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. If the vehicle is equipped with traction control a separate hydraulic line and specific solenoid valves are provided for each wheel. If the vehicle is not equipped with traction control then both front wheels have a seperate hydraulic line and specific solenoid valves, the rear wheels have only one hydraulic line controlled by one set of solenoid valves. 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.
When the EBCM senses the wheel slip, the EBCM closes the Inlet valve and keeps the Outlet valve closed in the BPMV in order to isolate the system. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.
If during the pressure hold mode the EBCM still senses wheel slip, the EBCM will decrease the pressure to the brake. The Inlet valve is left closed and the Outlet valve is opened. The excess fluid is stored in the accumulator until the return pump can return the fluid to the master cyclinder.
If during the pressure hold or the pressure decrease mode the EBCM senses that the wheel speed is too fast, the EBCM will increase the pressure to the brake. The Inlet is opened and the Outlet valve is closed. The increased pressure comes from the master cylinder and is related to the pressure applied to the brake pedal.
Traction control will not have any effect on the operation of the vehicle until the control module detects one or both of the rear wheels rotating faster than the front wheels. At this time the Electronic Brake and Traction Control Module (EBCM) will request the Powertrain Control Module (PCM) to retard timing. If the EBCM continues to detect the rear wheel are rotating faster than the front it will request the ASM to reduce the throttle angle. The last measure taken to control the rear wheel is for the EBCM to apply the rear brakes, thus reducing torque to the rear wheels. Once the rear wheels begin to rotate at the same speed as the front wheels, the system will return full control to the driver. During Traction Control mode, if the brake is applied to only one rear wheel, most of the torque from the engine will be directed to the other rear wheel which will improve the traction of the vehicle.
The braking is accomplished by closing the TCS Master Cylinder Isolating Valve, this isolates the master cylinder from the rest of the system. The TCS Prime valve opens 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 TCS On/Off switch. The system will remain deactivated until the ignition switch is cycled, or the switch is pressed again.
The compact spare tire rotates faster than the others. The EBCM compensates for this faster rotation. Refer to Tires and Wheels in Suspension for information on replacement tires.
The tire size is important for the performance of the ABS/TCS. The replacement tires should be the same size load range and construction as the original tires. Replace the tires in axle sets and only with tires of the same tire performance criteria specification number. Use of any other tire size or type may seriously affect ABS/TCS operation. For information on replacement tires for this vehicle, Refer to Tires and Wheels in Suspension.
The red BRAKE Indicator in the instrument cluster will illuminate to warn the driver of conditions in the brake system which may result in reduced braking ability. The red BRAKE indicator will also illuminate when the park brake is applied. The BRAKE warning Indicator will stay illuminated until the condition has been repaired, or the park brake is released. Refer to Brake System Testing in Hydraulic Brakes.
The ABS is located in the instrument cluster and will illuminate if a malfunction in the ABS is detected by the Electronic Brake and Traction Control Module (EBCM). The ABS Indicator informs the driver that a condition exists which results in turning off the Antilock brake and traction control function if equipped. If only the ABS Indicator is on, normal braking with full power assist is available. If the BRAKE and Antilock Indicators are on, a problem may exist in the hydraulic brake system. Refer to Brake System Testing in Hydraulic Brakes. Conditions for the Antilock Indicator to turn on are as follows:
The ASR Indicator (Chevrolet only) is located in the instrument cluster and will illuminate if a malfunction in the traction control system is detected by the Electronic Brake and Traction Control Module (EBCM). The ASR Indicator informs the driver that a condition exists which results in turning off traction. Conditions for the ASR Indicator to turn on are as follows:
The TCS Off Indicator (Pontiac only) is located in the instrument cluster and will illuminate if a malfunction in the traction control system is detected by the Electronic Brake and Traction Control Module (EBCM). The TCS Off Indicator informs the driver that a condition exists which results in turning off traction. Conditions for the TCS Off Indicator to turn on are as follows:
The Low Trac Indicator is located in the instrument cluster and will illuminate when the EBCM detects a low traction condition. Conditions for the Low Trac Indicator to turn on are as follows:
