See Figures 1 and 2
The Anti-Lock Braking System, designated ABS, prevents wheel lock-up. The ABS provides the shortest possible braking distance while maintaining full directional stability. The ABS controls the front wheels individually and the rear wheels together. The rear piston in the master cylinder operates the front brakes and the front piston operates the rear axle brakes.
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 Control Unit (CU) 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 immediately control the brake line pressures to the affected wheel(s) by issuing output signals to the hydraulic modulator.
The hydraulic modulator contains solenoids which react to the signals from the CU. When not activated, the solenoids allow brake line pressure to be modulated by the brake pedal in the normal fashion. At the direction of the (CU), the solenoids move to positions either isolating the brake line from pedal pressure (pressure hold) or isolating the line and opening a passage to relieve line pressure (pressure release). In this manner, brake application is controlled or actually lessened, dependent on the locking tendency of each wheel.
The decisions regarding these functions are made very rapidly and each solenoid can be cycled several times per second. Volvo employs a 3-channel control system. The front wheels are controlled separately; the rears are watched by a single sensor on all models except the 850/C70/S70/V70 which have two, and the common feed line to the rear brakes is controlled by one output on the hydraulic modulator.
The operator may feel a slight 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. Additionally, the final few feet of an ABS-engaged stop may be completed with the wheels locked; the system is inoperative below 3 mph.
When the ignition is ON and vehicle speed is over 3 mph (5 kph), the CU monitors the function of the system. Should a fault be noted, such a loss of signal from a sensor, the ABS system is immediately disabled by the CU. The Anti-Lock dashboard warning lamp is illuminated to inform the operator. When the ABS system is disabled, the vehicle retains normal braking capacity without the benefits of Anti-Lock.
Some vehicles have a Traction Control System (TRACS) incorporated into the ABS system. The CU monitors the speed sensors just as it does for the ABS, and if one of the drive wheels is detected moving faster than the other, it engages that wheels brakes to reduce the power to the wheel. This is accomplished by closing and opening the solenoids in the hydraulic modulator, causing the brake fluid to be pumped to that wheel, slowing the wheel rotation down and applying power equally.