When wheel slip is detected during a brake application, the ABS enters 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. The ABS cannot, however, increase hydraulic pressure above the amount which is transmitted by the master cylinder during braking.
During antilock braking, a series of rapid pulsations is felt in the brake pedal. These pulsations are caused by the rapid changes in position of the individual solenoid valves as the EBCM responds to wheel speed sensor inputs and attempts to prevent wheel slip. These pedal pulsations are present only during antilock braking and 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 cycle rapidly. During antilock braking on dry pavement, intermittent chirping noises may be heard as the tires approach slipping. These noises and pedal pulsations are 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 is no different than that of previous non-ABS systems. Maintaining a constant force on the brake pedal provides 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 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 the 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 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 switch. The system will remain deactivated until the ignition switch is cycled, or the switch is pressed again.
Stabilitrak® includes an additional level of control to the EBTCM. Stabilitrak® monitors the wheel speeds, lateral acceleration and steering sensor inputs in order to calculate a desired yaw rate and compare the desired yaw rate to the actual yaw rate reported by the yaw rate sensor. Steering maneuvers may cause the desired yaw rate to differ from the reported yaw rate by some amount. This difference may result from the following factors:
Stabilitrak® 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 in order to reduce power.
The following are 2 types of Stabilitrak®:
The ICCS2 uses normal force information from the CVRSS module in order to detect rough road conditions in order to help enhance vehicle braking.
The ICCS3 does not use normal force information from the CVRSS module in order to detect rough road conditions in order to help enhance vehicle braking. The ICCS3 is used on vehicles that are not equipped with CVRSS.
The normal force information is a function of the height of each front wheel relative to the body. The normal force information is sent from the CVRSS controller to the EBTCM via two dedicated data lines, and reflects the load at each front wheel. The EBTCM uses normal force information to detect rough road conditions while braking, eliminating false unwanted ABS cycling. This allows more aggressive braking when stopping on rough roads.
The Speed Dependent Steering System (Magna Steer®), incorporates its controller into the EBTCM. Refer to Variable Effort Steering Description in Variable Effort Steering.
Magna Steer® DTC C1241 will not cause any indicators to turn ON. The DIC will display the SERVICE STEERING SYS message.
When using the compact spare tire, it will rotate faster than the other tires. The EBTCM will compensate for this faster tire rotation.
For information on replacement tires for this vehicle, Refer to Tire Description in Tires and Wheels.
The red BRAKE lamp in the instrument cluster will illuminate to warn the driver of conditions in the brake system which may result in reduced braking ability. The CHECK BRAKE FLUID message will appear if the brake fluid level switch is closed. The BRAKE warning lamp will stay illuminated until the condition has been repaired. Refer to Brake Warning/Indicator Lamp Operation 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 ABS 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 Brake System Testing in Hydraulic Brakes. Conditions for the ANTILOCK Indicator to turn ON are as follows:
Several messages related to the ABS/TCS/ICCS may be displayed on the driver information center (DIC).
