ABS Operation

BPMV HYDRAULIC FLOW CHART

ANTILOCK BRAKE SYSTEM
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.

PRESSURE HOLD
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.

PRESSURE 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.

PRESSURE INCREASE
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
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(R) OPERATION (ICCS2 AND ICCS3)
Stabilitrak(R) includes an additional level of control to the EBTCM. Stabilitrak(R) monitor's 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:
^ Tire compliance
^ Suspension compliance
^ Wheel slip

Stabilitrak(R) 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(R):

ICCS2
The ICCS2 uses normal force information from the CVRSS module in order to detect rough road conditions in order to help enhance vehicle braking.

ICCS3
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.

NORMAL FORCE
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.

MAGNA STEER
The Speed Dependent Steering System (Magna Steer), incorporates its controller into the EBTCM. Refer to Variable Effort Steering Description in Steering and Suspension.

Magna Steer(R) DTC C1241 will not cause any indicators to turn ON. The DIC will display the SERVICE STEERING SYS message.

SPARE TIRES
When using the compact spare tire, it will rotate faster than the other tires. The EBTCM will compensate for this faster tire rotation.

REPLACEMENT TIRES
For information on replacement tires for this vehicle, Refer to Tire Description in Wheels, Tires and Alignment.

BRAKE WARNING LAMP
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.

ANTILOCK INDICATOR LAMP
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 areas 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 TCS have been disabled.
2. Instrument cluster bulb check. When the ignition switch is turned to RUN, 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 ABS braking is no longer available. Power assisted braking without ABS control is still available.
^ Refer to Brake System Testing in Hydraulic Brakes for diagnosis of any condition which causes the BRAKE warning Indicator to illuminate.

TRACTION CONTROL INDICATOR LAMP
^ Indicator in the instrument cluster notifies driver that the EBTCM has disabled the TCS.
^ Manually disabling the TCS will not activate this indicator.
^ Indicator is controlled by the instrument cluster, with messages via serial data class 2 from the EBTCM.

DRIVER INFORMATION CENTER (DIC) MESSAGES
Several messages related to the ABS/TCS/ICCS may be displayed on the driver information center (DIC).

TRACTION ENGAGED: This message is displayed when TCS has been active for at least 3 seconds.

TRACTION SUSPENDED: This message is displayed when TCS has been temporarily inhibited due to brake pad or catalytic converter over temperature conditions.

TRACTION OFF: This message is displayed when the traction control switch has been activated while the TCS is in the ON state.

TRACTION READY: This message is displayed when the traction control switch has been activated while the TCS is in the OFF state or TCS has been re-enabled while in a temporarily inhibited state.

SERVICE STABILITY SYS: This message is displayed when Stabilitrak(R) faults are detected. A single pulse chime will accompany the message.

STABILITY ENGAGED: This message is displayed when Stabilitrak(R) differential braking is active for at least 1 second.

SERVICE STEERING SYS: This message is displayed when Magna Steer(R) faults are detected. A single pulse chime will accompany the message.