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.
When the 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.
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.
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 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 (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 closing the throttle. The EBTCM will 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 Rear Master Cylinder Isolation Valve; this isolates the master cylinder from the rest of the system. The Rear 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.
Active Handling™ includes an additional level of control by the EBTCM. Active Handling™ monitors the Wheel Speed Sensors, Lateral Accelerometer, and the Steering Wheel Position Sensor inputs to calculate a desired vehicle yaw rate. The EBTCM then compares the desired vehicle yaw rate with the actual vehicle yaw rate supplied to the EBTCM from the Yaw Rate Sensor. The difference between the desired vehicle yaw rate and the actual vehicle yaw rate is the yaw rate error. Active Handling™ keeps the yaw rate error to a minimum by selectively applying individual wheel brakes as necessary.
Before Active Handling™ can be operational a Steering Wheel Position Sensor centering routine must be completed each time the vehicle is started and driven. The EBTCM runs a centering routine when the vehicle speed goes above 10 Km/h (6 mph). When the vehicle reaches 10 Km/h (6 mph), the EBTCM monitors the Steering Wheel Position Sensor inputs (Phase A, Phase B and Analog voltage) to see if the steering wheel is moving. If the steering wheel is not moving for a set period of time then the EBTCM assumes the vehicle is going in a straight line. At this point, the EBTCM looks at the analog voltage signal, this voltage, normally around 2.5V, is then considered the center position, the digital degrees also become zero at this time. This centering routine is necessary to compensate for wear in the steering and suspension. Wear in the steering and suspension can result in a change in the relationship between the steering wheel and the front tires when driving in a straight line, this can result in a false Steering Wheel Position Sensor signal sent to the EBTCM. By running the centering routine, the EBTCM can compensate for these changes by changing the digital and analog center position.
Once Active Handling™ is operational the EBTCM monitors the yaw rate error. If the EBTCM sees an increase in the yaw rate error it will use differential braking on the individual wheel brakes as necessary. To correct for oversteer differential braking is used on the left or right rear wheel brakes. To correct for understeer differential braking is used on the left or right front wheel brakes.
When the vehicle performs a high speed turn or curve, the EBCM will enter rear brake control mode. The vehicle speed is greater than 48 km/h (30 mph) and the vehicle lateral acceleration is greater than 0.6 g. The vehicle will exit rear brake control when the vehicle speed is less than 40 km/h (25 mph) or the vehicle lateral acceleration is less than 0.4 g.
During a rear brake control event, the EBCM performs a pressure increase on both rear brakes. The driver may feel a vibration in the brake pedal.
The Speed Dependent Steering System (Magnasteer®), incorporates its controller into the EBTCM.
Magnasteer® DTC C1241 will not cause any indicators to turn on.
Refer to Variable Effort Steering in Steering for DTC C1241 diagnostics and Magnasteer®2 description and operation.
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 indicator will also illuminate when the parking brake is applied or not fully released, or if the brake fluid level switch is closed. The BRAKE warning lamp will stay illuminated until the condition has been repaired. Refer to Hydraulic Brake Diagnostic System Check in Hydraulic Brakes.
The Antilock Indicator (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 (EBTCM). The Antilock Indicator informs the driver that a condition exists which results in turning off the Antilock brake and traction control function. If only the Antilock 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 Hydraulic Brake Diagnostic System Check in Hydraulic Brakes. Conditions for the Antilock Indicator to turn on are as follows:
Indicator in the IPC notifies driver that the EBTCM has disabled the TCS.
Indicator is controlled by the IPC, with messages via serial data from the EBTCM, no hard wires.
Several messages related to the ABS/TCS may be displayed on the Driver Information Center (DIC), they are:
