TCS modulation
Since TCS modulation braking occurs without depression of the brake pedal, the hydraulic unit must build up and modulate the pressure to the driven wheel which is to be braked if it develops wheel spin in excess of the permitted limit.
- Pressure Increase Valve Front Left
- Pressure Increase Valve
- Pressure Relief Valve Left Front
- Pressure Relief Valve Front Right
- Pump
- Wheel sensor, FL
- Wheel sensor, FR
- Wheel sensor, RL
- Wheel sensor, RR
- Inlet valve, FL (ABS)
- Inlet valve, FR (ABS)
- Inlet valve, RL (ABS)
- Inlet valve, RR (ABS)
- Outlet valve, FL (ABS)
- Outlet valve, FR (ABS)
- Outlet valve, RL (ABS)
- Outlet valve, RR (ABS)
- Accumulator chamber
- Pressure chamber
The control module performs TCS modulation with braking when the following criteria are met
- The driver does not depress the brake pedal, the brake light switch input is not active.
- Road speed is below 40 km/h.
- One of the driven wheels is spinning more than the permissible limit.
When TCS modulation with braking occurs, the vacuum servo, primary piston and secondary piston are in their rest positions. In the valve block the inlet and outlet valves are in their rest positions, that is to say the inlet valves are open and the outlet valves are closed.
The control module starts the pump while the pressure relief valve is closed in order to allow pressure build-up to occur. Pressure is built up by opening the pressure increase valve in order to supply the return pump with brake fluid. The control module closes the pressure increase valve when the pre-determined pressure has been reached. The pressure is then modulated as the control module opens the pressure increase valve to increase pressure, keeps both the pressure increase and pressure relief valves closed to retain pressure and opens the pressure relief valve to reduce pressure. The excess brake fluid resulting from opening the pressure relief valve is returned to the master cylinder.
When TCS modulation starts, a "filling pulse" is generated, which means that a small amount of pressure (same for both wheels) is built up in the wheel cylinders for both driven wheels irrespective of the wheel for which modulation with braking will occur.
Pressure will be maintained on the front wheel for which braking will not occur, i.e. both the pressure increase and pressure relief valves will be closed. This is in order to prepare for an application of the brakes if necessary.
The pressure increase valve on the wheel which is spinning will open. This causes an increase in the pressure to the wheel cylinder and the wheel is braked.
Modulation continues until:
- The friction between wheels and road surface changes so that wheel spin does not exceed the limit.
- The driver brakes (brake lights switch input active).
- Modulation is interrupted due to the risk for overheating the brakes.
When modulation is interrupted, the control module stops the pump, closes the pressure increase valve and opens the pressure relief valve, i.e. the valves and the return pump resume their rest positions.
If TCS modulation is interrupted by the driver braking, TCS modulation with braking will be disengaged and the braking function will be chosen instead. Any delay that may then arise will not affect the braking function since the brake fluid can flow through the check valve which is mounted parallel with the pressure relief valve.
Interruption of TCS modulation with braking due to a risk for overheating the brakes occurs as a result of continuous recording of the aggregate duration of TCS modulation with braking over a certain period of time by the control module.
The time is compared with a pre-programmed maximum value and modulation with braking is disengaged when this is exceeded.