REMOVAL. . . . . . . . . . . . . . . . . . . . . . . . . . . . .45INSTALLATION. . . . . . . . . . . . . . . . . . . . . . . . .45BRAKES - ABSDESCRIPTIONThe antilock brake system (ABS) is an electroni-cally operated, three channel brake control system.The vehicle has Electronic Variable Brake Propor-tioning (EVBP) designed into the system which elim-inates the combination/proportioning valve.The system is designed to prevent wheel lockupand maintain steering control during braking. Pre-venting lockup is accomplished by modulating fluidpressure to the wheel brake units.The hydraulic system is a three channel design.The front wheel brakes are controlled individuallyand the rear wheel brakes in tandem. The ABS elec-trical system is separate from other electrical circuitsinthevehicle.Aspeciallyprogrammedcontrollerantilock 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 LightOPERATIONBattery voltage is supplied to the CAB. The CABperforms a system initialization procedure at startup. A check of the ABS motor is performed at 15miles per hour. Initialization consists of a static anddynamic self check of system electrical components.The static and dynamic checks occurs at ignitionstart up. During the dynamic check, the CAB brieflycycles solenoids to verify operation. An audible noisemaybeheardduringthisselfcheck.Thisnoiseshould be considered normal. The ABS motor andpump are then checked at a speed of 15 mile perhour.If an ABS component exhibits a fault during ini-tialization, the CAB illuminates the amber warninglight and registers a fault code in the microprocessormemory.The CAB monitors wheel speed sensor inputs con-tinuously while the vehicle is in motion. However,the CAB will not activate any ABS components aslong as sensor inputs indicate normal braking.During normal braking, the master cylinder, powerbooster and wheel brake units all function as theywould in a vehicle without ABS. The HCU compo-nents are not activated.DRBRAKES - ABS5 - 39
The purpose of the antilock system is to preventwheel lockup. Preventing lockup helps maintain vehi-cle braking action and steering control.The antilock CAB activates the system wheneversensor signals indicate periods of wheel slip.Theantilocksystempreventslockupduringawheel slip condition by modulating fluid apply pres-sure to the wheel brake units.Brake fluid apply pressure is modulated accordingto wheel speed, degree of slip and rate of decelera-tion. Sensors at each front wheel convert wheel speedinto electrical signals. These signals are transmittedtotheCABforprocessinganddeterminationofwheel slip and deceleration rate.The ABS system has three fluid pressure controlchannels. The front brakes are controlled separatelyand the rear brakes in tandem. A speed sensor inputsignal indicating a wheel slip condition activates theCAB antilock program.