Lincoln Aviator: Anti-Lock Brake System (ABS) and Stability Control / Description and Operation - Anti-Lock Brake System (ABS) and Stability Control - System Operation and Component Description

System Operation

System Diagrams

Anti-Lock Brake System (ABS) and Stability Control

Steering Angle Sensor Module (SASM) — Vehicles With Adaptive Steering

Network Message Charts

Anti-Lock Brake System (ABS) Module - Network Input Messages

  Module Network Input Messages - ABS Module

Steering Angle Sensor Module (SASM) - Network Input Messages

  Network Input Messages - SASM

Anti-Lock Brake System (ABS) Function

The ABS module continuously monitors brake pedal input, lateral vehicle motion and the rotational speed of each wheel. The PCM sends the brake pedal switch information to the GWM over the HS-CAN1, the GWM sends the information to the ABS module over the HS-CAN2. The RCM sends lateral acceleration sensor information directly to the ABS module over the HS-CAN2. Wheel speed information is retrieved by the ABS module using 4 active wheel speed sensors. When the ABS module detects an impending wheel lock during a braking event, the ABS module modulates brake pressure to the appropriate brake calipers by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. Once the affected wheel returns to the desired speed, the ABS module deactivates the hydraulic pump motor and returns the solenoid valves to their normal position.

The ABS module has 2 self-test options, one uses a diagnostic scan tool and the other is carried out when the ABS module is initialized (ignition on). During either self-test the ABS module carries out a preliminary electrical check of the system sensors and activates the hydraulic pump motor for approximately one-half second. During this time, a buzzing or humming noise may be heard and a vibration may be felt in the brake pedal; this is a normal condition. During the module initialized self-test, the pump motor check is carried out at approximately 10 km/h (6 mph). Any malfunction detected in the system causes the ABS module to set a DTC, disable the ABS function and send a message over the HS-CAN2 to the GWM. The GWM then sends the message to the IPC over the HS-CAN3 to illuminate the ABS warning indicator. Diagnostic Trouble Codes (DTCs) which disable the ABS do not disable the base hydraulic power-assist braking system.

Auto Hold

The auto hold feature is activated and deactivated through the use of the auto hold switch located on the instrument panel. For the system to activate, the vehicle must not be moving, the driver safety belt must be buckled and the driver door must be closed.

The ABS module receives the driver safety belt buckle status from the RCM, driver door status from the BCM, while brake system pressure and the wheel speed are received from ABS sensors. This information allows the ABS module to determine if the vehicle is stopped. Once the previous conditions have been met the auto hold feature can be activated. When the switch is pressed, the HVAC module receives a ground signal from the auto hold switch and sends an auto hold message to the GWM over the HS-CAN3. The GWM relays the message to the ABS module over the HS-CAN2.

The ABS module then sends a message to the instrument cluster through the GWM indicating the auto hold feature has been activated and the instrument cluster illuminates the auto hold indicator.

The auto hold indicator illuminates when the IPC receives a message from the ABS module through the GWM.

Once the auto hold feature is activated and the driver presses the brake pedal, the ABS module closes the isolation valves in the HCU to maintain the current brake system pressure at the wheel ends. The ABS module maintains the pressure until the driver presses the accelerator pedal, shifts the transmission into PARK or after a specific time limit has been reached. The ABS module engages the parking brake after 2-10 minutes, depending on the grade of incline the vehicle is currently stopped on, the steeper the grade, the shorter the time.

Electronic Brake Force Distribution (EBD)

On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module uses wheel speed sensor inputs to evaluate rear wheel slip. Once the rear wheel slip exceeds a predetermined threshold, the ABS module commands the HCU to close the appropriate isolation valves 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. If the rear wheel slip continues and exceeds a second predetermined threshold, the ABS module commands the HCU to open the dump valves to decrease the rear brake pressure and allow the rear wheels to recover. A slight bump sensation may be felt in the brake pedal when EBD is active. If the ABS is disabled due to Diagnostic Trouble Codes (DTCs) being present in the ABS module, EBD continues to function unless the Diagnostic Trouble Codes (DTCs) are for wheel speed sensors or the HCU solenoid valves. When EBD is disabled, the ABS warning indicator, the red brake warning indicator and traction control indicator illuminate.

Emergency Brake Assist (EBA)

The EBA feature helps drivers in a severe braking event, such as an emergency, by applying the maximum possible braking force.

If the brake pedal is pressed very suddenly, the ABS module increases the hydraulic pressure to all of the brakes until the threshold for ABS intervention is reached. This generates the maximum braking power for the available traction. The ABS module monitors inputs from the brake pedal switch and from the pressure sensor within the HCU to check for sudden actuation of the brakes. With the brake pedal pressed, the ABS module triggers emergency braking if the rate of increase of hydraulic pressure exceeds the predetermined limit.

If the brake pedal is pressed so hard the ABS becomes active on the front wheels, the ABS module increases the pressure to the rear wheels up to the ABS intervention threshold.

EBA operation continues until the driver releases the brake pedal sufficiently for the hydraulic pressure in the HCU to drop below a specific threshold value. This threshold is saved in the ABS module.

HDC

When the hill descent control switch is pressed, the ATCM sends a hill descent braking assist message to the ABS module over the HS-CAN2. Once the vehicle speed is 3-32 km/h (2-20 mph) and the transmission is in REVERSE or any forward gear, the ABS module uses the hydraulic pump motor and the HCU solenoid valves to maintain the vehicle speed set by the driver. If the vehicle speed is increased or decreased outside of the operational window, the system remains active but descent speed cannot be set or maintained.

The hill descent control feature requires a cool down period after sustained use. The ABS module continually monitors the descent speed, angle of descent, pump motor run time and solenoid valve use. Once the ABS module determines a cool down period is needed, the module sends a message to the GWM over the HS-CAN2. The GWM relays the message to the IPC over the HS-CAN3 to display HILL DESCENT CONTROL OFF SYSTEM COOLING in the message center.

Hill descent control has 3 modes of operation:

• At 3-32 km/h (2-20 mph) when the hill descent control switch is pressed and conditions are correct for hill descent activation, the hill descent control system is enabled and active. The hill descent indicator flashes indicating the feature is active and the message center indicates hill descent control is active.
• At 32-64 km/h (20-40 mph) when the hill descent control switch is pressed and conditions are correct for hill descent activation, the hill descent control system is enabled, but not active. The hill descent control indicator illuminates solidly and a message is displayed in the message center indicating the reason hill descent is not active.
• At more than 64 km/h (40 mph) when the hill descent control switch is pressed, the hill descent control system is not enabled and the hill descent control indicator does not illuminate. The hill descent control feature can only be activated when vehicle speed is below 64 km/h (40 mph).

Hill Start Assist

When the vehicle is stopped on an incline greater than approximately a 5% grade, the ABS module holds the brake pressure for approximately 1.5 seconds while the driver transitions from the brake pedal to the accelerator pedal. This is accomplished by monitoring several HS-CAN messages and several sensors to determine if the vehicle is stopped and not parked, and if the vehicle is on an appropriate incline.

• The brake pedal message from the PCM and the wheel speed sensor inputs allow the ABS module to determine the vehicle has come to a complete stop.
• The transmission selector lever message sent by the GSM allow the ABS module to determine the vehicle is not parked.
• The stability sensor messages sent by the RCM allow the ABS module to determine if the vehicle is on an appropriate incline.

Once the above conditions have been met, hill start assist automatically engages. Before the driver releases the brake pedal, the ABS module commands the HCU to close the isolation valves which maintains the current brake system pressure, preventing the vehicle from rolling down the incline. Once the driver presses the accelerator pedal and the engine RPM increases, the ABS module gradually releases the brake pressure to make sure the vehicle is neither rolling back nor driving off until there is sufficient driving torque to accelerate the vehicle forward (or backward if reversing up the incline).

Hydraulic Fade Compensation and Brake Fluid Support

Hydraulic fade compensation and brake fluid support provides a function to counteract faded brakes by supplying additional wheel brake pressure, if the maximum achievable vehicle deceleration is not reached, while the driver applies (very) high pedal force (driver brake request remains longer than a predetermined time). Brake fluid support provides a software function to increase fluid flow to the wheel end brakes in the event the brakes have faded to the extent the primary or secondary master cylinder circuits can no longer supply fluid to the brake system.

Supplemental Braking Assist

In addition to preventing wheel lock up during braking events, the ABS module also provides supplemental hydraulic brake assist through the use of the hydraulic pump motor and the HCU. This is done in the event of a severe braking event, to maintain the distance gap set by the adaptive cruise control system or to assist with collision avoidance.

The ABS module continually monitors CAN messages and sensor inputs. When the messages or sensors indicate a severe braking event is about to occur or is occurring, the ABS module activates the hydraulic pump motor in the HCU to assist with vehicle braking.

On vehicles equipped with adaptive cruise control, the CCM monitors the area forward of the vehicle.

When an object enters this area and closes the distance gap set by the driver, the CCM sends a deceleration request to the IPMA over a LIN. The IPMA then sends the message to the ABS module over the HS-CAN2.

When the deceleration request message is received, the ABS module activates the hydraulic pump motor and solenoid valves in the HCU to slow the vehicle down to maintain the distance gap set by the driver. Once the distance gap set by the driver is achieved, the CCM stops sending the deceleration request message and the ABS module deactivates the hydraulic pump motor and solenoid valves in the HCU.

If the CCM determines the amount of braking provided by the ABS module is insufficient, the CCM sends a forward collision avoidance braking request message and warns the driver, both audibly and visually, through the use of the HUD. After receiving the braking request message, the ABS module waits for brake pedal input and, once received, applies maximum braking assist using the hydraulic pump motor and the HCU.

For additional information on the adaptive cruise control system,
Refer to: Cruise Control - System Operation and Component Description (419-03B Cruise Control, Description and Operation).

For additional information on the collision avoidance system,
Refer to: Collision Warning and Collision Avoidance System - System Operation and Component Description (419-03C Collision Warning and Collision Avoidance System, Description and Operation).

AdvanceTrac

The AdvanceTrac system is comprised of the ESC and traction control features.

Electronic Stability Control (ESC)

The ABS module continuously monitors the vehicle motion relative to the intended course. This is done by using sensors to compare the steering wheel input and the yaw rate sensor input with the actual vehicle motion.

The PSCM sends the steering wheel angle information to the ABS module over the HS-CAN2. The RCM sends yaw rate sensor and lateral accelerometer information to the ABS module also over the HS-CAN2. If the ABS module determines from the inputs the vehicle is unable to travel in the intended direction, it modulates brake pressure to the appropriate brake calipers 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 calculates how much engine torque reduction is required to help stabilize the vehicle and sends this torque reduction message to the GWM over the HS-CAN2 which relays the message to the PCM over the HS-CAN1. The ABS module also sends a vehicle stability event message to the GWM over the HS-CAN2 which relays this message to the IPC over the HS-CAN3. When the PCM receives the torque reduction message, it adjusts engine timing and decreases fuel injector pulses to reduce the engine torque to the requested level. When the IPC receives the vehicle stability event message, it flashes the stability-traction control indicator (sliding car icon).

Once the vehicle instability has been corrected, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and stops sending the traction event and torque reduction messages. The PCM returns engine timing and fuel injectors to normal operation and the IPC extinguishes the stability-traction control indicator (sliding car icon).

The ESC can be disabled by the driver using the stability-traction control switch. This is independent of ABS which cannot be disabled by the driver. The stability-traction control switch is hardwired to the IPC. When the driver presses the stability-traction control switch, it sends a ground signal to the IPC. The IPC responds by illuminating the stability-traction control disabled indicator (sliding car OFF icon) and sending a message to the GWM over the HS-CAN3. The GWM relays this message to the ABS module over the HS-CAN2. The ABS module takes no further action in regards to stability-traction until the driver presses the switch again in the same ignition cycle, the ignition is cycled from OFF to ON or vehicle speed exceeds 56 km/h (35 mph).

ESC does not operate with the transmission in REVERSE or with the transfer case in 4WD low. The ABS module disables ESC if there are any wheel speed sensor, stability sensor or steering angle sensor Diagnostic Trouble Codes (DTCs) present in the ABS module. If there is a communication error between the ABS module and the PSCM or the ABS module and the RCM, ESC also is disabled. When ESC is disabled, the ABS module sends a message to the GWM over the HS-CAN2. The GWM relays this message to the IPC over the HS-CAN3 to illuminate the stability-traction control indicator (sliding car icon).

Traction Control

The ABS module continuously monitors and compares the rotational speed of the drive wheels in relation to the non-driven wheels. When the drive wheels begin to spin faster than the non-driven wheels, the ABS module modulates brake pressure to the appropriate brake calipers 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 calculates how much engine torque reduction is required to eliminate the wheel slip and sends this torque reduction message to the GWM over the HS-CAN2 which relays the message to the PCM over the HS-CAN1. The ABS module also sends a traction event message to the GWM over the HS-CAN2 which relays this message to the IPC over the HS-CAN3. When the PCM receives the torque reduction message, it adjusts engine timing and decreases fuel injector pulses to reduce the engine torque to the requested level. When the IPC receives the traction event message, it flashes the stability-traction control indicator (sliding car icon).

Once the driven wheel speed returns to the desired speed, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and stops sending the traction event and torque reduction messages. The PCM returns engine timing and fuel injectors to normal operation and the IPC extinguishes the stability-traction control indicator (sliding car icon). Once vehicle speed exceeds 100 km/h (62 mph), traction control is accomplished only through the PCM torque control.

The traction control system can be disabled by the driver using the traction control switch (if equipped) or through the message center. This is independent of ABS which cannot be disabled by the driver. The traction control switch is hardwired to the IPC. When the driver presses the traction control switch, it sends a ground signal to the IPC. The IPC responds by illuminating the stability-traction control disabled indicator (sliding car OFF icon) and by sending a message to the GWM over the HS-CAN3. The GWM relays this message to the ABS module over the HS-CAN2. The ABS module takes no further action in regards to traction control until the driver presses the switch again in the same ignition cycle or until the ignition is cycled from OFF to ON. For additional information on disabling the traction control feature, refer to Disabling AdvanceTrac.

The ABS module disables traction control if there are any wheel speed sensor or solenoid valve Diagnostic Trouble Codes (DTCs) present in the ABS module. Traction control is also disabled if there is a communication error between the ABS module and the PCM. When traction control is disabled, the ABS module sends a message to the GWM over the HS-CAN2. The GWM relays this message to the IPC over the HS-CAN3 to illuminate the stability-traction control indicator (sliding car icon).

Curve Control

The ABS module continuously monitors the vehicle motion relative to the intended course. This is done by using sensors to compare the steering wheel input, the yaw rate sensor input, the lateral acceleration sensor input and the longitudinal acceleration sensor input with the actual vehicle motion.

The PSCM sends the steering wheel angle information to the ABS module over the HS-CAN2. The RCM sends yaw rate sensor and lateral accelerometer information to the ABS module also over the HS-CAN2. If the ABS module determines from the inputs the vehicle is experiencing over-steer or under-steer, the ABS module modulates brake pressure to the appropriate brake calipers 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 calculates how much engine torque reduction is required to help stabilize the vehicle and sends this torque reduction message to the GWM over the HS-CAN2 which relays the message to the PCM over the HS-CAN1. The ABS module also sends a curve control event message to the GWM over the HS-CAN2 which relays this message to the IPC over the HS-CAN3. When the PCM receives the torque reduction message, it adjusts engine timing and decreases fuel injector pulses to reduce the engine torque to the requested level. When the IPC receives the vehicle stability event message, it flashes the stability-traction control indicator (sliding car icon).

The curve control function does not operate with the transmission in REVERSE. The ABS module disables the curve control function if there are any wheel speed sensor, stability sensor or steering angle sensor Diagnostics Trouble Codes (DTCs) present in the ABS module. The curve control function is also disabled if there is a communication error between the ABS module and the PSCM, or the ABS module and the RCM. When the curve control function is disabled, the ABS module sends a message to the GWM over the HS-CAN2. The GWM relays this message to the IPC over the HS-CAN3 to illuminate the stability-traction control indicator (sliding car icon).

Roll Stability Control (RSC)

The ABS module continuously monitors the vehicle motion relative to the intended course. This is done by using sensors to compare the steering wheel input, yaw rate sensor input, lateral accelerometer input and roll sensor input with the actual vehicle motion.

The PSCM sends the steering wheel angle information to the ABS module over the HS-CAN2. The RCM sends yaw rate sensor, lateral accelerometer and roll rate sensor information to the ABS module also over the HS-CAN2. If the ABS module determines from the inputs the vehicle is becoming unstable, the ABS module modulates brake pressure to the appropriate brake calipers 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 calculates how much engine torque reduction is required to help stabilize the vehicle and sends this torque reduction message to the GWM over the HS-CAN2 which relays the message to the PCM over the HS-CAN1. The ABS module also sends a vehicle stability event message to the GWM over the HS-CAN2 which relays this message to the IPC over the HS-CAN3. When the PCM receives the torque reduction message, it adjusts engine timing and decreases fuel injector pulses to reduce the engine torque to the requested level. When the IPC receives the vehicle stability event message, it flashes the stability-traction control indicator (sliding car icon).

Once the vehicle instability has been corrected, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and stops sending the traction event and torque reduction messages. The PCM returns engine timing and fuel injectors to normal operation and the IPC extinguishes the stability-traction control indicator (sliding car icon).

RSC does not operate with the transmission in REVERSE or with the transfer case in 4WD low. The ABS module disables RSC if there are any wheel speed sensor, stability sensor or steering angle sensor Diagnostic Trouble Codes (DTCs) present in the ABS module. If there is a communication error between the ABS module and the SASM or the ABS module and the RCM, RSC also is disabled. When RSC is disabled, the ABS module sends a message to the GWM over the HS-CAN2. The GWM relays this message to the IPC over the HS-CAN3 to illuminate the stability-traction control indicator (sliding car icon).

Trailer Sway Control

Trailer sway is the undesirable yaw force a trailer can apply to the towing vehicle. Trailer sway control is a unique function of the stability control system that uses steering wheel angle information and yaw rate information to determine if a trailer sway event is taking place.

The PSCM sends the steering wheel angle information to the ABS module over the HS-CAN2. The RCM sends yaw rate sensor and lateral accelerometer information to the ABS module also over the HS-CAN2. If the ABS module determines from the inputs a trailer sway event is taking place, the ABS module modulates brake pressure to the appropriate brake calipers 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 calculates how much engine torque reduction is required to eliminate the trailer sway and sends this torque reduction message to the GWM over the HS-CAN2 which relays the message to the PCM over the HS-CAN1. The ABS module also sends a trailer sway event message to the GWM over the HS-CAN2 which relays this message to the IPC over the HS-CAN3. When the PCM receives the torque reduction message, it adjusts engine timing and decreases fuel injector pulses to reduce the engine torque to the requested level. When the IPC receives the vehicle stability event message, it flashes the stability-traction control indicator (sliding car icon) and displays TRAILER SWAY REDUCE SPEED in the message center.

Once the trailer sway has been corrected, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and stops sending the traction event and torque reduction messages. The PCM returns engine timing and fuel injectors to normal operation and the IPC extinguishes the stability-traction control indicator (sliding car icon) and stops displaying the trailer sway message in the message center. Trailer sway control only activates when vehicle speed is greater than 65 km/h (40 mph). Any malfunction disabling RSC also disables trailer sway control.

The driver can enable and disable the trailer sway control feature using the message center and steering wheel controls. For additional information, refer to the Owner's Literature.

Selectable Drive Modes

The selectable drive mode system optimizes driveability and comfort as well as maximizing traction while operating on different types of terrain. The ATCM controls the drive mode selection. When a drive mode is selected, the ATCM sends the drive mode message to the ABS module over the HS-CAN2. When the ABS module receives the message, it alters the intervention thresholds for ESC, RSC and traction control based on the selected mode. The ABS module also sends the drive mode message out to several other modules. The following modules receive the message and respond by altering their operation as follows:

• PSCM: adjusts steering effort and feel based on the selected mode.
• PCM: adjusts throttle response and gear changes to enhance powertrain response based on the selected mode.
• AWD module: automatically engages and disengages specific 4WD modes and raises or lowers torque output based on the selected mode.

The following drive modes are available:

• Conserve: For efficient and responsible driving. Conserve helps deliver maximum fuel efficiency and increases driving range.
• Deep Conditions: For crossing terrain that has deep, rutted surfaces such as mud or deep sand.
• Deep Sand: For driving on deep, soft and dry sand.
• Excite: For more aggressive on-road driving; increases throttle response, provides a sportier steering feel along with quicker shifting and a stiffer suspension. The transmission also holds gears longer, helping the vehicle accelerate faster when shifting gears.
• Normal: For normal, everyday on-road driving and towing. The Tow-Haul function can only be used in Normal mode.
• Slippery: For firm surfaces covered with loose or slippery material such as packed snow, ice, water, grass or a thin layer of gravel or sand.

Drive mode changes are not available when the ignition is off. If a mode is unavailable, the system defaults to Normal mode.

MyKey® Interaction

Through the MyKey® feature, traction control can be configured to be always on or to allow the driver to select the traction control function on or off. When the traction control function is configured to be always on and a MyKey® restricted key is in use, the IPC ignores any requests made by the driver to disable the traction control function and does not send any traction control disable messages to the GWM. Refer to the Owner's Literature for additional information on the various MyKey® settings.

Stability-Traction Control Indicator (Sliding Car Icon)

One or both of the stability/traction control indicators may illuminate as a result of momentary sensor disturbances due to environmental or driving conditions (including severe vehicle maneuvers or extreme off road usage). Once illuminated, the indicator remains on until the environmental or driving condition is no longer present and the ignition is cycled from ON to OFF and then back to ON again. If there are no other customer concerns, symptoms, indicators or Diagnostic Trouble Codes (DTCs), the stability/traction control indicator may have been illuminated due to these environmental or driving conditions.

Refer to: Instrument Panel Cluster (IPC) - System Operation and Component Description (413-01 Instrumentation, Message Center and Warning Chimes, Description and Operation).

Stability-Traction Control Disabled Indicator (Sliding Car OFF Icon)

Refer to: Instrument Panel Cluster (IPC) - System Operation and Component Description (413-01 Instrumentation, Message Center and Warning Chimes, Description and Operation).

Electronic Parking Brake Features

The ABS module is the controlling ECU for the electronic parking brake system and controls all parking brake features such as automatic drive away release. For additional information on the electronic parking brake system,
Refer to: Parking Brake - System Operation and Component Description (206-05 Parking Brake and Actuation, Description and Operation).

Component Description

Auto Hold Switch

The auto hold switch is a momentary contact, push button switch and is part of the electric parking brake switch.

Brake Fluid Level Switch

The brake fluid level switch is mounted on the mini-reservoir attached to the EBB assembly and is hardwired to the ABS module. The ABS module provides a reference voltage to the brake fluid level switch and the switch is grounded through the ABS module along a separate ground circuit.

Drive Mode Switch

The drive mode switch is a multi-position, rotating switch and is part of the ATCM.

Electric Brake Booster (EBB) Assembly

The EBB is serviced as a single assembly and should NOT be disassembled. The EBB assembly contains the ABS module, solenoid valves, pressure sensor and hydraulic pump motor; the EBB also takes the place of the brake master cylinder and the vacuum booster.

The ABS module is serviced as an assembly with the EBB. The ABS module is the ECU for the ABS and stability control systems. The module monitors all sensor inputs and all CAN messages relating to ABS and stability control, then directly controls the solenoid valves and the hydraulic pump motor in the EBB. When a new EBB assembly is installed, the ABS module must be programmed with the current vehicle configuration information. For additional information on module programming,
Refer to: Module Configuration - System Operation and Component Description (418-01 Module Configuration, Description and Operation).

When an ABS or stability control fault has been corrected or a new component has been installed, the ABS module must be calibrated using the ABS Calibration routine found on the diagnostic scan tool. ABS Calibration is required for the stability control sensors to learn the zero-position of the vehicle which means the vehicle must be on a level surface and not moving.

Hill Descent Control Switch

The hill descent control switch is a single-pole, momentary-contact switch and is part of the ATCM.

Stability Control Sensors

The stability control sensors for the traction control and ESC functions consist of the yaw rate sensor, lateral accelerometer, longitudinal accelerometer and roll rate sensor. The sensors are housed in the RCM which sends sensor information to the ABS module over the HS-CAN2. If any of the sensors are defective, a new RCM must be installed.

• The yaw rate sensor measures the yaw angle which is the difference between the direction the vehicle is pointing when cornering and the direction the vehicle is actually moving.
• The longitudinal accelerometer measures the acceleration and deceleration of the vehicle as it moves forward and backward.
• The lateral accelerometer measures the force created when a vehicle corners that tends to push a vehicle sideways.
• The roll rate sensor measures the rate of rotation of the vehicle along the centerline of the vehicle from front to back.

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 lateral acceleration due to gravity. However, if the vehicle is parked sideways on a bank or an incline, the sensor measures some lateral acceleration due to gravity, even though the vehicle is not moving.

Steering Wheel Angle Sensor

On vehicles equipped with adaptive steering, the steering wheel rotation sensor is part of the SASM and clockspring assembly. The steering wheel rotation speed, angle and direction of travel is derived based on the position of the steering wheel position sensor and is sent to the ABS module over the HS-CAN2.

On vehicles not equipped with adaptive steering, information from the wheel speed sensors, PSCM and RCM are used by the ABS module to determine steering wheel angle offset when the vehicle is driven after initial start up. The ABS module uses the information received to calculate the steering offset from center; the module saves the information in memory and transmits the information to the PSCM over the HS-CAN2. Then, using the information provided from the ABS module, the PSCM determines the steering angle and transmits the steering angle over the CAN. The ABS module must re-learn the steering center whenever power is lost, for example during service when the battery is disconnected. To re-learn the steering center, the vehicle must be driven over 20 km/h (12 mph) for at least 1 minute on a fairly straight road.

Wheel Speed Sensor

All 4 wheel speed sensors are active (magneto resistive) sensors that operate on the Hall-effect principle to generate a square wave signal proportional to the rotational speed of the wheel. Because these are active sensors, receiving voltage from the ABS module and sending a varying voltage back to the ABS module, they are able to detect much lower rotational speeds than passive (magnetic inductive) sensors. Each wheel speed sensor is connected to the ABS module by 2 circuits which are used for both sensor power and sensor signal return.

Wheel Speed Sensor Encoder Rings

The wheel speed sensor encoder rings are magnetized, toothed rings used to create a Hall-effect switch when combined with the wheel speed sensor. The encoder rings are part of the front and rear wheel hub and bearing assemblies and are serviced with the assemblies.