Hydraulic Control Assembly - Antilock Brakes: Description and Operation
Brake Control Modulator
The brake control modulator comprises:
- an ABS/TCCM
- a hydraulic control unit,
- an electrically driven hydraulic pump,
- two low pressure accumulators and damping chambers.
ABS/TCCM
The ABS/TCCM is integrated with the hydraulic control unit. It houses the solenoids that operate the hydraulic control unit's input and return valves; the valve stems locate into the ABS/TCCM's solenoids. There is no electrical connection between the ABS/TCCM and the hydraulic control unit.
The ABS/TCCM constantly monitors the brake electronic system by passing self-checking electrical pulses around the circuit. The self check procedures carried-out by the ABS/TCCM are listed below:
1. Monitoring of the operating voltage: the control module will shut-down should the voltages not be (approximately) in the range of 9 to 19V.
2. The pump is monitored for voltage supply and voltage feedback.
3. Monitoring for disturbance, where a failure may be caused by external radio frequency interference, internal leakage current, defective valve transistor or a faulty valve.
4. Internal controller failure, detected by a RAM and ROM test.
5. The Controller Area Network (CAN) micro-controller checks for correct operation of the communication network and performs a RAM self-check.
CAUTION: Disconnection of ABS/TCCM connector may introduce other fault codes. Following rectification and reconnection, ensure that no induced codes exist.
Signals processed by the ABS/TCCM are also sent to the CAN for vehicle use, such as the instrument cluster speed display, cruise control and stability/traction control.
Hydraulic Control Unit
Control of the fluid pressure to the front and rear brakes is by means of the valves contained in the hydraulic control unit. The valves are operated by solenoids housed in the ABS/TCCM.
Hydraulic Pump
The hydraulic pump is integrated with the hydraulic control unit.
It is a reciprocating two-circuit pump in which one brake circuit is assigned to each pump circuit.
It supplies the fluid pressure and volume to supply the brake circuits under ABS and Traction control.
The pump is driven by an electric motor, with the pump housing incorporating two low pressure accumulators and damping chambers for each brake circuit.
The accumulator stores the pressure and smooths out the output pulses from the pump.
ABS Warning Lamp
The system will be set in a fail-safe mode if a problem is detected, a warning lamp on the instrument panel will notify the driver there is a problem.
Anti-Lock Braking System (ABS)
The Antilock Braking System (ABS) components are combined with an hydraulic booster and a tandem master cylinder to provide a two circuit braking system. The system comprises, four wheel speed sensors, a brake control modulator and an ABS warning lamp.
Should a wheel start to lock-up during braking a signal transmitted from the wheel speed sensor to the brake control modulator will start the hydraulic pump. The brake control modulator will close the input valve of the line connected to the locked-wheel to stop any increase in fluid pressure to the brake caliper. If this fails to prevent the wheel locking, the pressure in the caliper will be decreased by opening the return valve until the wheel starts accelerating again. The fluid pressure from the return valve is directed into the low pressure accumulator, housed in the pump.
From the low pressure accumulator, fluid is pumped through the damping chamber to the brake master cylinder. The pressure to the brake caliper will then be increased in small steps to maintain maximum adhesion between the tire and road surface. This is achieved by closing the return valve, and opening the input valve and using the hydraulic pump to increase the pressure.
Although the system senses all four wheels independently, the rear wheels are regulated as a pair. If a sensor on a rear wheel detects a wheel decelerating rapidly, then the fluid pressure to both wheels is reduced. The fluid pressure is then dictated by the wheel having the lowest adhesion with the road surface.
The ABS system will be set in a fail-safe mode if a problem is detected a warning lamp on the instrument panel will notify the driver there is a problem. The brake system will still operate conventionally and with the same standard of performance as a vehicle not equipped with ABS.
Automatic Stability Control (ASC)
The Automatic Stability Control (ASC) utilizes the ABS sensing arrangement to provide the maximum traction force to propel the vehicle.
The ASC is switched on when the engine is started. The system can be switched off by pressing the 'ASC OFF' (the switched is marked 'TRAC OFF' on vehicles fitted with traction control). The switch, which is situated in the center console switchpack, lights up to warn that the system is switched off. An 'ASC' amber warning light flashes on the instrument panel when the system detects a spinning wheel.
The ASC system uses engine intervention to reduce the torque delivered to the drive wheels to prevent them spinning.
Engine torque is reduced in three ways:
- The throttle is moved towards the closed position.
- The ignition is retarded.
- Fuel is cut-off at the cylinder injectors.
Wheel spin is detected by the wheel speed sensors and communicated to the ABS/TCCM. The ABS/TCCM uses information from the CAN to calculate the torque that the engine should produce to stop the wheel spinning.
Torque reductions are then requested from the Engine Control Module (ECM) through the CAN. The throttle is then positioned to provide the target torque, which has been calculated to prevent wheel spin. During the transient phase of torque reduction the fuel is cut-off and the ignition retarded. Both the fuel cut-off and ignition retard will be restored to normal when the throttle is set to its new position.
The ASC uses a brake control modulator with six solenoid valves: three normally open inlet valves and three normally closed outlet valves.
Traction Control
Traction control has the same capabilities as ASC but with the added ability to actuate the brakes of the spinning wheel. This enhances ASC by improving acceleration particularly on surfaces with uneven friction, e.g. one wheel on ice the other on tarmac.
When wheelspin occurs, brake line pressure to the slipping wheel is generated, to operate that wheel's brake. The BCM achieves this by closing the cut-off solenoid valve and operating the hydraulic pump. Brake fluid is then drawn from the reservoir through the non-actuated master cylinder to the brake caliper.
A 'TRAC' amber warning light flashes on the instrument panel when the system detects a spinning wheel.
The traction control system uses a brake control modulator with nine solenoid valves: four normally open inlet valves and four normally closed outlet valves and one special isolating valve. The outlet valves control the brakes of each each rear wheel individually on the traction control system.