2. Pressure Maintaining (Hold) Phase: When the braking forces being applied are higher than can be transmitted to the road surface for a specific wheel the ECU will detects that wheel as tending towards lock (Slip relative to other wheels and a calculated vehicle reference speed). It will activate the relevant Inlet valve within the hydraulic modulator, switching it from its normally open position to closed, hence preventing further pressure being transmitted from the TMC to the affected wheel brake. As the outlet valve is also closed in this phase the pressure at the wheel is maintained. If the driver reduces, the brake pedal effort during ABS control brake fluid can return to the TMC through the one-way valve located within the modulator in parallel to the Inlet valve. 3. Pressure Reduction Phase: If the ECU detects that the percentage Slip relative to the other wheels and vehicle reference speed is still increasing during the pressure hold phase and the wheel is tending further towards lock it will start the pressure reduction phase, as the existing brake pressure in the wheel brake is too high. It will activate the relevant outlet valve within the hydraulic modulator, switching it from its normally closed position to open. This will allow fluid to pass from the wheel brake into the accumulator chamber within the modulator. At the same time, the ECU will activate the return pump integrated into the modulator to draw fluid from the accumulator and wheel brake and return it to the TMC. This fluid being returned to the TMC is what is felt as the pulsations at the pedal, and the amplitude of the pulsations is relative to the amount of fluid that needs to be reduced. Pressure reduction will continue until controlled wheel returns to the vehicle reference speed. SOLENO ID VALVE ELECTRI CITY STATUS VALVE OPEN OPEN- CLOSE CHANNE L CLOSE INLET ON CLOSE Master cylinder Wheel cylinder OUTLET ON OPEN Wheel cylinder Reservoir BRAKE_DIESEL 14
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