The Antilock Brake System (ABS) is an electronically operated, four channel brake control system which is part of Electronic Stability Control (ESC) including Electronic Roll Mitigation (ERM). The vehicle also has Electronic Variable Brake Proportioning (EVBP) designed into the system which eliminates the combination/proportioning valve. The combined Hydraulic Control Unit (HCU) and ABS module is located forward of the master cylinder, under the engine air box.
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 four channel design. All wheel brakes are controlled individually. The ABS electrical system is separate from other electrical circuits in the vehicle. The ABS module operates the system components.
ABS system major components include:
Battery voltage is supplied to the Antilock Brake System (ABS) module. The ABS module performs a system initialization procedure at start up along with an ABS motor check. 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 ABS module briefly cycles solenoids to verify operation. An audible noise may be heard during this self check. This noise should be considered normal.
If an ABS component exhibits a fault during initialization, the ABS module illuminates the amber warning light and registers a fault code in the microprocessor memory.
The ABS module monitors wheel speed sensor inputs continuously while the vehicle is in motion. However, the ABS module 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 Hydraulic Control Unit (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 ABS module 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 wheel convert wheel speed into electrical signals. These signals are transmitted to the ABS module for processing and determination of wheel slip and deceleration rate.
The ABS system has four fluid pressure control channels. Each wheel brakes are controlled separately. A speed sensor input signal indicating a wheel slip condition activates the ABS module antilock program.
There are Two solenoid valves (Isolation 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.
This system anticipates the potential for wheel lift by monitoring the driver's steering wheel input and the speed of the vehicle. When ERM determines that the rate of change of the steering wheel angle and vehicles speed are sufficient to potentially cause wheel lift, it applies the appropriate brake and may reduce engine power to lessen the chance that wheel lift will occur. ERM will only intervene during very severe or evasive driving maneuvers.
ERM can only reduce the chance of wheel lift occurring during severe or evasive driving maneuvers. It can not prevent wheel lift due to other factors such as road conditions, leaving the roadway or striking objects or other vehicles.
Ready Alert Braking is a feature that applies very low brake pressure to all calipers after a rapid accelerator pedal release is detected. This feature provides a firm confident pedal feel and optimal stopping distance in the case of rapid accelerator release.
The Rain Brake Support is a feature that removes the water film that builds up between the pads and the rotors when driving in wet conditions. When the windshield wipers are active, this feature applies a very low brake pressure to all calipers periodically. This maintains a firm, confident brake pedal feel and optimal stopping distance in wet conditions.
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