Antilock Brakes / Traction Control Systems: Description and Operation
System DescriptionABS Modulator-Control Unit Inputs and Outputs for 25P Connector:
ABS Modulator-Control Unit Inputs and Outputs for 25P Connector:
ABS Modulator-Control Unit Inputs and Outputs for 25P Connector
Features
When the brake pedal is pressed during driving, the wheels can 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. The ABS precisely controls the slip rate of the wheels to ensure maximum grip force from the tires, thereby ensuring the maneuverability and stability of the vehicle.
The ABS calculates the slip rate of the wheels based on the vehicle speed and the wheel speed, then it controls the brake fluid pressure to reach the target slip rate.
ABS Control Unit
Main Control
The ABS control unit detects the wheel speed based on the wheel sensor signal it received, then it calculates the vehicle speed based on the detected wheel speed. The control unit detects the vehicle speed during deceleration based on the rate of deceleration.
The ABS 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 reducing, pressure retaining, and pressure intensifying.
Self-diagnosis Function
1. The ABS control unit is equipped with a main CPU and a subCPU. Each CPU checks the other for problems.
2. The CPUs check the system circuits.
3. The ABS control unit turns on the ABS indicator when the unit detects a problem, and the unit stops ABS operation.
4. The self-diagnosis can be classified into these two categories:
^ Initial diagnosis
^ Regular diagnosis
On-board Diagnosis Function
The ABS can be diagnosed with the HDS.
The ALB Checker cannot be used with this system. For air bleeding, and checking wheel sensor signals, use the HDS. See the HDS Help menu for specific operating instructions.
ABS Modulator
The ABS modulator consists of the inlet solenoid valve, outlet solenoid valve, reservoir, pump, and the pump motor. The modulator reduces the caliper fluid pressure directly. It is a circulating-type modulator because the broke fluid circulates through the caliper, reservoir, and the master cylinder. The hydraulic control has three modes: Pressure intensifying, pressure retaining, and pressure reducing. The hydraulic circuit is an independent four channel type, one channel for each wheel.
Pressure intensifying mode:
Inlet valve open, outlet valve closed:
Master cylinder fluid is pumped out to the caliper.
Pressure retaining mode:
Inlet valve closed, outlet valve closed:
Caliper fluid is retained by the inlet valve and outlet valve.
Pressure reducing mode:
Inlet valve closed, outlet valve open:
Caliper fluid flows through the outlet valve to the reservoir.
Motor operation mode:
When starting the pressure reducing mode, the pump motor is ON.
When stopping ABS operation, the pump motor is OFF.
The reservoir fluid is pumped out by the pump, through the damping chamber, to the master cylinder.
Wheel Sensors
The wheel sensors are the magnetic contactless type. As the gear pulser teeth rotate past the wheel sensor's magnetic coil, AC current is generated. The AC frequency changes in accordance with the wheel speed. The ABS control unit detects the wheel sensor signal frequency and thereby detects the wheel speed.
When the wheel speed drops sharply below the vehicle speed, the outlet valve opens momentarily to reduce the caliper fluid pressure. The pump motor starts at this time. As the wheel speed is restored, the inlet valve opens momentarily to increase the caliper fluid pressure.