Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock ControlAnti-Lock Brake System (ABS)
The anti-lock brake system consists of the following components:
^ Hydraulic Control Unit (HCU)
^ Anti-Lock Brake System (ABS) module
^ rear wheel speed sensors
^ rear wheel speed sensor rings
^ front wheel speed sensors and sensor rings (part of the front hub and bearing assembly)
^ traction control switch
^ yellow ABS warning indicator
ABS with Traction Control and Stability Assist
The ABS with traction control and stability assist consists of these additional components:
^ accelerometer (contained in the sensor cluster)
^ yaw rate sensor (contained in the sensor cluster)
^ steering wheel rotation sensor
^ Trac off indicator
Principles of Operation
The ABS module, with or without stability assist, simultaneously manages the anti-lock braking, traction control and engine control systems to maintain vehicle control during deceleration and acceleration.
When the ignition switch is put in the RUN position, the module carries out a preliminary electrical check and, at approximately 20 km/h (12 mph), the hydraulic pump motor is turned on for approximately one half-second. Any malfunction of the anti-lock brake control system disables the traction control and stability assist (if equipped) and the anti-lock brake warning indicator illuminates. However, the power-assist braking system will function normally.
Traction Control with Stability Assist
With the ignition in the START or RUN positions, the stability assist module functions similarly to a conventional ABS module by monitoring and comparing the rotational speed of each wheel. The wheel speed sensors electrically sense each tooth of the anti-lock sensor ring as they pass through the sensor magnetic field. When the stability assist module detects an impending wheel lock, wheel spin or vehicle motion that is inconsistent with the driver commands, it modulates brake pressure to the appropriate brake caliper(s). The stability assist module triggers the HCU to open and close the appropriate solenoid valves. Once the affected wheel(s) return to the desired speed, the stability assist module returns the solenoid valves to their normal position, and normal base brake operation is restored.
The stability assist module communicates with the Powertrain Control Module (PCM) to assist with traction control. At speeds under 40 km/h (25 mph), when drive wheels lose traction and begin to spin, the stability assist module requests the PCM to reduce engine torque, while simultaneously applying and releasing the appropriate brake caliper(s) to maintain traction. The PCM accomplishes this by minor incremental timing changes and fewer fuel injector pulses until the stability assist module ends the request. The request ends when the driven wheel speed returns to normal. After the vehicle speed exceeds 40 km/h (25 mph), traction control is accomplished only through the PCM engine torque control.
The stability assist system constantly monitors vehicle motion relative to the intended course. This is done by using sensors to compare the steering input and brake application with that of the actual vehicle motion. The system does not activate when the vehicle is traveling in reverse, however, ABS and traction control continue to function as usual.
The stability assist/traction control switch allows the driver to control use of the stability assist system. This is independent of the ABS function, which cannot be switched off by the driver. The stability assist system status is indicated by a stability assist warning light in the stability assist/traction control switch. An illuminated stability assist/traction control switch indicates that the stability assist system is switched off. In the message center, ADVANCE TRAC OFF is displayed. If a failure is detected somewhere in the stability assist system which affects either the yaw control or traction control functions, the stability assist indicator will illuminate. The anti-lock brake function continues to work as designed unless the yellow anti-lock brake warning indicator is also illuminated. The normal brake function always occurs, unless the red brake warning indicator is illuminated.
When stability assist activates, any of the following may occur:
^ a rumble or grinding sound
^ a small deceleration or a reduction in the acceleration of the vehicle
^ the stability assist indicator flashes
^ if the brake pedal is applied, a vibration is felt in the pedal
^ if the event is severe and the brake pedal is not applied, the brake pedal will move to apply greater brake forces. A whooshing sound can also be heard from under the instrument panel when an event is severe.
There may be a slight movement of the brake pedal during the system self-test. The brake pedal moves when an active test of the brake booster is run. During this test, a small amount of pressure is generated at the master cylinder, but no pressure will be generated in the brake calipers. This test will occur above 48 km/h (30 mph) after the vehicle has been running approximately 8 minutes. The test is only run if the vehicle is stable, the brake pedal is not applied, and the accelerator pedal is slightly pressed. If the brake system is not bled correctly, the brake pedal movement can become more significant.
Yaw Rate Sensor
The yaw rate sensor (part of the accelerometer) measures the relative vehicle motion about the vertical axis through its center of gravity.
Lateral Accelerometer
The lateral accelerometer measures the acceleration that corresponds to the force involved while the vehicle slides sideways. This acceleration has two forms. The first is the centrifugal acceleration generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. The accelerometer only measures the acceleration along the lateral (or sideways) direction of the vehicle. On level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor will measure some lateral acceleration due to gravity, even though the vehicle is not moving.
Steering Wheel Rotation Sensor
The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. The steering wheel rotation sensor uses two signal lines to transmit information to the stability assist module: whether the steering wheel is being turned left or right and how far it is being turned. The stability assist system compares the steering wheel position to other input signals and stores them to determine a relative straight-ahead position. Stability assist confirms this position and modifies it as necessary during every new driving cycle.
Active Brake Booster
The active brake booster in vehicles equipped with stability assist can be actuated electrically by the stability assist module. The electrical brake booster actuation is necessary in severe stability assist events and makes sure that the HCU can generate enough brake pressure to maintain vehicle stability. The brake assist from the brake booster is especially useful in cold weather when the HCU cannot draw the brake fluid from the reservoir quickly at cold temperatures.
A solenoid within the active brake booster has a release switch to indicate when the driver is stepping on the brake pedal. The solenoid provides electrical actuation of the brake booster. Without the force of the input rod, the air valve is directly opened due to the movement of the energized solenoid. With increasing current applied to the solenoid, the air valve opens and output force is created. With decreasing current applied to the solenoid, the air valve is closed and the vacuum valve opens reducing output force. The release switch indicates when the brake pedal has been applied. It is integrated into the booster key that normally sits against the rear shell of the boot in its rest position and it adjoins the valve body when in its balance position. The switch itself provides indication from two positions. In its rest position, it supplies two signals: normally open and normally closed. When sufficient force is applied to initiate movement of the key away from the valve body, the brake booster switch changes state, signifying a driver application. When the stability assist module is activating the brake booster, the brake pedal is pulled forward as the valve body moves. The release switch is held in position by the input rod spring and remains against the valve body. Although the switch changes state when the brake pedal drops, the release switch remains inactive until it encounters an external force from the driver, whether in a stability assist event controlled by the stability assist module, or in a standard mechanical braking event.
The pedal travel switch sends brake application inputs to the stability assist module. This signal only changes state when the driver has applied the brake. If the ignition switch is in the RUN position, the brake booster is not electrically actuated, and no failure has been detected, the driver brake application signal will indicate a brake application when either the pedal travel switch switch or the release switch (built into the brake booster) indicates that the driver has applied the brake pedal. If the ignition switch is in the OFF position, the driver brake application signal will indicate a brake application only when the pedal travel switch switch indicates that the driver is applying the brake pedal.
When the ignition switch is in the OFF position, failures cannot be detected. If there are failures, several layers of redundancy will attempt to keep the driver brake application signal accurate. If the stability assist system detects faults on the driver brake apply signal line, it will also attempt to maintain correct stoplamp operation by communicating on the Standard Corporate Protocol (SCP) network. Control of the stoplamp operation is also required because the PTS switch can change states during a stability assist event when the driver is not applying the brake pedal. The switch can also change states during the system check of the brake booster. Only the stability assist system can interpret these changed states and determine which are actually due to driver brake application. Similarly, the stability assist system can take advantage of its other sensors to detect PTS switch failures.
Electronic Brake Distribution (EBD)
The EBD controls rear brake pressure and acts as an electronic proportioning valve. It is controlled by the ABS module. When the EBD is disabled, the amber ABS warning indicator and the red brake warning indicator will illuminate.
Panic Assist Braking
Panic assist braking functions by fully applying the brakes during a panic stop, using a specially developed pressure/pedal movement mechanism. Depending on vehicle speed, if pedal stroke is more rapid than normal, the brake booster automatically applies full brake boost. Panic assist will be disabled when the driver releases the brake pedal.
DTC C1963
This Diagnostic Trouble Code (DTC) is stored if the stability assist warning indicator has been turned ON because the system could not confirm the straight-ahead position for more than 30 seconds above 18 km/h (11 mph) or if the vehicle reaches 75 km/h (47 mph) before confirming the straight-ahead position. This condition will most likely occur if there is some other DTC or if driving aggressively immediately after turning the ignition on. If there is another DTC logged, the repair procedure is to follow the procedure for the first DTC. This DTC can be stored if:
^ the PRNDL or REVERSE switch has failed (check the operation of the PRNDL display and the back-up lights).
^ Unauthorized chassis modifications have been made.
In very rare circumstances this DTC can be stored by itself without the occurrence of any actual component failure. These conditions can cause DTC C1963 to set with no other DTC:
^ The steering wheel has been turned while the ignition is off and then the vehicle is started, and either the vehicle is driven continuously in a circle or the vehicle is driven with continuous swerving, sliding or tire spinning.
^ An uncalibrated stability assist module has been installed in the vehicle.
Stability Assist Module Calibration
The stability assist module needs to be recalibrated whenever a component specific to the stability assist system is disconnected or a new component is installed. If a DTC is logged for any component of the stability assist system, the DTC must be cleared before carrying out the recalibration procedure. The need to recalibrate the stability assist module is also indicated by the active/fail lamp flashing. The lamp will flash after clearing the DTCs that are indicated on the stability assist calibration list. If a DTC is retrieved after recalibration, refer to the Anti-Lock Brake System (ABS) Diagnostic Trouble Code (DTC) Index. If installing a new stability assist module, the calibration procedures as well as the configuration procedure must be carried out.
When carrying out the accelerometer or yaw rate sensor calibration using the diagnostic tool, the vehicle must be stationary and on a level surface. The ignition switch must be in the RUN position. When carrying out the steering wheel rotation sensor calibration on the scan tester, the ignition switch must be in the RUN position with the engine running and the steering wheel must be rotated back and forth from lock position to lock position.