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 stability control systems. The module monitors all sensor inputs and all HS-CAN messages relating to vehicle dynamics and directly controls the solenoid valves and the hydraulic pump motor.

The ABS module is available separately for service. When a new ABS module is installed, whether with a new HCU or not, the module must be programmed with the vehicle information. Refer to Description and Operation in Module Configuration .

When a stability control 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 diagnostic scan tool.

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 and can be serviced separately from the brake booster.

Hill Descent Control™ Switch 

The Hill Descent Control™ switch is a single-pole, momentary-contact switch. On Expedition vehicles, the Hill Descent Control™ switch is part of a 3-switch pack containing the stability-traction control switch, the Hill Descent Control™ switch (if equipped) and the power liftgate switch (if equipped). On Navigator vehicles, the Hill Descent Control™ switch is part of the Front Controls Interface Module (FCIM) and is serviced with the FCIM.

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 stability control systems. The ABS module and the HCU are attached directly together. A new HCU also comes equipped with a new ABS module.

Stability Control Sensors 

The stability control sensors for the stability control 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 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 Switch 

The stability-traction control switch is a single-pole, momentary-contact switch. On Expedition vehicles, the stability-traction control switch is part of a 3-switch pack containing the stability-traction control switch, the Hill Descent Control™ switch (if equipped) and the power liftgate switch (if equipped). On Navigator vehicles, the stability-traction control switch is part of the Front Controls Interface Module (FCIM) and is serviced with the FCIM.

Steering Wheel Angle Sensor 

The steering wheel angle sensor is an internal component of the power steering gear and is serviced with the power steering gear. The PSCM uses the internal sensors to calculate the steering wheel angle, rotation speed and direction of travel and then sends this information to the ABS module over the HS-CAN.

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 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.

Wheel Speed Sensor Encoders 

The wheel speed sensor encoders are magnetized, toothed rings used to create a Hall-effect switch when combined with the wheel speed sensor. On RWD vehicles, the front encoders are part of the front wheel hub and bearing while the rear encoders are part of the rear axle halfshaft. On 4WD vehicles, all 4 encoders are part of the front and rear halfshafts.