Principles of Operation
Anti-Lock Control
Principles of Operations
Anti-Lock Control
The anti-lock control system is controlled by the ABS module which continuously monitors and compares the rotational speed of each wheel. The wheel speed sensors are connected to the ABS module by 2 circuits. One circuit provides voltage for sensor operation and the other circuit provides sensor input to the ABS module. As the wheels spin, the wheel speed sensor tone rings pass through the magnetic field generated by the active wheel speed sensor. This causes the strength of the magnetic field to increase (as a tooth passes the sensor) or decrease (as a gap passes the sensor) and generates a square-wave signal that is sent to the ABS module to indicate individual wheel speed.
When the ABS module detects an impending wheel lock-up, brake fluid pressure to the appropriate brake caliper will be modulated (pulsated). This is accomplished by opening and closing the appropriate solenoid valves inside the Hydraulic Control Unit (HCU) while the hydraulic pump motor is also actuated. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal position and the hydraulic pump motor is deactivated.
The ABS module is self-monitoring and will carry out self-tests at pre-determined times. When the ignition switch is turned to the RUN position, the ABS module will carry out a preliminary electrical check of the wheel speed sensors and their circuits by sending voltage through the sensor and checking for the voltage to return. When the vehicle is traveling at speeds above 20 km/h (12 mph), and the brake pedal is not being pressed, the pump motor will be commanded ON for approximately 0.5 second to check pump motor operation. Also, during all phases of operation while the vehicle is in motion, the ABS module checks for correct operation of the wheel speed sensors by comparing wheel speed input to other sensor input. If a malfunction is detected, the ABS module will deactivate the anti-lock control system and will also send a message over the High Speed Controller Area Network (HS-CAN) bus, when the Instrument Cluster (IC) receives this message it will illuminate the yellow ABS warning indicator and (if equipped) the stability/traction control indicator "sliding-car icon".
Electronic Brake Distribution (EBD)
On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module then uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a predetermined threshold, the ABS module closes the appropriate isolation valves in the HCU to hold the rear brake pressure constant while allowing the front brake pressure to build. This creates a balanced braking condition between the front and rear wheels and minimizes the chance of rear wheel lockup during hard braking. As the vehicle decelerates, the valves are opened to increase the rear brake pressure in proportion to the front brake pressure.
A slight bump sensation may be felt in the brake pedal when Electronic Brake Distribution (EBD) is active.
If ABS is disabled due to DTCs being present in the ABS module, EBD will continue to function unless the DTCs are for wheel speed sensors or the HCU. When EBD is disabled, the red brake warning indicator, the ABS warning indicator and (if equipped) the stability/traction control indicator "sliding-car icon" will be illuminated.
Traction Control System
The traction control system is controlled by the ABS module and uses the same wheel speed sensors and tone rings that are used for the anti-lock control system. The ABS module continuously monitors and compares the rotational speed of each wheel and when it detects a wheel spinning excessively, modulates brake pressure to the appropriate brake caliper. This is accomplished by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN bus that a traction control event is taking place. When the PCM receives this message, it will assist with traction control by adjusting engine timing and decreasing fuel injector pulses, when the IC receives this message, it will flash the stability/traction control indicator "sliding-car icon". Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN bus indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the stability/traction control indicator.
Once vehicle speed reaches or exceeds 100 km/h (62 mph), traction control is accomplished only through the PCM.
If the anti-lock control system is disabled due to DTCs being present in the ABS module, the traction control system will also be disabled. When the traction control system is disabled due to DTCs being present, both the ABS warning indicator and the stability/traction control indicator will be illuminated.
Electronic Stability Control (ESC) System
The Electronic Stability Control (ESC) system is controlled by the ABS module and uses the same wheel speed sensors and tone rings that are used for the anti-lock control system. The ESC system also uses input from the steering wheel rotation sensor, the stability control sensor cluster (yaw rate, longitudinal and lateral acceleration) and information from other modules sent over the HS-CAN bus to help maintain vehicle stability. The ABS module uses all of these inputs to continuously monitor the vehicle's direction of travel relative to the driver's intended course. If the ABS module determines from all these inputs that the vehicle is unable to travel in the intended direction, it modulates brake pressure to the appropriate brake caliper(s) by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN bus that a stability event is taking place. When the PCM receives this message, it will assist with vehicle stability by adjusting engine timing and decreasing fuel injector pulses. When the IC receives this message, it will flash the stability/traction control indicator "sliding-car icon". Once the stability condition has been corrected, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN bus indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the stability/traction control indicator.
If the anti-lock control system is disabled due to DTCs being present in the ABS module, the ESC system will also be disabled. When the ESC system is disabled due to DTCs being present, both the ABS warning indicator and the stability/traction control indicator will be illuminated.
Stability/Traction Control Switch
Unlike the standard anti-lock control and ESC system, the traction control system can be deactivated by the driver through the stability/traction control switch. The switch is hard-wired to the IC and when pressed sends a ground signal to the IC. The IC then sends a message over the HS-CAN bus indicating that the driver has requested that the system be deactivated and illuminates the stability/traction control indicator "sliding-car icon". After receiving this message, the ABS module and PCM will ignore any traction control events until the driver presses the switch again, the ignition key is cycled or the vehicle speed reaches or exceeds 70 km/h (43 mph). Once the vehicle speed reaches or exceeds 70 km/h (43 mph), the traction control system will re-activate but the indicator will remain illuminated.
Stability/Traction Control Indicator "Sliding-Car Icon"
Status of the traction control, ESC system is indicated by a stability/traction control indicator "sliding-car icon" located in the IC (or message center if equipped). When the driver disables the traction control systems through the use of the stability/traction control switch, this indicator will illuminate solidly, when the ESC or traction control system is active, the indicator will flash and when the system is disabled due to DTCs being present in the ABS module, the indicator will illuminate in conjunction with the yellow ABS warning indicator.
If the traction control system has been disabled through the stability/traction control switch and the vehicle speed reaches or exceeds 70 km/h (43 mph), the system will activate but the indicator will remain illuminated until the driver presses the switch again or the ignition switch is cycled.
Stability Control Sensor Cluster
The sensor cluster (also called the accelerometer) consists of the yaw rate sensor, lateral accelerometer and longitudinal accelerometer. The sensor cluster is usually mounted as close as possible to the center point of the vehicle to get the most accurate readings. The sensor cluster sends information from the sensors to the ABS module over a dedicated Controller Area Network 2 (CAN2) bus.
- The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.
- The roll rate sensor measures the rate of rotation along the front to rear horizontal axis.
- The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle moves sideways.
- The longitudinal accelerometer measures the acceleration corresponding to the force involved when the vehicle moves forward and rearward in the horizontal plane, along the centerline of the front and rear wheels.
Lateral acceleration has 2 forms. The first is the centrifugal acceleration that 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.
Steering Wheel Rotation Sensor
The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring a slotted-wheel as it passes through the sensor gap. The rotation sensor uses 2 signal lines to transmit information to the ABS module about whether the steering wheel is being turned left or right and how far it is being turned. The rotation sensor does not indicate the position of the steering wheel relative to straight-ahead.
ABS Module Configuration
The ABS module must be configured when a new ABS module or HCU are installed.
To configure the ABS module, refer to Programmable Module Installation (PMI) in Information Bus. Programming and Relearning