ABS/TCCM
Wheel Speed Sensors
Each wheel is monitored by a wheel speed sensor, to detect the speed of movement of a toothed gear that is driven by the wheel. Signal's generated by the toothed gear passing the sensor are sent to the ABS/TCCM to communicate the speed of wheel rotation and any rapid change of wheel speed.
The frequency of the sinusoidal output voltage signal from the sensor, is proportional to road wheel rotational speed and the number of teeth on the toothed gear. The amplitude of the voltage is dependent upon the 'air gap' between the sensor and toothed wheel, therefore it is important that this preset gap remains clear and undisturbed.
Brake Control Modulator
The brake control modulator comprises:
a 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 connectors 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 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 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 switch pack, 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 controller area network (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.