Antilock Brake System Description

ABS

The Controller Antilock Brake (CAB) 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 and maintain vehicle stability. The CAB also provides a vehicle speed signal WSS) 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.

The CAB has a special software program called Electronic Variable Brake Proportioning (EVBP), that monitors the wheel speeds) and when certain criteria are met the software will enable the HCU to perform the same brake fluid management control as the combination/ proportioning valves.

The EBC 125 (RWAL) system uses Electronic Brake Apportioning (EBA). The HCU replaces the conventional proportioning valve as a means of balancing the front-to-rear braking effort under normal braking conditions.

EBA makes more effective use of the rear brakes when the truck is lightly loaded, balancing front-to rear lining wear and minimizing instances of rear wheel Antilock action. As with a proportioning valve, EBA uses the rear axle speed sensor to determine how much, if any, to reduce the rear brake hydraulic pressure based on the deceleration of the rear wheels. EBA automatically adapts to variations in the vehicle loading and road surface, where as a proportioning valve is fixed.

The EBC 325 (ABS) system uses Dynamic Rear Proportioning (DRP). Like EBA on the RWAL system, the HCU replaces the conventional proportioning valve as means of balancing the front-to-rear braking effort under normal braking conditions. Unlike EBA, DRP adjusts hydraulic pressure to the rear brakes based on the amount of slip indicated by the wheel speed sensors when in activation.

Both EBC 125 (RWAL) and EBC 325 (ABS) use the CAB and HCU to make an integral electronic/ hydraulic unit which shares data with other electronic modules on the vehicle via the PCI BUS network.

During an RWAL stop, the system still uses the front/rear hydraulic split; however, the brake system pressure is further split into one control channel. During RWAL operation, the front wheels are not assisted and brake pressure is applied from the master cylinder, while the rear wheels are controlled together through one channel by the HCU.

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 channel. By using separate control channels for the front wheels, more steering control is maintained during maximum braking.

During an Antilock stop, "wheel lock-up" doesn't 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.