ABS/TCS/Iccs System Operation

ANTILOCK BRAKE SYSTEM
When wheel slip is noted during a brake application, the ABS will enter Antilock mode. During Antilock braking, hydraulic pressure in the individual wheel circuits is controlled to prevent any wheel from slipping. A separate hydraulic line and specific solenoid valves are provided for each wheel. The ABS can decrease, hold, or increase hydraulic pressure to each wheel brake. However, it cannot increase hydraulic pressure above the amount which is transmitted by the master cylinder during braking.

During Antilock braking, a series of rapid pulsation's will be felt in the brake pedal. These pulsation's are caused by the rapid changes in position of the individual solenoid valves as they respond to their desired wheel speed. This pedal pulsation is present during Antilock braking and will stop when normal braking is resumed or when the vehicle comes to a stop. A ticking or popping noise may also be heard as the solenoid valves rapidly cycle. During Antilock braking on dry pavement, the tires may make intermittent chirping noises as they approach slipping. These noises and pedal pulsation's should be considered normal during Antilock operation.

Vehicles equipped with ABS may be stopped by applying normal force to the brake pedal. Brake pedal operation during normal braking should be no different than previous systems. Maintaining a constant force on the pedal will provide the shortest stopping distance while maintaining vehicle stability.

PRESSURE HOLD
When the Electronic Brake and Traction Control Module (EBTCM) senses a wheel slip the EBTCM isolates the system by closing the Inlet valve and keeps the Outlet valve closed in the Brake Pressure Modulator Valve (BPMV). This holds the pressure steady on the brake so hydraulic pressure does not increase or decrease.

PRESSURE DECREASE
If during the pressure hold mode the EBTCM still senses wheel speed slip it will decrease pressure to the brake. It does this by leaving the Inlet valve closed and opening the Outlet valve in the BPMV. The excess fluid is stored in the accumulator until it can be returned to the reservoir by the return pump.

PRESSURE INCREASE
If during the pressure hold or decrease mode the EBTCM senses that the wheel speed is too fast it will increase pressure to the brake. It does this by opening the Inlet valve and closing the Outlet valve in the BPMV. The increased pressure comes from the master cylinder and is related to the pressure applied to the brake pedal.

TRACTION CONTROL
Traction control will not have any effect on the operation of the vehicle until the control module detects one or both of the front wheels rotating faster than the rear wheels. At this time the Electronic Brake and Traction Control Module (EBTCM) will request the Powertrain Control Module (PCM) to reduce the amount of torque applied to the drive wheels. The PCM does this by retarding timing and selectively turning off fuel injectors (up to a maximum of 5). The EBTCM will apply the front brakes, thus reducing torque to the front wheels. Once the front wheels begin to rotate at the same speed as the rear wheels, the system will return full control to the driver. During Traction Control mode, if the brake is applied to only one front wheel, most of the torque from the engine will be directed to the other front wheel which will improve the traction of the vehicle.

The braking is accomplished by closing the Traction Control System (TCS) Master Cylinder Isolating Valves, this isolates the master cylinder from the rest of the system. The TCS Prime valves open to allow the pump to get brake fluid to build pressure for braking. The drive wheel circuit solenoids are energized as needed to allow for pressure hold, pressure increase, or pressure decrease.

The TCS may be deactivated by the driver if desired. In order to deactivate the TCS with the engine running, depress the Traction Control disable switch. The system will remain deactivated until the ignition switch is cycled, or the switch is pressed again.

STABILITRAK OPERATION (ICCS2 AND ICCS3)
Stabiltrak includes an additional level of control to the EBTCM. Stabiltrak monitors the wheel speeds, lateral acceleration and steering sensor inputs to calculate a desired yaw rate, and compare it to the actual yaw rate reported by the Yaw Rate Sensor. Steering maneuvers can cause the desired yaw rate to differ from the reported yaw rate by some amount. This difference can result from tire compliance, suspension compliance, and wheel slip. Stabiltrak tries to bring these yaw rates into agreement by selectively applying the front wheel brakes. This is known as Active Brake Control (ABC). If wheel slip rates are excessive, traction control may activate to reduce power.

The following are two types of Stabiltrak: these are ICCS2 and ICCS3.
^ The ICCS2. The ICCS2 uses normal force information from the CVRSS module to detect rough road conditions to help enhance vehicle braking.
^ The ICCS3. The ICCS3 does not uses normal force information from the CVRSS module to detect rough road conditions to help enhance vehicle braking. ICCS3 is used on vehicles that are not equipped with CVRSS.