Basic System Operation
When brakes are applied with normal to moderate pressure, brake fluid is routed from master cylinder through HCU unrestricted to wheels. During heavy breaking conditions, brake fluid pressure is modulated through HCU to individual wheels by using separate hydraulic channels for each wheel.
During an ABS stop, hydraulic pressure is controlled to all 4 wheel circuits. ABS can build or release pressure at each wheel, depending on signal generated by wheel speed sensor. ABS is operational with vehicle speeds of 3-5 MPH or more.
Amber ABS warning light is controlled by CAB and/or Body Control Module (BCM). CAB, along with Hydraulic Control Unit (HCU), are combined in one Integrated Control Unit (ICU) located behind steering rack, on left side of suspension crossmember. See Fig 1 . Hydraulic pump is an integral part of HCU. CAB controls ABS warning light by grounding circuit. BCM grounds ABS warning light when it determines that CAB cannot control lamp. If CAB connector is disconnected, a shorting bar in connector will illuminate ABS warning light.
When ignition is turned on (system initialization), ABS warning light is on until CAB completes its self tests. At 5-10 MPH, a dynamic check is performed. At a speed of 14 MPH, ABS will attempt to operate pump motor. If brake pedal is being depressed, pump motor test will be run at 25 MPH regardless of brake pedal position. If any component exhibits a trouble condition during system initialization or dynamic check, CAB will illuminate ABS warning light or TRAC OFF light (if equipped).