The antilock brake system or the ABS helps to maintain control and directional stability of an automobile in case of extreme braking circumstances. This is achieved by controlling the rotational speed of every wheel by metering the brake line pressure at the time of extreme braking. The system works on most types of road surfaces and decreases the risk of an accident and severity of an impact. Research has shown that an antilock brake system can decrease the chance of a vehicle accident by 18%. ABS brake systems were introduced in the late seventies and have enjoyed great technological advances since that time. Not only does ABS provide non-skid functionality but it also supports electronic stability control, brake assist, traction control, etc. Recently, additional sensors have been added to the system, gyroscopic sensors and steering wheel angle sensors. Both synchronize to match the direction of the car with the direction of the steering wheel. The wheel angle sensor also helps the ABS system control the outer wheels to have a more positive braking effect when compared to the inner wheels on the curve.
The ABS assembly is made up of a central electronic unit, four solenoid valves and two or more electric hydraulic pumps. The function of the electric hydraulic pump is to supply brake fluid pressure to the braking system by forcing hydraulic pressure to a reservoir located in the accumulator. The four solenoid pressure valves control brake fluid pressure for each individual wheel. During an ABS operation event, one or more of the solenoid valves dump brake line pressure to a particular wheel allowing it to start turning.
Each wheel has a dedicated speed sensor which sends an electronic signal to the ABS controller, from this signal the rotational speed of the wheel can be determined. The location of wheel speed sensor may vary, on most vehicles the front sensor is made into the steering knuckle, near the front wheel hub or outer CV joint. In the rear of the vehicle the sensor rings are bolted to the rear driver assembly on the ring gear or attached to the outer axle flange on each side, some even use the vehicles speedometer (Vehicle Speed Sensor) to read rear wheel speed.
When the ABS system detects a problem a fault code is stored in the ABS system control unit. When an error occurs the ABS warning light located on the dashboard flashes or stays on steadily. The ABS computer stores a diagnostic code until the malfunction has been repaired. To retrieve ABS trouble codes on older vehicles can be tricky because of the variation of retrieval methods; in this case a car repair manual is needed. On newer vehicles a diagnostic information connector is located under the dash on the driver's side; a trouble code scanner can be attached to retrieve ABS codes similar to engine trouble codes. After the problem has been repaired codes should erase automatically with no procedure from the scan tool. (Real time system) after the vehicle has been moved about 6 feet and the brake pedal has been pressed for five seconds. When repairs have been completed the ABS, MIL (malfunction indicator lamp) should not be illuminated, if the ABS light remains on there is still a problem, re-scan the ABS computer for further repairs.
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