Antilock Brake System Description

SYSTEM DESCRIPTION
The controller-antilock brake module is used to monitor wheel speeds and modulates (control) hydraulic pressure in each brake channel. The modulated hydraulic pressure is used to prevent wheel lock up during braking. The CAB also provides a vehicle speed signal (VSS) to the powertrain control module.

During a non-ABS stop, the system functions as a standard front/rear split configuration. The primary supplies brake fluid pressure to the front brakes, and the secondary supplies the rear brakes.

All 2500 and 3500 Series vehicles with EBC 325 do not use a conventional combination/proportioning valve. This system uses the existing ABS solenoids to replace and perform the same functions that the combination and proportioning valves do. The CAB has a special software program Electronic Variable Brake Proportioning (EVBP), that monitors the wheel speeds and when certain criteria are met the software will enable the solenoids to perform the same brake fluid management control as the combination/proportioning valves.

During an ABS stop, the system still uses the front/rear hydraulic split; however, the brake system pressure is further split into three control channels. During ABS operation, the front wheels are controlled independently and are on two separate control channels. The rear wheels are controlled together through one control channel. By using separate control channels for the front wheel, more steering control is maintained during maximum braking.

During an antilock stop, "wheel lock-up" does not necessarily mean that the wheel has locked, it means only that the wheel is turning slower than the vehicle speed. This is called "wheel slip" and is indicated as a percentage. 0% slip means that the wheel is rolling free and 100% slip means that the wheel is locked. The antilock system maintains an average of approximately 20% wheel slip.