System Description
PURPOSEThe purpose of ABS is to prevent the vehicle wheels locking during brake application, thus 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.
CAUTION:
- 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 the tire and road surface.
- The additional control provided by ABS must never be exploited in a dangerous or reckless manner which could jeopardize the safety of the driver or other road users.
System Description
The brake system is hydraulically power assisted with an integrated, electronically controlled four channel anti-lock brake system (ABS).
The use of a power assisted brake system means that during brake application, additional hydraulic energy is provided by a hydraulic power unit.
This hydraulic power unit consists of an electrically driven pump and an accumulator which stores hydraulic energy in readiness for brake application. A pressure switch controls the hydraulic pump to maintain fluid pressure in the accumulator.
The hydraulic system comprises two completely independent circuits.
The rear calipers and upper pistons of the front calipers form the POWER CIRCUIT. The lower pistons in the front calipers form the COMBINED POWER/HYDROSTATIC CIRCUIT.
Hydraulic Components:
Key to Diagram
A Fluid feed/return
B Power circuit
C Hydrostatic (master cylinder) circuit
D Combined hydrostatic/power circuit
E Component enclosure
NOTE: Numbers in parentheses (1), (1.1), etc, correspond to components in the diagram.
Hydraulic booster unit (1)--Mounted in the same position as the conventional master cylinder/servo unit, the booster unit contains the following components: Fluid reservoir, power valve, master cylinder, isolating valve, ABS solenoid control valves, servo cylinder.
NOTE: The hydraulic booster 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. Extreme care must be taken when changing seals to avoid ingress of debris.
Fluid reservoir (1.1)--Mounted on top of the booster unit, the plastic reservoir is subdivided internally to provide separate capacity for the brake fluid used in the hydrostatic and power circuits. A central tube incorporates a filter and a low fluid warning level switch.
Power valve (1.2)--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.
Master cylinder (1.3)--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. A tilt valve is incorporated to supply fluid to the master cylinder from the reservoir connection.
Isolating valve (1.4)--The isolating valve consists of two solenoid valves controlling fluid inlet and outlet. Their function is to disconnect the master cylinder from the servo cylinder and to connect the servo cylinder to the reservoir return during ABS function.
ABS solenoid control valves - 8 of (1.5)--Each pair, comprising inlet and outlet solenoid valves, controls 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 of (1.6)--have four functions.
1. To provide combined energy from both the hydrostatic and the power circuit to the brake calipers.
2. To provide brake feel at the brake pedal.
3. To provide hydrostatic (master cylinder) braking through the servo cylinder to the calipers in the event of no power circuit pressure to the servo cylinder.
4. To provide braking from both the power circuit and from hydrostatic fluid remaining in the servo cylinder, in the event of no hydrostatic circuit pressure from the master cylinder.
Hydraulic power unit (2)--The hydraulic power unit comprises an electrically driven pump and a pressure switch. The pressure switch incorporates three electro-mechanical switches: one for the pump, another, at a different presure setting, to illuminate the pressure warning light. The latter switch plus the third switch inform the ECU of low pressure and that ABS function should cease whilst pressure remains low.
The pump also incorporates a non-return valve, a low pressure inlet filter, and a pressure relief valve to protect the system.
Accumulator (3)--The diaphragm accumulator is precharged with nitrogen, its function is to store hydraulic energy.
Pressure Conscious Reducing Valve (PCRV) (4)--The PCRV is located between the power valve and the ABS solenoid valves for the rear axle. Its function is to limit the brake pressure to the rear axle.
Brake calipers - front - (5) rear - (6)
NOTE: To identify the separate hydraulic circuits, they are referred to as Hydrostatic and Power circuits.
Power circuit - The rear calipers and the upper pistons in the front calipers form one circuit, supplied by direct hydraulic power from the power valve.
Hydrostatic circuit - The lower pistons in the front calipers form the other circuit, supplied with hydraulic energy from the servo cylinders comprising a combination of master cylinder pressure and direct hydraulic power.
Electronic control unit (ECU)--ABS operation is controlled by the ECU located under the left hand front seat, and grounded to the center tunnel.
The ECU is connected to the ABS harness by a 35 way connector.
The ECU is a non-serviceable item. It must be replaced if failure occurs.
Sensors and exciter rings - 4 of--A sensor is mounted at each wheel, sensing a 60 tooth exciter ring. When the vehicle is in motion the inductive sensors send signals to the ECU. The front exciter ring is fitted to the outside diameter of the constant velocity joint in each front hub. The rear exciter ring is bolted to the rear of each brake disc.