Theory Of Operation-Braking Systems

BRAKING SYSTEM OPERATION:  The Integrated Control Unit (ICU) consists of the Anti-lock Brake Module (ABS) and Hydraulic Control Unit (HCU). The ABS Module monitors the Master Cylinder Pressure (MCP) Sensor signal, mounted inside the HCU to detect when the Brake Pedal is applied. The ABS Module also monitors the pressure sensors in each hydraulic circuit to calculate the Brake Torque signal. The calculation is based on the sum of pressure applied to the caliper in each hydraulic circuit. These signals are broadcast on the CAN bus. When the ABS Module receives an autonomous brake input it controls a motor/actuator inside the Hydraulic Assembly to apply hydraulic pressure to the calipers to brake the vehicle.

WHEEL SPEED SENSOR OPERATION:  Wheel Speed Sensors (WSS) are used to monitor vehicle speed and wheel slip conditions. The Wheel Speed Sensors are hard-wired to the Brake System Control Module. The ABS Module broadcasts the WSS signals on the CAN bus. The ABS Module performs circuit fault diagnostics for the WSS sensors and wiring. The ABS Module and PCM perform rationality checks against the outputs from the sensors. This is described in detail in the DIAGNOSTIC OVERVIEW information below.  The WSS signals are also compared to the Steering Wheel Angle and Dynamic Sensors. Comparing these signals together can determine when the vehicle is in a turning event or if there is a fault in one of the WSS signals.

ANTI-LOCK BRAKING EVENT:  During a braking event, the ABS Module monitors the WSS signals to detect a wheel lock condition due to a skid event. If a wheel is determined to be locked, the ABS Module cycles the Outlet Valve. Cycling the valve off/on relieves and applies the pressure to the wheel to correct the condition. The pressure will be regulated until the wheel lock condition is no longer present.

TRACTION CONTROL AND ELECTRONIC STABILITY CONTROL (ESC):  If a wheel spin condition is detected during driving, and the brakes are not being applied, the ABS Module will cycle the Inlet Valve in the system, applying braking to the wheel to correct the wheel slip condition. The system monitors the steering wheel angle, yaw rate and lateral acceleration (Dynamic Sensor) and Wheel Speed Sensors to determine an over/under steer condition. The ABS Module will open the appropriate Inlet Valves, applying braking to counteract the condition.