When wheel slip is detected during a brake application, the ABS enters antilock mode. During antilock braking, hydraulic pressure in each wheel hydraulic circuit is controlled to prevent a wheel from slipping. A separate hydraulic line and specific solenoid valves are used for each wheel. The ABS can decrease, hold, or increase hydraulic brake pressure to each wheel.
The ABS cannot, however, increase hydraulic pressure above the amount which is transmitted by the master cylinder from the brake pedal during braking.
During antilock braking, a series of rapid pulsations is felt in the brake pedal. These pulsations are caused by the rapid changes in position of the individual solenoid valves as the Electronic Brake Control Module (EBCM) responds to wheel speed sensor inputs and attempts to prevent a wheel slip condition. These pedal pulsations are present only during antilock braking and stop when normal braking is resumed or when the vehicle comes to a stop. A ticking or popping noise may also be heard as the solenoid valves cycle. During antilock braking on dry pavement, intermittent chirp noises may be heard as the tires approach the slipping point. These noises and pedal pulsations are normal during antilock operation.
Vehicles equipped with ABS may be stopped by applying normal force to the brake pedal. There is no need to pump the brake pedal. Brake operation during normal braking is no different than that of previous non-ABS systems. Maintaining a constant force on the brake pedal provides the shortest stopping distance while maintaining vehicle stability.
The IPC illuminates the ABS indicator when a faulty is detected in any of the related circuits.