Vacuum On Demand System

WARNING: This page is about a different car, the 2018 Ford EcoSport. However, it is still accessible from the selected car via links, so may be relevant.

Vehicles equipped with a 2.0L engine supply vacuum to the brake booster through the use of a valve solenoid and an aspirator in the brake booster vacuum tube. When the ABS module detects a low vacuum condition in the brake booster, a message is sent to the PCM requesting additional vacuum. The PCM responds by opening the valve solenoid which allows the engine to draw air from the brake booster, increasing vacuum in the booster. Once the vacuum in the booster has reached the required level, the ABS module stops sending the message and the PCM closes the valve solenoid.

Component Description

Anti-Lock Brake System (ABS) Module

The ABS module is attached directly to the HCU and is the ECU for the ABS. The ABS module monitors all sensor inputs and all HS-CAN messages relating to anti-lock brake control, then directly controls the solenoid valves and the hydraulic pump motor in the HCU.

The ABS module and HCU are connected together but are serviced separately. When a new ABS is installed, the module must be programmed with the vehicle information. Refer to: Communications Network - System Operation and Component Description .

Hydraulic Control Unit (HCU)

The HCU contains the solenoid valves, the hydraulic pump motor and the pressure sensor used by the ABS module for ABS operation. The ABS module and HCU are connected together but are serviced separately.

Wheel Speed Sensor

The 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 then sending a varying voltage back to the 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 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 wheel 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.