Antilock 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.

When the EBTCM senses the wheel slip, the EBTCM 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 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.

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 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 TCM 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 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 TC 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 ECM has not set any current DTC(s). The TCS will be disabled under the following conditions:

Using the full size spare tire supplied with the vehicle will not affect the operation of the ABS or TCS. Refer to Tire Description in Tires and Wheels for information on replacement tires.

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.

(1)	ABS Indicator
(2)	TC Indicator

TCS Switch and TC Indicator