System overview: Introduction

The Mark 25 brake control system 

The Mark 25 brake control system with the brake control module (BCM) is an electronic control system which prevents wheel lock up during braking (ABS). By preventing wheel lock up, steering control is maintained and maximum braking effect is utilized.

The brake control system is equipped with:

• Antilock Braking System
• Electrical Brake force Distribution (EBD) for the rear wheel brakes.

The brake control system, as a factory installed option, can be supplemented with:

• TRACS (TRAction Control System)
• STC (Stability and Traction Control)
• DSTC (Dynamic Stability and Traction Control).

EBD 

EBD (Electrical Brake force Distribution) is a function for electronically controlling the brake force on the rear wheels and is integrated with the ABS system. The purpose of the control module for electronic brake force distribution (EBD) control is to control the hydraulic pressure in the brake circuit for the rear wheels if the rear wheels tend to lock under braking. This ensures optimal and stable braking. The EBD function replaces the relief valve used in the Mark IV version to reduce the pressure in the brake circuit for the rear wheels.

TRACS 

TRACS is a traction control system which is integrated with the ABS system. TRACS is only available on all-wheel drive (AWD) cars. The traction control system distributes driven power between the wheels so that the wheel with the best road traction receives the greatest amount of driven power. The system is mainly designed to assist with moving off on slippery surfaces at speeds up to approximately 60 km/h. TRACS has only a very limited effect at speeds over 60 km/h. TRACS (traction control system) is always engaged when the ignition is switched on. TRACS (traction control system) is engaged and disengaged using a switch under the climate control module (CCM). From model year 2003 inclusive, the TRACS function cannot be disabled manually.

STC 

STC (stability and traction control) consists of two components:

• Traction Control - this function is described above under TRACS. The traction control function is always engaged when the ignition is switched on. The traction control function together with the stability function is engaged and disengaged using a switch under the climate control module (CCM). From model year 2003 inclusive, the Traction Control function cannot be disabled manually
• Stability Control - stability system which is integrated in the ABS system and stabilizes the car in the event of wheel spin. The brake control module (BCM) transmits a signal to the engine control module (ECM) to reduce engine torque until the slipping wheel(s) has/have stopped spinning. The stability function is always engaged when the ignition is switched on. The stability function together with the traction control function is engaged and disengaged manually using a switch under the climate control module (CCM).

DSTC 

DSTC (Dynamic Stability and Traction control) - a dynamic anti-slip system which is integrated in the STC (stability and traction control) system. The DSTC (dynamic stability and traction control) function is engaged when the ignition is switched on but can be disengaged or engaged manually using the switch on the climate control module (CCM).

The DSTC (Dynamic stability and traction control) system consists of the following:

• Traction Control - This function is described above under TRACS
• Stability Control - Described under STC above
• AYC (Active Yaw Control). The system is based upon a number of sensors which sense the driver's steering movements (steering wheel angle) and the directional, both straight and lateral, movement of the car. The information is used to correct the car in the event of oversteer or understeer
• RSC (Roll over stability control) is a function which monitors the tilt angle rate. If the driver turns the steering wheel quickly, during an avoidance maneuver for example, and the car is likely to tip over, the tilt angle sensor registers the rate at which the tilt angle is changing. The DSTC system rights the car. RSC (roll over stability control) is only available on the XC90
• Emergency Brake Assistance is a new function found in cars equipped with DSTC (dynamic stability and traction control) from model year 2002-. The function is integrated in the brake control module (BCM) and monitors the movement of the brake pedal. A sudden movement of the brake pedal (emergency braking) deploys the emergency brake assistance function to achieve optimum braking (more effective than normal braking). Information from the pedal position sensor is used to start the emergency brake assistance function. Full braking force is maintained for as long as the driver maintains or increases the pressure on the brake pedal. If the driver releases the brake pedal the emergency brake assistance function is immediately interrupted. Emergency brake assistance operates from approximately 10km/h up to the maximum speed. This function cannot be switched off.