Antilock Brakes / Traction Control Systems: Description and Operation
ANTI-LOCK BRAKING SYSTEMDESCRIPTION
The anti-lock braking system (ABS) components are combined with a hydraulic booster, tandem master cylinder (TM C) and ventilated disc brakes on all four wheels, to provide a two circuit braking system. The front brakes are fitted with four-piston calipers; the rear brakes have single piston calipers and drums for the cable operated handbrake. The anti-lock braking system comprises the following:
1. Hydraulic control module comprising electric motor driven pump, two low-pressure accumulators, modulator valve block and ABS control module (ABS CM)
2. Four inductive wheel speed sensors, hub end mounted
3. ABS warning indicator, mounted on the instrument panel
4. Auxiliary inputs providing information to the ABS CM
5. Diagnostic ISO communication BUS input/output link.
The valve block houses solenoid operated valves which are activated by voltage signals from the control module. The signals are generated using wheel speed information received from the wheel speed sensors.
The valves regulate the supply of pressure individually to the front wheels and collectively to the rear wheels, as necessary, to prevent wheel locking during braking.
When the ignition is switched on, an ABS selftest is initiated. During this test, the ABS warning indicator is lit for approximately 1.7 seconds and then extinguished. After this time delay the control module is ready to process signals provided from the various input sources and, using the software defined algorithm, control the electrical and hydraulic circuits. A fault is indicated if the warning indicator remains lit or comes on whilst the vehicle is being driven. Under fault conditions the system is inhibited or disabled, although conventional braking is unaffected.
The fluid level indicator lamp, mounted on the instrument panel, is lit when the brake fluid falls below the MIN mark on the brake fluid reservoir.
Fault conditions detected by the ABS CM disable the ABS until the fault is rectified. The system will be disabled when any of the following conditions occur:
1. Valve failure
2. Sensor failure
3. Main driver failure (internal ABS CM fault)
4. Redundancy error (internal ABS CM fault)
5. Over-voltage / under-voltage
6. Pump motor failure
7. Under-voltage condition.
The input frequency from each wheel speed sensor signal is translated by the ABS CM into a comparable wheel speed. The ABS CM continually monitors the system. False wheel speed information, such as sudden speed changes or excessive speeds, is detected as a sensor malfunction. The ABS CM reacts to fault conditions in the following ways:
Inhibit - ABS is inhibited until the sensed speed returns to within an acceptable limit, whereupon ABS is restored. Conventional braking is unaffected. Depending on vehicle speed, the ABS warning indicator may come on.
Disable - ABS is disabled (switched off) and the ABS warning indicator comes on. The system will not be restored until the engine is switched off and restarted or the fault has been rectified. After the system has been disabled, the warning indicator remains on until the vehicle has reached a speed of 20 km/h (12.5 mph) during the first ignition cycle after fault rectification.