Traction Control System (TCL)
GENERAL DESCRIPTIONIf the throttle is opened excessively when a vehicle is started or accelerated, the driving wheels may slip due to excessive drive torque. This adversely affects startability, accelerating ability or controllability. To ensure startability, accelerating ability and controllability, this TCL system will prevent wheel slip by controlling (applying) brake fluid pressure to the drive wheels to adjust the driving torque according to road conditions.
Each wheel-speed sensor detects wheel speed and converts it to an electric signal. The ABS-ECU determines which wheel begins to slip according to that signal. If the ABS-ECU determines wheel slip, it activates the solenoid valves and the pump inside the hydraulic unit to prevent the wheel slip by applying brake ensuring controllability. This TCL system is controlled by the ABS-ECU, and the TCL components are shared by the ABS system. If the ABS hydraulic unit is faulty, the ABS-ECU must be replaced.
CONSTRUCTION DIAGRAM
FLUID PRESSURE DIAGRAM
OPERATION OF SYSTEM
The hydraulic unit incorporates eight ABS solenoid valves, two (IN, OUT) for each wheel, and four TCL solenoid valves, two (IN, OUT) for each front wheel (driving wheel).
Described below is the functioning of these components when one of the front wheels is about to slip.
1. When the TCL is inactive
The TCL solenoid valve (OUT) and ABS solenoid valve (IN) are open while the TCL solenoid valve IN and ABS solenoid valve (OUT) are closed. If the brake pedal is depressed, the fluid pressure in the master cylinder flows through the TCL solenoid valve (OUT), check valve (1) and ABS solenoid valve (IN) to the wheel cylinder, operating the brake.
If the brake pedal is released, the fluid pressure in the master cylinder falls, and the fluid returns through the ABS solenoid valve (IN), check valve (2) and TCL solenoid valve (OUT) to the master cylinder, releasing the brake.
2. When the TCL is active
1. Pressure increasing mode
If the ABS-ECU determines that either of the front wheels is slipping due to abrupt acceleration, etc., it commands the TCL solenoid valve (OUT) to close and the TCL solenoid valve to open. Furthermore, it energizes the motor to drive the pump. The pump then sucks out the fluid in the master cylinder through the TCL solenoid valve (IN) and the inlet valve, then sends it through the outlet valve and the ABS solenoid valve (IN) to the wheel cylinder, increasing the fluid pressure in that cylinder.
2. Pressure holding mode
If the ABS-ECU determines that the fluid pressure has been increased to the optimum level, it stops supplying power to the motor and the TCL solenoid valve (IN). This causes the pump to stop its operation and the TCL solenoid valve (IN) to close. As a result, the fluid passage is blocked and the fluid pressure in the wheel cylinder is retained.
3. Pressure reducing mode
If the ABS-ECU determines that wheel slipping has been prevented, it commands the TCL solenoid valve (OUT) to open. This causes the fluid in the wheel cylinder to flow through the ABS solenoid valve (IN), check valve (2) and TCL solenoid valve (OUT) back to the master cylinder, reducing the fluid pressure in the brake circuit.