ABS System Operation
ABS/TCS BPMV HYDRAULIC FLOW
ABS/TCS/ACTIVE HANDLING (RPO JL4) BPMV HYDRAULIC FLOW
ANTILOCK BRAKE SYSTEM
When wheel slip is noted during a brake application, the Antilock Brake System (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 pulsations will be felt in the brake pedal. These pulsations 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 pulsations 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 Electronic Brake and Traction Control Module (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 Brake Pressure Modulator Valve (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 Electronic Brake and Traction Control Module (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 Brake Pressure Modulator Valve (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 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 Traction Control System (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 (RPO JL4)
Active Handling includes an additional level of control by the Electronic Brake and Traction Control Module (EBTCM). Active Handling monitors the Wheel Speed Sensors (WSS), Lateral Accelerometer, and the Steering Wheel Position Sensor (SWPS) 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 an SWPS 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 30 Km/h (18 mph). When the vehicle reaches 30 Km/h (18 mph), the EBTCM monitors the SWPS 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 SWPS 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.
MAGNASTEER
The Speed Dependent Steering System (Magnasteer), incorporates its controller into the Electronic Brake and Traction Control Module (EBTCM).
Magnasteer Diagnostic Trouble Code (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.
REPLACEMENT TIRES
The tire size is important for the performance of the Antilock Brake System/Traction Control System (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.
BRAKE WARNING INDICATOR
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.
ANTILOCK INDICATOR
The Antilock Indicator (Antilock Brake System) (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:
1. ABS malfunction detected. As previously described, the Antilock Indicator turns on when a problem has been found in the ABS. The Antilock Indicator indicates that the ABS and Transmission Control System (TCS) have been disabled.
2. Instrument Panel Cluster bulb check. When the ignition is turned to on, the Antilock Indicator will turn on for approximately three seconds and then turn off.
^ Illumination of the BRAKE warning Indicator may indicate reduced braking ability.
^ Illumination of the Antilock Indicator without the BRAKE warning Indicator on indicates only that Antilock braking is no longer available. Power assisted braking without Antilock control is still available.
^ Refer to Hydraulic Brake Diagnostic System Check in Hydraulic Brakes for diagnosis of any condition which causes the BRAKE warning Indicator to illuminate.
TRACTION CONTROL INDICATOR (CAR ICON)
- Indicator in the Instrument Panel Cluster (IPC) notifies driver that the Electronic Brake and Traction Control Module (EBTCM) has disabled the Transmission Control System (TCS).
- Indicator is controlled by the IPC, with messages via serial data from the EBTCM, no hard wires.
DRIVER INFORMATION CENTER (DIC) MESSAGES
Several messages related to the Antilock Brake System/Transmission Control System (ABS/TCS) may be displayed on the Driver Information Center (DIC), they are:
^ ABS ACTIVE: This message is displayed when ABS is active.
^ SERVICE ABS: This message is displayed continuously when an ABS fault exists.
^ SERVICE TRACTION SYSTEM: This message is displayed continuously when a TCS fault exists.
^ TRACTION SYS ACTIVE: This message is displayed when TCS is active.
^ TRACTION SYSTEM-OFF: This message is displayed continuously when TCS is turned off.
^ TRACTION SYSTEM-ON: This message is displayed for 3.5 seconds when TCS is turned on.
^ TRAC/ACT HNDLG-ON (JL4): This message is displayed for 3 seconds when TCS and Active Handling is turned on.
^ ACTIVE HANDLING (JL4): This message is displayed when Active Handling is active.
^ ACT/HNDLG-WARMING UP (JL4): This message is displayed if the Steering Wheel Position Sensor (SWPS) has not been centered after 30 seconds. This message is also displayed if the underhood temperature sent from the Body Control Module (BCM) is too cold.
^ COMPETITIVE DRIVING (JL4): This message is displayed continuously when in competitive driving mode.
^ SERVICE ACTIVE HNDLG (JL4): This message is displayed continuously when an Active Handling fault exists.
^ TRAC/ACT HNDLG-OFF (JL4): This message is displayed continuously when TCS and Active Handling is turned off.
^ WARM UP COMPLETE (JL4): This message is displayed for 3.5 seconds after the SWPS is centered.