ABS/TCS System Operation
ANTILOCK BRAKING MODEAntilock Brake System (ABS)
When wheel slip is noted during a brake application, the ABS will enter the antilock mode. During antilock braking, the hydraulic pressure in the individual wheel circuits is controlled in order 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 the hydraulic pressure to each wheel brake. However the ABS cannot increase the hydraulic pressure above the amount that the master cylinder transmits during braking. During antilock braking, a series of rapid pulsations will be felt in the brake pedal. The rapid changes in the position of the individual solenoid valves as they respond to the desired wheel speed causes the pulsations. 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 nose 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 are normal during antilock operation. Brake pedal operation during normal braking should be no different than previous systems. Maintaining a constant force on the pedal provides the shortest stopping distance while maintaining vehicle stability.
Pressure Hold
When the Electronic Brake and Traction Control Module (EBTCM) senses the wheel slip, the EBTCM closes the inlet valve and keeps the outlet valve closed in the Brake Pressure Modulator Valve (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.
Pressure Decrease
If during the pressure hold mode the EBTCM still senses wheel slip, the EBTCM 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 cylinder.
Pressure Increase
If during the pressure hold or the pressure decrease mode the EBTCM senses that the wheel speed is too fast, the EBTCM will increase the pressure to the brake. The inlet valve 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 MODE
Traction Control System (TCS)
The TCS is designed to limit wheel slip during acceleration when one or more of the drive wheels are accelerating too rapidly and the brake switch is off.
The EBTCM monitors wheel speed slip through the ABS wheel speed sensors then processes wheel speed sensor data and sends a desired wheel torque value to the Engine Control Module (ECM) and Transmission Control Module (TCM) via a Pulsewidth Modulated (PWM) signal. The ECM then calculates and employs control of the wheel slip/wheel torque by utilizing the following methods:
1. By retarding ignition timing.
2. By shutting off up to three injectors.
The TOM will prevent transmission downshifting if necessary, thereby reducing torque to the rear wheels providing stability and traction.
If the EBTCM determines that the engine torque reduction is not sufficient to minimize wheel spin, it will apply the rear brakes in order to reduce the 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 the traction control mode, if the brake is applied to only one rear wheel, most of the engine torque will be directed to the other rear wheel, thus improving the vehicle traction. Closing the Traction Control System (TCS) master cylinder isolating valves isolates the master cylinder from the rest of the system. The TCS prime valves open in order to allow the pump access to the brake fluid to build the pressure for braking. The drive wheel circuit solenoids are energized as needed in order to allow for pressure hold, pressure increase, or pressure decrease. The driver can deactivate the TCS, if desired. In order to deactivate the TCS with the engine running, depress the TCS switch. The TO indicator in the instrument cluster will illuminate. The system remains deactivated until the ignition switch is cycled, or the switch is pressed again.
The TCS will be enabled when the TCS switch is in the ON position (TC indicator not illuminated), and the ECU has not set any current DTC(s). The TCS will be disabled under the following conditions:
^ The TCS switch is in the OFF position
^ The EBTCM senses a valid brake switch input
^ The park brake is engaged
^ The PCM sets a Diagnostic Trouble Code (DTC) that turns on the Malfunction Indicator Lamp (MIL)
^ The EBTCM sets any DTC that effects the TCS operation
Tires with ABS/TCS
Using the full size spare tire supplied with the vehicle will not affect the operation of the ABS or TCS. Refer to "Replacement Tires Description" in Tires and Wheels for information on replacement tires.
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 Indicator will stay illuminated until the condition has been repaired. Refer to "Brake Warning System Check" in Hydraulic Brakes. Testing and Inspection
ABS INDICATOR
The ABS indicator 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 ABS indicator informs the driver that a condition exists which results in turning off the antilock brake and traction control function. If only the ABS indicator is on, normal braking with full power assist is available. If the BRAKE and ABS indicators are on, a problem may exist in the hydraulic brake system. Refer to "Brake Warning System Check" in Hydraulic Brakes. Testing and Inspection Conditions for the ABS indicator to turn on are as follows:
1. ABS malfunction detected. As previously described, the ABS indicator turns on when a problem has been found in the ABS. The ABS indicator indicates that the ABS and TCS have been disabled.
2. Instrument panel cluster bulbs.check. When the ignition is turned to RUN, the ABS 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 ABS 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 Brake Warning System Check in Hydraulic Brakes for diagnosis of any condition which causes the BRAKE warning Indicator to illuminate.
TC INDICATOR
The TC indicator located in the instrument cluster will illuminate to notifies the driver of the following:
^ The driver has switched off TCS by pressing the TCS switch
^ The EBTCM sets a DTC that effects the operation of the TCS
As a bulbs.check procedure, the EBTCM will command the TC indicator on for three seconds when the EBTCM receivers Ignition power