluid is released back to the brake fluid reservoir via the master cylinder.

Scheme 41

Stability Control function

Monitors and controls wheelspin in the drive wheels at all speeds to improve the cars traction and stability.

If one or both of the drive wheels begins to spin (rotates faster than the rear wheels), an order is sent to the engine control module (ECM) to limit the engine torque.

Id deactivated if the driver depresses the brake pedal.

The function is similar to the DSA S/V 40.

Note. The Stability control function cannot be ordered separately, but is part of Stability traction control (STC).

Stability and Traction Control (STC) function

STC = Traction control + Stability control

Stability and traction control (STC) maximizes the cooperation between the traction control and stability control depending on the circumstances and driving conditions.

Is deactivated if the driver depresses the brake pedal.

The activation/deactivation switch is on the dashboard.

Standard/option depending on market.

Software downloading

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