Anti-lock Brake Systems (ABS) are designed to prevent locked-wheel skidding during hard braking or during braking on slippery surfaces. The front wheels of a vehicle cannot apply steering force if they are locked and sliding; the vehicle will continue in the previous direction of travel. The 4-wheel ABS system used on Volkswagen vehicles holds the wheels just below the point of locking, thereby allowing some steering response and preventing the rear of the vehicle from sliding sideways.
There are conditions for which the ABS system provides no benefit. Hydroplaning is possible when the tires ride on a film of water, losing contact with the paved surface. This renders the vehicle totally uncontrollable until road contact is regained. Extreme steering maneuvers at high speed or cornering beyond the limits of tire adhesion can result in skidding which is independent of vehicle braking. For this reason, the system is named anti-lock rather than anti-skid. Wheel spin during acceleration on slippery surfaces may also fool the system into detecting a system failure and entering the fail-safe mode.
Under normal conditions, the ABS system functions in the same manner as a standard brake system and is transparent to the operator. The system is a combination of electrical and hydraulic components, working together to control the flow of brake fluid to the wheels when necessary.
The Electronic Control Unit (ECU) is the electronic brain of the system, receiving and interpreting signals from the wheel speed sensors. The unit will enter anti-lock mode when it senses impending wheel lock at any wheel, and will immediately control the brake line pressures to the affected wheel(s) by issuing output signals to the hydraulic modulator assembly.
The hydraulic modulator contains solenoids which react to the signals from the ECU. Each solenoid controls brake fluid pressure to one wheel. The solenoids allow brake line pressure to build according to brake pedal pressure, hold (by isolating the system from the pedal and maintaining current pressure) or decrease (by isolating the pedal circuit and bleeding some fluid from the line).
The decisions regarding these functions are made very rapidly, as each solenoid can be cycled up to 10 times per second. Volkswagen employs a 3-channel control system; the front wheels are controlled separately, while the rear wheels are controlled together, based on the signal of the wheel with the greatest locking tendency.
The operator may feel a pulsing in the brake pedal and/or hear popping or clicking noises when the system engages. These sensations are due to the valves cycling and the pressures being changed rapidly within the brake system. While completely normal and not a sign of system failure, these sensations can be disconcerting to an operator unfamiliar with the system.
Although the ABS system prevents wheel lock-up under hard braking, as brake pressure increases, wheel slip is allowed to increase as well. This slip will result in some tire chirp during ABS operation. The sound should not be interpreted as lock-up but rather as an indication of the system holding the wheel(s) just outside the locking point. The final few feet of an ABS-engaged stop may be completed with the wheels locked, as the system is inoperative below 3 mph (5 kph).
When the ignition is ON and vehicle speed is over 3 mph (5 kph), the ECU monitors the function of the system. Should a fault be noted, such as loss of signal from a sensor, the ABS system is immediately disabled by the ECU. The ANTI-LOCK dashboard warning lamp illuminates to inform the operator. When the ABS system is disabled, the vehicle retains normal braking capacity without the benefits of anti-lock.