Anti-Lock Brake System (ABS) and Stability Control - System Operation and Component Description

ABS module must be calibrated. The calibration procedure (IVD Initialization) is carried out using a diagnostic scan tool and is required for the stability control sensors to learn the zero-position of the vehicle. The calibration procedure requires the vehicle to be on a level surface and not moving.

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.

Hydraulic Control Unit (HCU)

The HCU contains the solenoid valves, the hydraulic pump motor and the pressure sensor used by the ABS module for the various stability control systems. The ABS module and the HCU are attached directly together. The HCU is available separately for service.

Selectable Driving Modes Switch

The selectable driving modes switch is a momentary-contact, toggle style switch and is part of the FCIM. If a new switch is required, then a new FCIM must be installed.

Stability Control Sensors

The stability control sensors for the vehicle dynamic system consist of the yaw rate sensor, lateral accelerometer and longitudinal accelerometer. 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.

Lateral acceleration has 2 forms. The first is the centrifugal acceleration 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.

Stability-Traction Control OFF Switch

The stability-traction control OFF switch is either a momentary-contact, push-button style switch or a momentary- contact, toggle style switch depending on optional vehicle features. Regardless of switch type, the stability- traction control switch is part of the FCIM and if a new switch is required, then a new FCIM must be installed.

Steering Wheel Rotation Sensor

The steering wheel rotation sensor is part of the EPAS gear assembly. The steering wheel rotation speed, angle and direction of travel is calculated by the PSCM and is sent to the ABS module over the HS-CAN2.

Wheel Speed Sensors

All 4 wheel speed sensors are active (magneto resistive) sensors operating 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. One circuit provides voltage for sensor operation and the other circuit provides sensor input to the ABS module.

Wheel Speed Sensor Encoders

The front wheel speed senor encoders are non-magnetized, toothed rings pressed into the front wheel hub. The rear wheel speed sensor encoders are magnetized, toothed rings pressed onto the rear wheel hub. As the wheel hub rotates the wheel speed sensor is exposed to magnetic fields or alternating strength. The encoders are part of the wheel hubs and are serviced with the hubs.