Design
Design
Control Module And ABS Hydraulic Modulator
The brake control module (BCM) is mounted on the hydraulic modulator in the engine compartment.
The control module is supplied with power via two fuses and connected to ground by two ground terminals.
There are diagnostics for the valve coils.
The brake control module (BCM) is available in two different versions.
ABS control module
The control module contains two different micro-processors which receive signals from different sensors in the system. The micro-processors operate in parallel and use a program to compute the necessary ABS and electronic brake force distribution functions. The ABS control module has eight valve coils for the hydraulic valves, four outlet valves and four inlet valves.
The control module has a connector for connection to the car wiring.
Control module for STC (stability and traction control), DSTC (dynamic stability and traction control), and TRACS (traction control system)
The control module for STC/DSTC/TRACS has twelve valve coils for the hydraulic valves; four are outlet valve coils, four are inlet valve coils and four are used for stability and traction control.
When replacing brake control modules (BCM) with DSTC, the control module must be calibrated according to the instructions in VADIS vehicle communication or by a special test drive (see test drive schedule DSTC/BCM, description).
ABS hydraulic modulator
The hydraulic modulator is available in three versions; ABS, STC (stability and traction control) and DSTC (dynamic stability and traction control)/TRACS (traction control system). The ABS hydraulic modulator is located in the engine compartment.
The ABS hydraulic modulator consists of hydraulic valves, check valves and a pump motor. The pump motor builds up the brake pressure required during ABS or stability control.
There are diagnostics for the pump motor.
STC/DSTC switch
The switch for STC/DSTC is positioned next to the other buttons on the lower section of the climate control module (CCM). The stability and traction control functions can be switched on and off manually (this applies only to the stability function from model year 2003 inclusive). When the ignition is switched on all functions are engaged. The switches have a green LED indicator which display the ON position. When STC/DSTC is switched on or off, this is confirmed by a message in the driver information module (DIM). If the STC- /DSTC function is disabled, the other functions work normally.
NOTE: The figure displays the switch for cars equipped with STC.
Warning lamps
The brake system uses four warning lamps positioned in the combined instrument panel.
ABS warning lamp
The ABS warning lamp indicates an ABS system fault.
Brake warning lamp
The brake warning lamp indicates:
- faults in the electronic brake force distribution (EBD) function
- that the brake fluid level is too low.
Traction control warning lamp (only cars with STC/DSTC) model year 1999-2002
The traction control warning lamp is on continuously when:
- STC/DSTC is deactivated because a diagnostic trouble code (DTC) is stored
- The STC/DSTC system has been manually switched off
- the brake disc temperature is too high.
The traction control warning lamp flashes when:
- a drive wheel starts to spin during acceleration. The control module is activated to regain traction
- the lateral stability limit is exceeded (the car oversteers or understeers) during active yaw control.
Traction control warning lamp (only cars with STC/DSTC) model year 2003-
The traction control warning lamp is lit continuously when the road surface is slippery.
The traction control warning lamp flashes when:
- there is a risk of a loss of traction
- a drive wheel starts to spin during acceleration. The control module is activated to regain traction
- the lateral stability limit is exceeded (the car oversteers or understeers) during active yaw control
- the limit for the anti-roll function is exceed and active yaw control is activated to regain stability.
General warning lamp model year 1999-2002
The general warning lamp (red/yellow) lights only for those faults in the brake control system which disable emergency brake assistance.
General warning lamp model year 2003-
- STC/DSTC is deactivated because a diagnostic trouble code (DTC) is stored
- The brake disc temperature is too high.
Text display model year 2003-
The stability function has been manually disabled using the STC/DSTC switch.
Wheel sensors
There are four wheel sensors, one at each wheel. The wheel sensors supply the control module with information about the rotation speed, acceleration and retardation of each wheel. By recording the pulses, the control module can calculate the wheel speed.
The wheel sensors are active sensors. When the pulse wheel rotates, the sensor generates a pulsed current (quadratic wave) where the strength of the current depends on the position of the pulse wheel. The signals from the coils in the wheel sensors are then affected by a magnetic resistance element, which generates a current which changes between 7 mA and 14 mA, and whose frequency increases with speed.
On the front wheels, the pulse wheel is pressed onto the outer constant velocity joint and the wheel sensors are located radially in the shaft housing relative to the pulse wheel. On the rear wheels, the pulse wheel is pressed onto the hub and the wheel sensors are located axially relative to the pulse wheel. The rear pulse wheels can be replaced. Both the front and rear pulse wheels have 48 teeth. In the event of a fault in any of the wheel sensors the following systems will be disengaged or will not engage:
- ABS
- TRACS
- STC
- DSTC.
Electronic brake force distribution (EBD) is available if one wheel sensor is faulty. Electronic brake force distribution (EBD) is disengaged if there is a fault in more than one wheel sensor simultaneously.
The control module calculates the vehicle speed using the signals from the wheel sensors. There are diagnostics for the wheel sensors.
Stop (brake) lamp switch
The stop (brake) lamp switch is a single pole switch positioned at the brake pedal lever. The stop (brake) lamp switch self-adjusts during installation.
The brake control module (BCM) receives information about the status of the stop (brake) lamp switch via the engine control module (ECM). The stop (brake) lamp switch signal is used by the brake control module (BCM) to diagnose faults in the pedal position sensor.
The stop (brake) lamp switch is supplied with power from the battery. When the switch is closed, the signal is grounded through the engine control module (ECM). The engine control module (ECM) then transmits a signal on the Control area network (CAN) indicating that the stop (brake) lamp switch has been activated.
There are diagnostics for the stop (brake) lamp switch.
Sensor module (Only DSTC)
The sensor module is installed under the right-hand front seat and contains two to four sensors, depending on which of the three versions is installed. The information from the sensor module is used for stability calculations in the brake control module (BCM). The sensor module for 2 wheel drive cars contains a yaw angle sensor which measures the yaw angle speed in degree/s, and a lateral acceleration sensor which measures lateral acceleration in m/sec.sq. The sensor module all-wheel drive (AWD) also contains a yaw angle sensor and lateral acceleration sensor as well as an acceleration sensor which measures longitudinal acceleration in m/sec.sq.
The XC90 also has a tilt sensor in the sensor module. The tilt sensor gauges the tilt rate in degree/s. The sensor is installed using a new bracket, turned 90 degrees.
The sensor module communicates with the brake control module (BCM) via an internal control area network (CAN).
The supply voltage to the sensor is 12 V and is connected to ground.
There are diagnostics for the sensor module.
When replacing the sensor module, the brake control module (BCM) must be calibrated according to the information in VADIS vehicle communication or manually according to the test drive.
Active power brake booster, pedal pressure sensor (DSTC only)
The power brake booster is positioned on the firewall in the engine compartment. It consists of a vacuum cylinder (1), pedal position sensor (2), pressure build-up valve (3) and pedal pressure sensor (4). The pressure build-up valve and the pedal pressure sensor (activation unit) have a common connector which cannot be replaced.
The pedal pressure sensor is positioned in the power brake booster and provides the brake control module (BCM) with information about brake activity. If the driver presses the brake pedal during stability control, the control module will take account of this.
The active power brake booster is controlled by the brake control module (BCM) and builds up an initial brake pressure. This is so to allow braking during active yaw control. An active brake pressure is obtained within 0.1 seconds. This occurs without the driver needing to brake. The pressure build-up valve opens and allows atmospheric pressure to enter the rear chamber in the power brake booster. The brake pressure sensors (5) continuously monitor the brake pressure. The pulse width controlled pressure build-up valve in the power brake booster maintains the brake pressure at approximately 10 bar during stability control.
Pedal position sensor
The pedal position sensor is mounted on the power brake booster in the engine compartment. The sensor has several functions. It is used to:
- register rapid pedal movements for emergency brake assistance
- shut off the traction control during braking
- to diagnose the function of the stop (brake) lamp switch
- it is used by the engine control module (ECM) for cruise control. The signal is transmitted via the control area network (CAN).
The sensor, which responds to the push rod for the pedal, is a sliding potentiometer. 0 mm corresponds to 0 V and 38.5 mm movement corresponds to approximately 95% of battery voltage (Ubat).
Brake pressure sensor (Only DSTC)
Two brake pressure sensors are positioned on the master cylinder. One sensor is located on the primary circuit and one on the secondary circuit. The information is transmitted via two analog signals (measurement range 0-5 V) and is used by the brake control module (BCM) to calculate the brake pressure during active yaw control, and when the driver applies the brakes.
The sensors are supplied with voltage via the brake control module (BCM). There are diagnostics for the sensors.
When replacing the brake pressure sensor, the brake control module (BCM) must be calibrated according to the information in VADIS vehicle communication or manually according to the test drive.