Part 2

Traction control when one or more drive wheels is spinning




If one or more drive wheels tends to spin once the vehicle has been started and driven off, the brake control module (BCM) registers this and compares the signals from the wheel sensors with a calculated reference speed. The control module then closes one or both SV valves and starts the hydraulic pump. The SV valve then blocks the connection between the pump's pressure side and the master cylinder's primary/secondary circuit. The intake valve of the spinning wheel is closed so that the wheel is not braked. The hydraulic pump draws brake fluid from the brake fluid reservoir via the ESV valve and increases pressure until the bypass valve opens. The hydraulic pressure then reaches the spinning wheel and decelerates it so that the driving force is distributed between the drive wheels so that the drive wheel with the best grip receives the largest amount of driving force.
Since the pump usually delivers more flow than required for traction control, excess brake fluid will be directed to the master cylinder via the bypass valve or will be suctioned up directly by the pump.
Once the spinning wheel has been decelerated to a condition with normal spin, the intake valve is closed. Depending on the wheel's acceleration, the outlet valve will open (brake fluid flows back to the induction side of the pump) so that there is a drop in pressure in the brake circuit or the outlet valve remains closed to maintain pressure (pressure hold). Pressure can once again be increased in the brake circuit by closing the outlet valve and opening the intake valve.

This control keeps the wheel in a condition of optimal spin until one of the following occurs:
- the spin tends to cease as a result of increased friction with the surface
- control is terminated by the brake control module (BCM) to prevent brake overheating
- the vehicle is braked
- the vehicle reaches a speed at which traction control ceases.
The SV valve is activated throughout the entire traction control process.
Activating the brakes, which the brake control module (BCM) registers via a signal from the brake light switch, terminates traction control and causes the SV valve to open. The ESV valve is closed so that the pump cannot suck brake fluid from the master cylinder. The hydraulic unit now works as a normal ABS hydraulic modulator.
The traction control function is only operable if there is a difference in speed between the drive wheels. This difference varies with vehicle speed. When vehicle speed is 0 km/h (0 mph), there must be an 8 km/h (5 mph) difference between the drive wheels for traction control to engage. A difference of 18 km/h (11.2 mph) is required at a vehicle speed of 20 km/h (12.4 mph) and a 25 km/h (15.5 mph) difference is required at 40 km/h (25 mph). At high speeds, too a great difference is required between the drive wheels. Thus, use of traction control is not possible at speeds above 60 km/h (37.3 mph).

Active yaw control

S60 (11-)





Other




The brake control module (BCM) with DSTC system continually uses information from various system sensors to calculate the position of the steering wheel and the vehicle. The brake control module (BCM) calculates the driving style of the driver by measuring:
- steering angle
- steering angle speed
- engine torque
- the vehicle speed
- brake pressure.
The brake control module (BCM) also calculates vehicle behavior by monitoring the vehicle's:
- yaw angle speed
- lateral acceleration
- forward acceleration.
The brake control module (BCM) receives information on current steering angle from the steering wheel angle sensor module (SAS).
When the differences between the driver's intended direction of travel and the vehicle's behavior exceed a threshold, active yaw control (AYC) regulates engine torque (stability function) in order to maintain the vehicle's lateral stability during most driving conditions. The AYC function also activates the individual brake control to regulate the SV and ESV valves of the hydraulic unit as well as the intake and outlet valves. These correct the vehicle in the event of a skid.
For information on steering angle and steering angle speed, see Design and Function, Steering Wheel Angle Sensor Module (SAS).









During AYC, in other words when the vehicle is skidding, the hydraulic pump starts to build up brake pressure regardless of whether the driver is activating the brake pedal. The brake control module (BCM) then activates the valves in the hydraulic unit and checks individual wheel brake force. This is done by increasing, maintaining or lowering brake pressure individually to stabilize the vehicle. If the driver depresses the brake pedal during stability control, the control module will use the brake pressure sensor integrated in the hydraulic unit to incorporate the driver's brake actuation into stability control.
If the driver depresses the brake pedal during stability control, the brake control module (BCM) will use the brake pressure sensor integrated in the hydraulic unit to incorporate the driver's brake actuation into stability control.
The calculations performed by the control module include the driver's desired brake pressure and functions for:
- Active yaw control (AYC)
- ABS
- EBD
- Stability control

Engine Drag Control (EDC) (applies for XC60 and S60 (11-))
Engine Drag Control (EDC) is a function that is part of the DSTC function. It is a stability function that prevents the wheels locking when engine braking. These situations can occur on slippery surfaces when the driver quickly releases the accelerator pedal or when down shifting. To prevent the wheels from locking, the Brake control module (BCM) requests an increase in wheel torque from the Engine control module (ECM). EDC can also prevent the wheels locking when braking with ABS on slippery surfaces.

AYC function, oversteering




Oversteering, which is when the vehicle turns to a greater degree than the steering wheel has been turned, is an extremely dangerous situation that is hard for the average driver to control. If not remedied, oversteering can cause the vehicle to start spinning out of control.
In the example above, the vehicle makes a right-hand turn and oversteers. To counteract the oversteering, the DSTC system closed the SV valve (separation valve), opened the ESV valve (electronic shuttle valve) and started the pump. Intake valve (C1) remains open while the other intake valves are closed. As a result, the brakes are applied to the left front and rear wheels. In the event of oversteering, the brake control module (BCM) sends a request for torque reduction to the engine control module (ECM) via the CAN network.

AYC function, understeering




Understeering is when the vehicle has the tendency to try to continue straight ahead even though the steering wheel has been turned. In the event of understeering, the brake control module (BCBM) sends a request for torque reduction to the engine control module (ECM) via the CAN network.
In the example above, the steering wheel was turned to the right, but the vehicle tends to continue straight ahead. The SV valve (separation valve) closes, the ESV valve (electronic shuttle valve) opens and the pump starts. The DSTC system applies the brakes to the right rear wheel by leaving intake valve C2 open and closing the other intake valves (C1, C3 and C4).

Roll control RSC
Roll control RSC (Roll over stability control) is a function that monitors the vehicle's angular speed.
If the driver suddenly turns the steering wheel, e.g., to swerve, and the vehicle tends to roll, the angle sensor registers the speed of the angle. The vehicle is stabilized using the DSTC-system.

Advanced stability control (ASC) (Applies for XC60 and S60 (11-))
The function advanced stability control (ASC) monitors and controls the rear axle's sideways movement to improve the vehicle's stability in oversteering situations with high lateral forces. Advanced stability control (ASC) is a part of the DSTC-system.

DSTC sport mode
To permit more dynamic driving the driver can choose DSTC sport mode. In DSTC sport mode the driver will experience a sportier drive experience. The function stability control is off and the function antispin (AYC) has lower thresholds for action. Choose DSTC sport mode via the menu in Infotainment control module (ICM).

Other functions
The Mark 25E1 brake system also includes a number of functions to assist the driver in various braking situations.
Brake assistance
Emergency brake assistance (EBA) is a function that helps the driver obtain maximum braking effect in an emergency situation. The brake control module (BCM) monitors the pressure in the brake servo with the help of a pressure sensor in the hydraulic unit. Quick movement of the brake pedal (emergency braking) generates a rapid change of pressure in the brake servo. When the control module registers the pressure change, the brake system activates the hydraulic pump. The hydraulic pump increases brake pressure in the brake system to obtain optimum braking effect (more powerful than during normal braking). Full braking effect is maintained as long as the driver maintains or increases pressure on the brake pedal. The function ceases immediately if the driver releases the brake pedal. The function is active from approx. 10 km/h (6 mph) to top speed. The function cannot be disengaged.
Hill descent control (only XC60 AWD and XC70 AWD)
Hill Descent Control (HDC) is a function to help the driver handle the vehicle when driving "off-road".
For manual transmissions the function is active in 1st gear as well as reverse gear (R). For automatic transmissions the function is active in 1st gear on the condition that Geartronic is used and when reverse gear (R) is selected. The function brakes the vehicle automatically without the driver stepping on the brake pedal. The characteristics for the throttle pedal change as well.
The function is activated with a switch on lower part of Climate control module (CCM). The function can be activated and deactivated manually. The switch has a visual green LED-indicator which shows position ON. When the function is activated and deactivated, this is confirmed with a message in Driver information module (DIM).
HSA- Hill Start Assist (only vehicles with transmission MPS6)
HSA (Hill start assist) is a function to help the driver when starting on a hill.
The function prevents unwanted vehicle movement from the time that the driver releases the brake pedal and presses down the throttle pedal. The function is active in position D (to prevent the vehicle from rolling backward) as well as in position R (to prevent the vehicle from rolling forward).
In order for the function to engage, the incline of the slope must be at least 4%. After the brake pedal has been released, brake pressure is maintained for 1.5-2 seconds. For further information about the function see Design and function, HSA - Hill Start Assist.
Trailer stability assistance (TSA) (Applies for X60 and S60 (11-))
The purpose of Trailer stability assistance (TSA) is to stabilize the vehicle with connected trailer in situations where the trailer has started to swerve (self-induced oscillations). The phenomena with vehicles with trailers experiencing self-induced oscillations can arise for all combinations of cars and trailers. Normally very high speeds are required to generate self-induced oscillations, but if the trailer is overloaded or if the load is distributed incorrectly, there is a risk of self-induced oscillations even at lower speeds, 70-90 km/h.