Component Description

Anti-Lock Brake System (ABS) Module 

The ABS module is attached directly to the HCU and is the ECU for all of the vehicle dynamic systems. The module monitors all sensor inputs and all HS-CAN messages that relate to vehicle dynamics and then directly controls the solenoid valves and the hydraulic pump motor.

On vehicles equipped with adaptive cruise control, the ABS module and HCU are serviced as an assembly. When a new ABS module is installed, whether with a new HCU or not, the module must be programmed with the vehicle information. For additional information on module programming, REFER to Module Configuration , PMI.

When a vehicle dynamic system fault has been corrected or a component has been replaced, the ABS module requires calibration. The calibration procedure 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 must not be moving. The calibration procedure is carried out using a scan tool.

Hydraulic Control Unit (HCU) 

The HCU contains the solenoid valves, the hydraulic pump motor and the pressure sensor used by the ABS for the various vehicle dynamic systems. The ABS module and the HCU are attached directly together. On vehicles equipped with adaptive cruise control, the ABS module and HCU are serviced as an assembly.

Wheel Speed Sensor 

On vehicles not equipped with active park assist, all 4 wheel speed sensors are active (magneto resistive) sensors that operate on the Hall-effect principle to generate a square wave signal that is proportional to the rotational speed of the wheel. Because these are active sensors, receiving voltage from the ABS module and then 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. One circuit provides voltage for sensor operation and the other circuit provides sensor input to the ABS module.

On vehicles equipped with active park assist, both rear wheel speed sensors are active, bi-directional sensors. Each of the 2 sensors contain 2 sensing elements mounted side-by-side. Because the 2 sensing elements are mounted next to each other the 2 voltage signals are slightly out of phase, which causes one element to generate a voltage signal before the other element. This allows the ABS module to not only determine wheel speed, but also wheel direction for active park assist.

Wheel Speed Sensor Encoders 

The wheel speed sensor encoders are several magnets arranged in a circle around one side of the wheel bearing in alternating poles. As the bearing rotates, the wheel speed sensor is exposed to alternating north-south magnetic fields. The encoder is part of the wheel bearing and is serviced with the bearing.

Stability Control Sensors 

The stability control sensors for the vehicle dynamic system consist of the yaw rate sensor, roll rate sensor, lateral accelerometer and longitudinal accelerometer. The sensors are housed in the RCM which sends sensor information to the ABS module over a private HS-CAN. If any of the sensors are inoperative, 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 incline, the sensor measures some lateral acceleration due to gravity, even though the vehicle is not moving.

Steering Wheel Angle Sensor 

On vehicles not equipped with active park assist, the steering wheel angle, rotation speed and direction of travel is calculated by the PSCM and is sent to the ABS module over the HS-CAN.

On vehicles equipped with active park assist, the steering wheel angle, rotation speed and direction of travel are directly measured by the steering wheel angle sensor. The sensor is mounted between the clockspring and the SCCM, the steering wheel information is sent to the ABS module over the HS-CAN.

Brake Booster Vacuum Sensor 

The brake booster vacuum sensor is a piezoelectric device used by the ABS module to monitor the vacuum in the brake booster. The sensor is hardwired to the ABS module by 3 circuits. One circuit is for the 5 volt sensor supply, one circuit is for sensor ground and one circuit is for sensor output. The sensor output ranges from 0.2 volt to 4.9 volts, depending on the amount of vacuum in the booster. The sensor is located on the front of the brake booster at the 7 o'clock position and can be serviced separately from the brake booster.

All Terrain Control Module (ATCM) 

The ATCM houses the hill descent switch. The hill descent switch is a momentary contact switch with a LED used to indicate when the system has been enabled. For additional information on the ATCM, REFER to Four Wheel Drive (4WD) Systems , ATCM.