Antilock Brakes / Traction Control Systems: Description and Operation
PURPOSE/OPERATIONGrip Force Of Tire And Road Surface:
Slip Rate:
ABS Solenoid Operation:
When the brake pedal is depressed 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, and it thereby ensures 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 attain the target slip rate.
ABS 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 it transmits the control signal to the modulator unit solenoid valve when the slip rate is high.
- The pressure reduction control has three-modes: pressure reduction, pressure retaining, and pressure intensifying.
Self-Diagnosis
- The ABS control unit is equipped with a CPU and a sub CPU, that check each other for problems.
- The CPUs check the circuit of the system.
- Self-diagnosis can be classified into two categories.
- Initial diagnosis: Performed right after the engine starts and until the ABS indicator goes off.
- Regular diagnosis: Performed right after the initial diagnosis until the ignition switch is turned OFF.
- When a problem is detected by self-diagnosis, the system:
- Turns the fail-safe relay OFF
- Turns the solenoid valve OFF
- Turns the pump motor OFF
- Turns the ABS indicator ON
Self-Diagnosis Table:
Operation Mode Table:
On-Board Diagnosis System Function
- The ABS control unit is connected to the 16P Data Link Connector.
- The ALB Checker CANNOT be used with this system.
- For air bleeding, and checking wheel sensor signals, use the Honda PGM Tester.
- The ABS CAN be diagnosed with the Honda PGM Tester.
- See the Honda PGM Tester user's manuals for specific operating instructions.
ABS Modulator
IN: Inlet Valve (Normally Open). OUT: Outlet Valve (Normally Closed):
- The ABS modulator consists of the inlet solenoid valve, outlet solenoid valve, reservoir, pump, pump motor and the damping chamber.
- The modulator reduces the caliper fluid pressure directly. It is a circulating-type modulator because the brake fluid circulates through the caliper, reservoir and the master cylinder.
- The hydraulic control has three modes: pressure reduction, pressure retaining and pressure intensifying.
- The hydraulic circuit is the 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 Reduction Mode:
- Inlet valve closed, outlet valve open
- Caliper fluid flows through the outlet valve to the reservoir.
Motor Operation Mode:
- When starting the pressure reduction 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 Speed and Modulator Control
- When the wheel speed drops sharply below the vehicle speed, the inlet valve closes to retain the caliper fluid pressure.
- When the wheel speed drops further, 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.
Wheel Sensor
- The wheel sensors are the magnetic contact less type. As the gear purser 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.