Brake Control System - Design and Function (Convertible): Other Volvo C70 I

Braking without ABS

When the brake pedal is depressed, the force of the pedal is transmitted via the air hydraulic brake power assist unit (brake servo) to the master cylinder pistons and the brake light switch triggered. The center valve in the secondary piston and the seal on the primary piston close the connections to the brake fluid reservoir. In cars with TRACS the primary piston also has a center valve.

The hydraulic pressure in the two brake circuits increases and triggers the pistons in the brake calipers which press the brake pads against the brake discs.

When the brake pedal is released and returns to its resting position, the pistons in the master cylinder return to their resting position. The connection to the brake fluid reservoir opens as the center valve, the seals and the braking system are depressurized. The pistons in the brake calipers return to their resting position because of their seals.

Scheme 267

EBD is integrated with the ABS system and controls the braking force to the brakes on the rear wheels.

EBD stands for Electronic Brakeforce Distribution. With EBD, the task of the control module is to control the hydraulic pressure in the rear wheel brake circuit (1) if the rear wheels are tending to slip more than the front wheels when braking. This ensures that the car brakes smoothly and safely without ABS coming on.

The control module controls the brake pressure by operating the inlet and outlet valves in the rear wheel brake circuit (1).

The EBD pressure sensor is connected to the master cylinder primary circuit and indicates if the primary circuit is under pressure. EBD can only be used when the EBD pressure sensor (2) is activated. EBD replaces the reducer valve which would otherwise reduce the pressure in the rear wheel brake circuit.

Depending on how much the rear wheels are slipping relative to the front wheels, the brake pressure in the rear wheel circuit will be controlled differently. The control system will try to keep rear wheel slip 0-2% higher than front wheel slip. It can therefore be said to be load-responsive.

Generally speaking, the difference in slip between the rear wheels and front wheels when braking mainly depends on how hard the braking is and how loaded the car is. Hard braking/no load can produce serious slipping, light braking/heavy load may give less slip.

As EBD does not require as much brake fluid as ABS/TRACS, the hydraulic pump is not activated. Fluid returning from the brake circuits is taken up by the low pressure accumulators.

No braking

When the driver is not braking the car, the braking system is in stand-by mode: that is, the master cylinder is in its resting position and the connection between the brake fluid reservoir and the rest of the hydraulic system is open. The system is then completely unpressurized. In the hydraulic unit valve block, the hydraulic valves are in their resting position: that is, the inlet valves are open and the outlet valves are closed.

Control functions

Based on input signals, the control module controls the following functions and components

1. ABS warning light on the instrument panel (dashboard).
2. The hydraulic unit hydraulic pump and the inlet and outlet valves. (4)
3. ABS and EBD systems

And in cars with TRACS

1. TRACS system.
2. TRACS warning light on the instrument panel for faults which only affect the TRACS system and when the TRACS function is disabled using the switch.

Scheme 268

The control module has two different microprocessors which receive signals from the different sensors in the car. The microprocessors work in parallel, using a program which operates the ABS, EBD and TRACS systems.

The control module for ABS, EBD and TRACS is integrated with the hydraulic unit and is located in the engine compartment.

The main function of the control module is to process signals from the different sensors and control the hydraulic pump and the electromagnetic hydraulic valves (inlet and outlet valves) in the hydraulic unit when using ABS, EBD or TRACS. The control module houses the electrical coils for the inlet and outlet valves.

The control module checks its calculations and all input and output signals via the built-in diagnostics functions. If the control module detects a fault, ABS, EBD or TRACS will be switched off completely or disabled, depending on how serious the fault is, and the ABS (TRACS) light will come on. When disconnecting, the control module aborts or stops all functions (not road speed signal); disabling prevents certain functions from being performed.

Any faults will be recorded in the control module memory. Data can be read out using the Volvo ST connected to the DLC in the passenger compartment in front of the gear lever.

When the car is driven off, the control module checks that it is receiving signals from the wheel sensors. When the car first reaches 20 km/h (12.5 mph) (or 40 km/h (25 mph) if the brake light switch is activated), the control module checks the pump engine by activating it briefly. This means the hydraulic pump will be audible. This is completely normal.

ABS is not activated until the car's road speed exceeds approx. 7 km/h (4.4 mph).

Scheme 269

The control module receives input signals on the car's operating conditions from the following components

1. Wheel rotation speed, acceleration and retardation from wheel sensor and pulse wheel. (1, 2)
2. Signal from EBD pressure sensor when the primary circuit is pressurized. (3)
3. Signal from brake light switch when the driver depresses the brake pedal. (5)

And in cars with TRACS

1. When TRACS cuts in or out from the switch on the instrument panel (dashboard). (9)

The following are needed for the control module to function

1. Voltage supply, 30-supply, from the battery. (6)
2. Voltage supply, 15I-supply, from the ignition switch. (7)
3. Ground connections.

Scheme 270

The control module sends a speed signal to the instrument panel (dashboard) based on the input signals.

The road speed signal is the average of the front wheel speeds.

Scheme 271

When braking, the control module (4) receives a signal from the brake light switch (5) that braking has started. The control module then switches to stand-by mode for ABS control. The signal from the brake light switch is not necessary for ABS control, but makes for gentler adjustment when ABS first cuts in.

A wheel sensor (1) by each wheel sends signals to the ABS control module. The control module uses these signals to calculate the car's reference speed. If there is a risk of the wheels locking, the control module triggers the ABS system hydraulic unit (4) and the hydraulic pressure to the wheel concerned is adjusted to avoid the wheel locking.

The pressure in the circuit is controlled so that the maximum possible braking effect is transmitted to the rod, which occurs when the wheel is rotating at approx. 20% slippage relative to the road surface.

The ABS system does not engage until the car's road speed exceeds approx. 7 km/h (4.4 mph) (i.e. there is nothing to stop the wheels locking under approx. 7 km/h (4.4 mph)).

ABS can be divided into three different stages

1. Maintaining brake pressure. (Constant pressure) The wheel is threatening to lock. The control module closes the inlet valve to the circuit to prevent the brake pressure increasing any further. If the wheel still continues to slow down, the brake pressure must be increased.
2. Reducing the brake pressure. (Pressure dumping) The control module opens the outlet valve to reduce the brake pressure and allow the wheel to speed up. If the wheel starts turning too quickly, the brake pressure must be increased.
3. Increasing brake pressure. (Pressure build-up) The control module closes the outlet valve and opens the inlet valve, increasing the pressure and slowing the wheel down.

Steps 1, 2 and 3 are then repeated until braking ends or ABS cuts out.

Scheme 272

This is located on the instrument panel (dashboard) and is activated if the ABS system control module detects a fault in the ABS system. When the ignition is switched on or the engine starts, the control module switches the warning light on. The light then goes out after approx. 2 seconds if no fault is detected. If the warning light is on, the ABS system is not working but normal braking power is still available. The warning light flashes when the ABS system DTM is activated.

When the control module DTM is disengaged, the warning light comes on for approx. 2 seconds and then goes out if no fault is detected.

The warning light is controlled by a relay on the instrument panel (dashboard). When the ABS control module grounds the signal to the relay, the relay breaks the light's ground and the light goes out. When the ABS control module increases the voltage over approx. 7 V, the relay opens and the light comes on.

Scheme 273

The hydraulic unit (1) is integrated with the control module and is located in the engine compartment. The hydraulic unit consists of a hydraulic pump (2) of the eccentric piston type and an electric motor (3) in a valve block with inlet valves (4) and outlet valves (5) and a low-pressure accumulator (6). The electric coils for the inlet and outlet valves are located in the control module.

The hydraulic unit must not be taken apart for repairs, but must always be replaced as a complete unit.

The hydraulic pump, which is operated by a 12 V DC motor, supplies brake fluid to the brake circuits when ABS/TRACS is in use.

The pressure in the brake circuits when using ABS and EBD is governed by the pressure in the master cylinder, and is directly proportional to the force the driver applies to the brake pedal. The hydraulic unit is oversized so that it can always deliver the necessary flow and pressure.

Scheme 274

The control module controls, via the inlet and outlet valves and the hydraulic pump, the pressure to the different brake calipers when braking under ABS, EBD or TRACS.

The valve block has three internal circuits, one for the LH front wheel, one for the RH front wheel and a common circuit for the rear wheels.

The front wheels are controlled individually, while the rear wheels are under common control, depending on which wheel threatens to lock first.

The valve block has six valves for controlling the brake pressure, three inlet valves and three outlet valves. So, each front wheel has an inlet valve and an outlet valve, while the rear wheels have a common inlet valve and outlet valve between them. In stand-by mode, the inlet valves are open and the outlet valves closed. The outlet valves act as non-return valves between the brake calipers and the hydraulic pump.

The valve block has two low-pressure accumulators, one for the primary circuit (1) and one for the secondary circuit (2).

The accumulators are located between the outlet valves and the hydraulic pump and quickly absorb the brake fluid returning from the brake calipers when using ABS, EBD or TRACS and which the hydraulic pump cannot suck back. This ensures that the brake calipers can release quickly and that the wheels can speed up quickly.

Scheme 275

Cars fitted with TRACS have a different hydraulic unit. The difference is that

1. The brake pipe (1) between the master cylinder primary circuit and the hydraulic unit is larger in diameter. This gives the brake fluid pipe more capacity and ensures that the hydraulic pump has sufficient brake fluid when using TRACS.
2. The valve block has an extra inlet valve (2) (TRACS inlet valve) which closes the connection between the hydraulic pump and master cylinder primary circuit when TRACS is activated. The valve is electrically operated and is normally open.
3. The valve block has an extra hydraulic valve (3) (TRACS valve) which closes the connection between the master cylinder primary circuit and the hydraulic pump inlet side when the brake pedal is depressed and pressure builds up in the primary circuit. The valve is hydraulically operated and normally open.
4. The valve block has an overflow valve (4). If the hydraulic pump supplies more brake fluid than is normally required for control purposes, the excess brake fluid is fed back to the hydraulic pump inlet side via the overflow valve. The valve block has an overflow valve (4). If the hydraulic pump supplies more brake fluid than is normally required for control purposes, the excess brake fluid is fed back to the hydraulic pump inlet side via the overflow valve. The overflow valve also controls the maximum pressure in the system under TRACS control.

Scheme 276

This is located on the instrument panel (dashboard) and comes on if TRACS is disengaged via the switch, if there is a fault in the TRACS system (when the brake discs on the front wheels are in danger of overheating) or if there is a fault in the ABS system. When the ignition is switched on or the engine starts, the control module switches the warning light on. The light then goes out after approx. 2 seconds if no fault is detected.

When the control module DTM is disengaged, the warning light comes on for approx. 2 seconds and then goes out.

If the rear wheels lock while TRACS is engaged (e.g. if the parking brake is applied), the control module will disengage TRACS and switch to ABS. The TRACS warning light may come on and then go out when ABS is disengaged.

Activating components/functions

Components and functions can be activated individually with this function. Activation is only possible if all wheel speeds are 0 km/h. Activation is disabled at speeds above 0 km/h.

The following components can be activated

1. ABS/TRACS lamp
2. Pump motor
3. Rear wheel outlet valve
4. Rear wheel inlet valve
5. Right front wheel outlet valve
6. Right front wheel inlet valve
7. Left front wheel outlet valve
8. Left front wheel inlet valve
9. TRACS inlet valve

Note. If diagnostic trouble code (DTC) ABS-445 Pump motor is permanently stored, activation is not possible.

For further information about activation, see Frozen values with DTCs

Read Control module ID

The Volvo System Tester identifies the control module by reading a code from the control module's memory. This code is made up of two figures, a software number and a hardware number.

Reading input and output signals

This service can be used to read out the values and status of the control module input and output signals continuously. The parameters are divided into three predefined lists. The division is made so that parameters in list 1 and 2 are updated by the control module at the same time. The parameter values thus reflect driving conditions at the same time and are directly comparable with one another.

The following parameters can be read out

List 1

1. Wheel speed LH front wheel
2. Wheel speed RH front wheel
3. Wheel speed LH rear wheel
4. Wheel speed RH rear wheel
5. Extrapolation counter LH front wheel
6. Extrapolation counter RH front wheel
7. Extrapolation counter LH rear wheel
8. Extrapolation counter RH rear wheel

List 2

1. The car's reference speed
2. The car's reference acceleration
3. Wheel speed LH front wheel
4. Wheel speed RH front wheel
5. Wheel speed LH rear wheel
6. Wheel speed RH rear wheel
7. Wheel acceleration LH front wheel
8. Wheel acceleration RH front wheel
9. Wheel acceleration LH rear wheel
10. Wheel acceleration RH rear wheel

List 3

1. ABS warning light
2. Brake light switch, position
3. EBD pressure sensor, position

And in cars with TRACS

1. TRACS warning light
2. TRACS switch

For more information on parameters see Running values, Volvo System Tester .

Reading out and erasing DTCs

This service can be used to read out and erase DTCs posted, counters and frozen values. The diagnostic system can identify 33 different faults in the form of DTCs (35 with TRACS). For fault-tracing DTCs see X. DTC table. No DTCs can be erased until all DTCs posted have been read out at least once during the same diagnostic session.

For each DTC, the frozen values can be read out for the following parameters

1. The car's reference speed
2. Brake light switch status
3. ABS/EBD/TRACS status

For more details of frozen values see Frozen values with DTCs . Frozen values with DTCs.

Scheme 277

Data on any faults and other information can be read out with the Volvo ST connected to the DLC on the console in front of the gear lever. The Volvo ST communicates with the control module via a standardized communications program.

You can use the Volvo ST to

1. Read out and erase posted DTCs, the counter and frozen values.
2. Read out control module input and output signal values and status continuously.
3. Read the control module ID.

Scheme 278

The DLC is common to both Volvo Diagnostics and the less sophisticated diagnostic system OBD II, On Board Diagnostics II (OBD II some markets only, USA/CDN).

Connecting the Volvo ST

1. Ignition off.
2. Install memory module
3. Connect Volvo ST by leads to DLC in console in front of gear lever.
4. Ignition on
5. Switch power on to Volvo ST.
6. Select language, model, function group.
7. Select system.
8. Select operation (read, erase, DTCs, activate components etc.).

To activate Volvo Diagnostics

1. Ignition must be on.
2. Battery voltage must be normal.
3. Speed of all wheels must be less than 10 km/h (6 mph).
4. The control module must not have activated ABS, EBD or TRACS.

Once the control module has activated Volvo Diagnostics, the ABS warning light will start flashing at a frequency of 2 Hz.

When activation of Volvo Diagnostics ends, the warning light stops flashing and the control module returns to its normal position for ABS, EBD and TRACS.

Scheme 279

The control module has a built-in diagnostics system (Volvo Diagnostics) which monitors itself and input and output signals continuously.

If the control module detects a fault, it posts a DTC and switches the ABS (TRACS) warning light on (there are exceptions to this, see D. DTC table). At the same time as the DTC is posted, the value of three different parameters, which are also posted, is frozen. The control module can post up to 33 (TRACS 35) DTCs.

The most serious faults switch the warning lights on and cut out ABS, EBD and TRACS completely. Less serious faults switch the warning lights on and cut some functions out. When disconnected, the control module aborts or disables all functions (not the road speed signal); disabling prevents certain functions from being carried out.

If the faulty components only affect TRACS, TRACS alone is cut out. If the ABS light is on, this means the ABS system is not working normally and in some cases, EBD will not be working either.

With some DTCs, the control module runs a diagnostic check as soon as the ignition is switched on, which means that if the warning light stays on, the fault has persisted and ABS/EBD will not work. With other DTCs, the control module only runs a check when some function is activated, the car is being driven or if the road speed exceeds 20 km/h (12 mph). This means that the warning light may go out as normal when the ignition is switched on or when the engine is started but the fault may persist. Only when the warning light comes on will ABS/TRACS cut out.

Should a fault disappear for any reason once the DTC has been posted permanently in the control module, data on the fault will be retained in the control module.

For each DTC posted, there is a counter which counts the number of fault-free driving cycles since the DTC was posted (intermittent fault). A driving cycle is defined as the first time the car passes 20 km/h (12 mph) since the ignition was last switched on.

If the counter reads 0, this tells the control module that the fault is permanent. If the counter reading is greater than 0, the control module treats this as if the fault does not exist (intermittent fault). If the fault reoccurs, the counter is reset to 0. If the counter exceeds 63, more than 63 driving cycles without the fault re-occurring, the DTC is erased, the counter and the frozen value are reset and the warning light goes out.

Air hydraulic brake power assist unit (brake servo)

Air hydraulic brake power assist unit (brake servo)

Brake circuit warning light

Brake circuit warning light

Brake pedal and brake light switches

Brake pedal and brake light switches

Brake pipes

Brake pipes

Master cylinder

Master cylinder

Scheme 280

The brake light switch sends a signal to the control module telling it that braking has started, so that the control module disengages TRACS, if it is in use, and stands by to use ABS/EBD if necessary, which means that the control module makes all calculations more precisely. If the control module does not receive any signals from the brake light switch, it will engage the ABS system, although the control will be rougher when ABS first cuts in.

The brake light switch is powered from the battery. When contact is made, the brake light switch comes on and the control module receives a signal telling it braking has started.

Scheme 281

The EBD pressure sensor tells the control module when the brakes are being used, to enable the control module to activate EBD. The EBD pressure sensor takes power from the control module at a no-load voltage of approx. 11 V. When the primary circuit is not under pressure (no braking), the pressure sensor is unaffected and the contacts closed, which means that the voltage is reduced to approx. 5 V by the control module. When the contacts are closed, the voltage is pulsed to ground. When the primary circuit is under pressure (braking in progress), the contacts break and the signal goes to high, which the control module interprets to mean that braking is in progress and that EBD can be used. The EBD pressure sensor is connected to the master cylinder primary circuit.

Power supply and ground points

The control module is powered continuously at 2 x 30-supply from the battery and a 15I-supply from the ignition when the key is turned to position 11. The control module has two ground connections.

Scheme 282

The TRACS switch (5) switches TRACS on and off (cars with TRACS only). Once the ignition is switched on, TRACS is on at all times. TRACS only disengages when the switch is pressed. The switch is spring-loaded and always returns to its normal position with the contacts broken.

The signal lead between the control module and TRACS switch is common to both the warning light and TRACS switch. The signal from the instrument panel (dashboard) warning light (4) to the control module is normally approx. battery voltage. When the switch is pressed, contact is made and the signal is grounded, causing the warning light to come on. The control module then disconnects TRACS and keeps the signal grounded, so the warning light stays on even when the switch is released.

The control module switches off the TRACS warning light on the instrument panel (dashboard) when TRACS is engaged via the switch.

Scheme 283

On the front wheels, the pulse wheel is pressed onto the outer driveshaft and the wheel sensor is located in the spindle casing radially relative to the pulse wheel.

On the rear wheels, the pulse wheel is pressed onto the hub and the wheel sensor is located axially relative to the pulse wheel. The rear pulse wheels can be exchanged. Both the front and rear pulse wheels have 48 teeth.

The wheel sensor's task is to provide the control module with information on the wheel rotation speed. When a wheel rotates, the teeth on the pulse wheel induce a voltage in the wheel sensor coil. This generates an AC voltage signal whose frequency and voltage depend on the number of teeth passing per second. The frequency and voltage increase as the speed of rotation increases. The voltage induced is normally approx. 300mV AC when the wheel is rotating at 1 turn per second. By recording the frequency, the control module can calculate the wheel's acceleration and retardation.

If there is a fault in any wheel sensor, ABS and TRACS will be disconnected or not engaged in the first place. However, EBD (Electric Brakeforce Distribution) remains on the whole time.

Using the signals from the wheel sensors, the control module calculates the car's road speed.

Instrument panel (dashboard)

Speedometer

The control module converts the signal from the front wheel sensors to a pulsed signal which is sent to the instrument panel (dashboard). The road speed signal is the average of the front wheel speeds.

The road speed signal is sent to the instrument panel (dashboard) even if ABS/EBD or TRACS are disengaged.

Parking brake system

Parking brake system

Parking brake warning light

Parking brake warning light

TRACS not working

If no drive wheel is spinning, the braking system is unaffected, i.e. the master cylinder is in its resting position and the connection between the brake fluid reservoir and the rest of the hydraulic system is open. The system is then completely depressurized. In the hydraulic unit valve block, the hydraulic valves are in their resting position, the inlet valves and TRACS valve are open and the outlet valves are closed.

Wheel brakes

Wheel brakes

Scheme 284

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

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 illustration 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 285

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 286

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.