ABS Anti-Lock Brakes

The antilock brake system or the ABS control unit 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 anti-lock 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.

ABS Brake System
Typical ABS Brake System

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.

There are several types of ABS systems; some of the most popular models are based from the Bosch ABS Actuator (BAA), Nippon-Denso ABS Actuator (NAA) systems. The basic design and assembly is the same, the Nippon-Denso system has a separate solenoid relay pack and ABS computer, where as in the Bosch system both the components are combined. Also, there is an immediate connection between the wheel speed sensors and the ECU (engine control unit), these systems are connected by the CAN (Controller Area Network) bus, this system communicates by sending multiple signals to multiple devices at any given time

Each wheel has a dedicated wheel 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 sensor VSS (vehicle speed sensor) to read rear wheel speed.

Wheel Speed Sensor
Wheel Speed Sensor (configurations may vary)

Common Problems and Fixes

Today's brake systems are reliable and easy to maintain, most systems are designed for easy service and brake pad replacement. If the brake pedal becomes hard to push, there can be a possibility of a broken vacuum supply line or a failed power brake booster. It is also very important to use the correct type of brake fluid; otherwise the brake system can malfunction. This can lead to problems such as spongy feeling pedal and the brake pedal travel becoming excessive. Only the highest quality brake fluid must be used in a brake system and must never be reused and must always be discarded immediately after the fluid storage container is opened to avoid moisture contamination. When adding brake fluid to the master cylinder care must be taken make sure brake fluid is not spilled on the paint of the car, brake fluid is very corrosive and will destroy the paint.

Squealing brakes is an indication of a problem with the brake system. Most of the time a squealing noise can be a warning that the brake pads are worn down and need to be replaced. Other causes can be overheated brake pads or rotors. Sometimes rust can get impregnated on the brake lining material and cause grumbling or squeaking noises. When cleaning brake components prior to service precautions must be taken. Most brake systems contain microscopic fibers that are extremely hazardous to your health. While cleaning with brake system cleaner there is an increased chance of inhaling these fibers that accumulate on the brake components. Cleaning should be done in a well ventilated area, use an air filter respirator if possible for best results.

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 in most cases. (Real time system) when the vehicle has been moved about 6 feet and the brake pedal has been pressed for five seconds the codes should clear. 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.


Written by
Technical Writer


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Article first published (Updated 2015-01-06)