ABS System Description
ABS System Description
ABS Modulator-Control Unit Inputs and Outputs for 47P Connector
System Outline
This system is composed of the ABS modulator-control unit, the wheel speed sensors, and the system indicators in the gauge control module. The ABS modulator-control unit controls the Anti-Lock Brake System (ABS), the Electronic Brake Distribution (EBD), the Creep Aid System (CAS), and brake assist with the brake pressure of each wheel.
ABS (Anti-lock Brake System) Features
Anti-lock Control
Without ABS, when the brake pedal is pressed while driving, the wheels sometimes lock before the vehicle comes to a stop. In such an event, the maneuverability of the vehicle is reduced if the front wheels are locked, and the stability of the vehicle is reduced if the rear wheels are locked, creating an extremely unstable condition. With ABS, the system precisely controls the slip rate of the wheels to ensure maximum grip force from the tires, and it thereby ensures maneuverability and stability of the vehicle. The ABS calculates the slip rate of the wheels based on the four wheel speeds, and then it controls the brake fluid pressure to reach the target slip rate.
Grip force of tire and road surface
Main Control
The control unit detects the wheel speed based on the wheel speed sensor signals it receives, then it calculates the vehicle speed based on the detected wheel speed. The control unit detects the vehicle speed during deceleration based on the wheel speeds.
The control unit calculates the slip rate of each wheel, and transmits the control signal to the modulator unit solenoid valve when the slip rate is high.
The hydraulic control has three modes: Pressure retaining, pressure reducing, and pressure intensifying.
EBD (Electronic Brake Distribution) Features
The EBD feature helps control vehicle braking by adjusting the rear brake force in accordance with the rear wheel load before the ABS operates. Based on the wheel speed sensor signals, the control unit uses the modulator to control the rear brakes individually. When the rear wheel speed is less than the front wheel speed, the ABS modulator-control unit retains the current rear brake fluid pressure by closing the inlet valve in the modulator. As the rear wheel speed increases and approaches the front wheel speed, the ABS modulator-control unit increases the rear brake fluid pressure by momentarily opening the inlet valve. This whole process is repeated very rapidly. While this is happening, there may be kickback at the brake pedal, and you may also hear a muted buzzing sound from the ABS modulator-control unit. This is normal.
Brake Assist Features
Brake assist helps ensure that any driver can achieve the full braking potential of the vehicle by increasing brake system pressure in a panic situation, bringing the vehicle into a full ABS stop.
If during a panic stop the ABS modulator-control unit determines that the brake system pressure increases above a threshold in less than a certain amount of time, the ABS modulator-control unit engages brake assist.
Because the brake system pressure crossed the pressure threshold before the time threshold had expired, the ABS modulator-control unit goes into brake assist mode.
Creep Aid System (CAS) Features
This system prevents the vehicle from rolling forward or backward on a hill during which the driver releases the brake pedal by controlling the brake pressure.
When the ABS modulator control unit receives a CAS activation request signal from the PCM, the system assists the driver to get the vehicle started by holding the brake pressure. This control is done not only on hills, but also on level roads. When the driver lifts his foot off the brake pedal to get the vehicle started, the brake switch is turned off, and then the brake pressure is held by operating the TCS valve and the ESV valve.
Modulator Unit
The modulator unit consists of the inlet solenoid valve, the outlet solenoid valve, the TCS NO (normally open) solenoid valve, the ESV NC (normally closed) solenoid valve, the reservoir, the pump, and the pump motor.
The hydraulic control has three modes of ABS action; pressure intensifying, pressure retaining, and pressure reducing. Pressure adding mode is combined at brake assist action.
The hydraulic circuit is an independent four channel type; one channel for each wheel.
