ABS Description and Operation
ANTILOCK BRAKE SYSTEMDESCRIPTION
The Antilock Brake System (ABS) is an electronically operated, three channel brake control system. The vehicle has Electronic Variable Brake Proportioning (EVBP) designed into the system which eliminates the combination/proportioning valve. The system is designed to prevent wheel lockup and maintain steering control during braking. Preventing lockup is accomplished by modulating fluid pressure to the wheel brake units.
The hydraulic system is a three channel design. The front wheel brakes are controlled individually and the rear wheel brakes in tandem. The ABS electrical system is separate from other electrical circuits in the vehicle. A specially programmed controller antilock brake unit operates the system components.
ABS system major components include:
- Controller Antilock Brakes (CAB)
- Hydraulic Control Unit (HCU)
- Wheel Speed Sensors (WSS)
- ABS Warning Light
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 (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.
The CAB has a special software program called Electronic Variable Brake Proportioning (EVBP), that monitors the wheel speed(s) 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 breaking effort under normal braking conditions.
EBA makes more effective use of the rear brakes when the truck is lightly loaded, balancing front-torear 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 controls 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.
OPERATION
Battery voltage is supplied to the CAB. The CAB performs a system initialization procedure at start up. A check of the ABS motor is performed at 15 miles per hour. Initialization consists of a static and dynamic self check of system electrical components. The static and dynamic checks occurs at ignition start up. During the dynamic check, the CAB briefly cycles solenoids to verify operation. An audible noise may be heard during this self check. This noise should be considered normal. The ABS motor and pump are then checked at a speed of 15 mile per hour.
If an ABS component exhibits a fault during initialization, the CAB illuminates the amber warning light and registers a fault code in the microprocessor memory.
The CAB monitors wheel speed sensor inputs continuously while the vehicle is in motion. However, the CAB will not activate any ABS components as long as sensor inputs indicate normal braking.
During normal braking, the master cylinder, power booster and wheel brake units all function as they would in a vehicle without ABS. The HCU components are not activated.
The purpose of the antilock system is to prevent wheel lockup. Preventing lockup helps maintain vehicle braking action and steering control.
The antilock CAB activates the system whenever sensor signals indicate periods of wheel slip.
The antilock system prevents lockup during a wheel slip condition by modulating fluid apply pressure to the wheel brake units.
Brake fluid apply pressure is modulated according to wheel speed, degree of slip and rate of deceleration. Sensors at each front wheel convert wheel speed into electrical signals. These signals are transmitted to the CAB for processing and determination of wheel slip and deceleration rate. The ABS system has three fluid pressure control channels. The front brakes are controlled separately and the rear brakes in tandem. A speed sensor input signal indicating a wheel slip condition activates the CAB antilock program.
There are two solenoid valves (isolation valve and dump valve) which are used in each antilock control channel. The valves are all located within the HCU valve body and work in pairs to either increase, hold, or decrease apply pressure as needed in the individual control channels.
During an ABS stop, the ISO valve is energized which acts to prevent further pressure build-up to the calipers. Then the Dump valve dumps off pressure until the wheel unlocks. This will continue until the wheels quit slipping altogether.