Principles of Operations

Principles of Operations

Anti-Lock Control
The ABS module receives wheel speed readings from each wheel speed sensor and processes this information to determine if an ABS event is necessary. The wheel speed sensor detects the magnetic field of the sensor ring as it passes the face of the active wheel speed sensor.

The active wheel speed sensor generates a signal that is sent to the ABS module. The wheel speed sensor circuitry connects to the ABS module through 2 wires and a connector at each wheel speed sensor. When the ignition is turned to the RUN position, the ABS module carries out a self-test by sending a reference voltage to all of the wheel speed sensors through their circuitry to determine if they are functional.

The ABS module continuously monitors and compares the rotational speed of each wheel and when it detects an impending wheel lock, modulates brake pressure to the appropriate brake caliper. This is accomplished by triggering the hydraulic control unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal position and normal (base) braking resumes.

The HCU has additional internal solenoid valves incorporated to enable control of the wheel brake pressures.

The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module will carry out a preliminary electrical check, and above 32 km/h (20 mph) the pump motor is turned on for approximately 1/2 second. Also, during all phases of operation the ABS module, with the vehicle in motion, checks for correct operation of the wheel speed sensors. Any malfunction of the ABS will cause the ABS to shut off. Normal power assisted braking, however, remains.

ABS Module Configuration
The ABS module must be configured when any one of the following components are installed new:
^ ABS module
^ HCU
^ Stability control sensor cluster
^ Steering wheel rotation sensor

To configure the ABS module, refer to Programmable Module Installation in Information Bus.

Stability Assist System
With the ignition switch in the START or RUN positions, the ABS module functions similarly to a conventional ABS by monitoring and comparing the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor, which detects the magnetic field of the sensor ring as it passes the face of the active wheel speed sensor. When the ABS module detects an impending wheel lock, wheel spin or vehicle motion that is inconsistent with the driver commands, brake pressure is modulated to the appropriate brake caliper(s) and the PCM modulates engine torque. The ABS module triggers the HCU to open and close the appropriate solenoid valves and the PCM regulates fuel and spark to the cylinders. Once the affected wheel(s) return to the desired speed or the undesired vehicle motion stops, the ABS module returns the solenoid valves to their normal position, the PCM returns the fuel and spark to their normal operation and normal ABS brake operation is restored.

Engine Only Traction Assist
Some vehicles may be equipped with an engine only traction control (EOTC) system. In order to achieve maximum traction, the EOTC system is designed to limit wheel spin by modulating engine torque when driving on slippery or loose surfaces. Engine torque modulation is achieved by regulating fuel and spark to the cylinders. This means that the traction control system is controlled by the PCM and not the ABS module. The only function of the ABS module in an EOTC system is to provide wheel speed information to the PCM along the high-speed controller area network (HS-CAN) bus. The PCM uses the wheel speed information to determine when traction control is needed.

The traction assist system can be disabled by pressing the stability/traction control switch (located on the instrument panel center finish panel) and is indicated by the yellow traction control indicator on the instrument cluster. The traction assist system will reset and return to normal operation when the ignition switch is cycled or when the switch is pressed and released a second time during the same ignition cycle.

The traction assist system is contained in the PCM and is configured when the PCM is configured. Refer to Programmable Module Installation in information Bus.

Interactive Vehicle Dynamics (IVD)
The ABS module monitors the stability control sensor cluster and the steering wheel rotation sensor and calculates if the vehicle may be approaching a situation where vehicle stabilization is needed. If such a situation exists, the ABS module activates the interactive vehicle dynamic (IVD) system to apply preemptive action. When activated, the IVD system adjusts the brake torque at specific wheels in response to direct measurement of the driver intended vehicle motion. By adjusting brake torque, the system can reduce the cornering forces on the vehicle.

Information from the stability control sensor cluster is fed into the ABS module. The module uses information from the wheel speed sensors, the throttle position sensor, the steering wheel rotation sensor and the sensor cluster to measure the change in vehicle direction. If the module determines from all these inputs that conditions exist for a potential instability event, the system applies one or more brakes and reduces engine torque to make the vehicle more stable.

The ABS module constantly monitors the 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, the ABS and traction assist continue to function as usual. The stability/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 lamp in the stability/traction control switch. An illuminated stability/traction control switch indicates that the stability assist system is switched off. The anti-lock brake function continues to work as designed unless the yellow ABS warning indicator is also illuminated. Normal braking function always occurs, unless the red brake warning indicator is illuminated.

When stability assist activates, any of the following can occur:
^ A rumble or grinding sound much like ABS
^ A small deceleration or a reduction in the acceleration of the vehicle
^ The stability assist indicator flashes
^ A vibration is felt in the pedal much like ABS

Stability Control Sensor Cluster
The stability control sensor cluster consists of the yaw rate sensor and a lateral accelerometer. The lateral accelerometer measures the acceleration which corresponds to the force involved when the vehicle slides sideways. This acceleration has 2 forms. The first is the centrifugal acceleration which is generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. On level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor measures some lateral acceleration due to gravity even though the vehicle is not moving.

The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.

The sensor cluster receives power from the battery junction box (BJB) and is grounded to the vehicle chassis. The sensor communicates with the ABS module along a secondary, dedicated controller area network (CAN2) bus.

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. On vehicles equipped with IVD the steering wheel rotation sensor is hard-wired to the ABS module and sends the information about whether the steering wheel is being turned left or right and how far it is being turned to the ABS module. The ABS module then sends the information to the power steering control (PSC) module along the HS-CAN. On vehicles without IVD, the steering wheel rotation sensor is hard-wired to the PSC module instead of the ABS module.

The rotation sensor receives power from the BJB and is grounded through the ABS module (on vehicles with IVD) or through the PSC module (on vehicles without IVD).