Traction Control Valve Operation

PRESSURE BUILD
If a wheel speed sensor indicates a wheel spin (traction loss) tendency and Traction Control function has not been switched off by the driver, the system will begin traction control operation. The normally open Traction Control valves are closed to isolate the drive wheel brake channels from the master cylinder and the rear brake channels. The Controller Antilock Brakes CAB will also turn on the pump and close the inlet valves to the brake channel it does not need to apply. For example: if a traction loss condition is detected at the right front wheel only, the CAB will close the left front inlet valve to prevent unneeded application of the brake at that wheel. Fluid pressure/volume from the pump will be applied to the right front brake caliper via the normally open inlet valve.

PRESSURE HOLD
As soon as the traction loss at the spinning wheel has been braked down to acceptable limits. The CAB will close the normally open inlet valve to prevent any additional pressure from being applied at the wheel brake. Since the pump can produce more volume than required for traction control operation. Pressure relief valves in the Hydraulic Control Unit HCU will direct the excess fluid volume to the master cylinder primary and secondary circuits. This fluid will enter the reservoir through the open center valves in the primary and secondary pistons.

PRESSURE DECAY
When the wheel spin tendency is no longer detected, the CAB will open the outlet valve to lower the pressure in the brake channel. When the outlet valve is opened, fluid from the brake channel is permitted to flow back to the reservoir at the HCU.