System Description -1
ANTI-LOCK BRAKE SYSTEM - DESCRIPTION
Anti-lock Braking System (ABS)
NOTE: On vehicles up to 99MY, the Anti-lock Brake System (ABS) is a standard feature, with Electronic Traction Control (ETC) optional. On vehicles from 99MY the ABS and ETC is standard on all models.
The brake system is hydraulically power assisted with an integrated, electronically controlled four channel ABS system. The use of a power assisted brake system means that, during brake application, additional hydraulic energy is provided by the hydraulic power unit.
Fluid pressure is distributed from the brake booster/ABS modulator unit to each of the four brake calipers. Foot pedal pressure is assisted by pressure created in the master cylinder and direct pressure from the power valve.
The hydraulic system comprises two completely independent circuits, with a vertical i.e. front/rear split. The combined POWER and HYDROSTATIC CIRCUIT supplies the front calipers. The POWER CIRCUIT supplies the rear calipers.
The purpose of ABS is to prevent the vehicle wheels locking during brake application, maintaining vehicle steerability and stability. This allows the vehicle to be steered while the brakes are applied, even under emergency conditions, and to avoid obstacles where there is sufficient space to redirect the vehicle.
The ABS system on vehicles up to 99MY uses a 'C' series ABS ECU, which includes an optional two wheel electronic traction control feature. The ECU has a single connector and is located on the bulkhead behind the glove box.
On vehicles from 99MY a 'D' series ABS ECU is used. The 'D' series ECU features four wheel electronic traction control, which is a standard feature on all models. The ECU has three connectors and is located in the same position as the 'C' series ECU.
The ABS ECU receives wheel speed information from four wheel speed sensors. The ECU monitors the deceleration of each wheel during braking and, in the event of one or more wheels being outside the expected values, controls via the hydraulic system the braking force available to that wheel.
When the wheel speed has returned to within the expected limits, the ECU restores the hydraulic pressure to the wheel. The ABS system allows optimal stopping distances to be achieved and prevents the wheels from locking.
The system is active after the ignition is switched to position 11 and has completed a bulb check. ABS and ETC warning lamps in the instrument pack warn of system operation and failure. Normal (non-ABS) braking remains available in the event of an ABS system failure.
When the ABS system operates, the driver will experience an audible noise from the ABS modulator and vibration transmitted through the brake pedal.
WARNING: ABS IS AN AID TO RETAINING STEERING CONTROL AND STABILITY WHILE BRAKING.
ABS CANNOT DEFY THE NATURAL LAWS OF PHYSICS ACTING ON THE VEHICLE.
ABS WILL NOT PREVENT ACCIDENTS RESULTING FROM EXCESSIVE CORNERING SPEEDS, FOLLOWING ANOTHER VEHICLE TOO CLOSELY OR AQUAPLANING, I.E. WHERE A LAYER OF WATER PREVENTS ADEQUATE CONTACT BETWEEN TIRE AND ROAD SURFACE.
THE ADDITIONAL CONTROL PROVIDED BY ABS MUST NEVER BE EXPLOITED IN A DANGEROUS OR RECKLESS MANNER WHICH COULD JEOPARDIES THE SAFETY OF DRIVER OR OTHER ROAD USERS.
Electronic Traction Control (ETC) - Up to 99MY
ETC is an option, available as an extension to ABS. The system operates on the rear axle only, to prevent loss of traction where one wheel has more grip than the other. The system works by applying the brake to a spinning rear wheel. This transfers torque to the wheel with grip. By applying the brake, ETC supplies the torque resistance, which the wheel cannot.
An example of when the system would operate is where one side of the vehicle is on ice, with the other side on tarmac. ETC will control the spinning rear wheel.
If both wheels spin the system does not operate, as braking one wheel will not aid traction.
The system switches itself out at 50 km/h (30 mph). A vehicle travailing above this speed will not need ETC.
ETC system operation is smooth and continuous and will not affect the comfort of the vehicle.
ETC is inhibited when the brakes are applied. When the ETC feature is operating the 'TO' warning lamp in the instrument pack will illuminate for a minimum of two seconds, a single audible warning will sound and on earlier vehicles 'TRACTION' will be displayed in the message center.
Electronic Traction Control (ETC) - From 99MY
ETC is standard on all models from 99MY onwards. ETC operates in conjunction with the ABS system to give improved traction for all wheels where one wheel has more grip than the other. The system operates by applying the brake to a spinning wheel. This transfers torque to the remaining wheels with grip. By applying the brake, ETC supplies the torque resistance, which the wheel cannot.
An example of when the system would operate is where one side of the vehicle is on ice, with the other side on tarmac. The ABS ECU monitors the speed of the each wheel. If any wheels are rotating faster than the others, brake pressure is applied to that wheel, slowing it down to match the other wheel speeds.
If all wheels spin at the same speed the system does not operate, as braking one wheel will not aid traction.
The system switches itself out at 100 km/h (62.5 mph). A vehicle travailing above this speed will not need ETC.
ETC system operation is smooth and continuous and will not affect the comfort of the vehicle. No driver intervention is required.
ETC is inhibited when the brakes are applied. When the ETC feature is operating the 'TC' warning lamp in the instrument pack will illuminate for a minimum of two seconds and a single audible warning will sound.
Hydraulic Circuit Diagram
Brake booster/ABS modulator unit - up to 99MY
Hydraulic Circuit Diagram
Brake booster/ABS modulator unit - from 99MY
Hydraulic components
Numbers refer to location of components illustrations
Brake booster/ABS modulator unit
Mounted in the same position as a conventional brake master cylinder/servo unit, the brake booster/ABS modulator contains the following components: fluid reservoir, power valve, master cylinder, isolating valve, ABS control valves and servo cylinders. It also contains the ETC solenoid control valves
NOTE: The brake booster/ABS modulator unit is not a serviceable item, if internal failure occurs a new unit must be fitted. The fluid reservoir and its seals may be changed in the event of damage.
WARNING: Extreme care must be taken when changing reservoir seals to avoid ingress of debris.
Fluid reservoir - 1.
Mounted on top of the unit, the plastic reservoir is subdivided internally to provide separate capacity for brake fluid used in the hydrostatic and power circuits. A central tube incorporates a filter. A built in fluid level warning switch and a finer filter for the power circuit fluid are also incorporated. The fluid level warning switch is closed when there is sufficient fluid in reservoir.
On manual vehicles, the reservoir also supplies fluid to the clutch system.
Master cylinder - 2.
Operation of the master cylinder displaces a volume of brake fluid into the servo cylinders and increases fluid pressure. Piston movement inside the master cylinder will also activate the power valve.
Power valve - 3.
The power valve is an extension of the master cylinder, it controls fluid pressure in the power circuit in direct proportion to pressure in the master cylinder. The power valve is of spool valve design.
Isolating valve - 4.
The isolating valve consists of two solenoid valves controlling fluid inlet and outlet. Their function is to disconnect the master cylinder from the servo cylinders and to connect the servo cylinders to the reservoir return during ABS function.
ABS solenoid control valves, 8 off- 5.
Each pair, comprising inlet and outlet solenoid valves, control ABS braking to each wheel. In response to signals from the ECU, the valves decrease, hold or increase brake pressure according to the need to retain wheel rotation and obtain optimum braking. The solenoid valves are designed to respond rapidly to ECU signals.
Servo cylinders, 2 off- 6.
Servo cylinders have five functions:
1. To provide combined energy from both hydrostatic and power circuit to brake calipers.
2. To provide 'brake feel' at the brake pedal.
3. To provide hydrostatic (master cylinder) braking through the servo cylinders to calipers in the event of no power circuit pressure to servo cylinders.
4. To provide braking from the power circuit and from hydrostatic fluid remaining in servo cylinder, in event of no hydrostatic circuit pressure from master cylinder.
5. To provide ABS control to the front calipers in response to pressure modulations in the power circuit.
ETC control valves - up to 99MY - 7 & 8
The ETC control valves are optional on vehicles up to 99MY. The ETC inlet solenoid valve (7) is normally closed and the ETC isolating solenoid valve (8) is normally open when ETC operation is not required.
When ETC operation is required, the inlet solenoid valve opens allowing fluid from the power circuit to flow to the rear ABS solenoid control valves. Simultaneously, the ETC isolating solenoid valve closes, isolating the connection to the PCRV. The ABS ECU energizes the applicable ABS solenoid valve to pressurize the applicable rear wheel brake.
ETC control valves - from 99MY- 7 & 8
The ETC control valves are standard on vehicles from 99MY. The ETC inlet solenoid valve (7) is normally closed and the ETC isolating solenoid valve (8) is normally open when ETC operation is not required.
When ETC operation is required, the inlet solenoid valve opens allowing fluid from the power circuit to flow to the power valve. The power valve is actuated by the pressure and opens to allow fluid from the power circuit to flow directly to the front ABS solenoid control valves and to the rear solenoid control valves via the PCRV. The ETC isolating solenoid valve closes isolating the return flow from each ABS solenoid control valve. The ABS ECU energizes the applicable ABS solenoid valve(s) to pressurize the applicable front and/or rear wheel brake.