BRAKES (EPB) 14 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. FAIL-SAFE FUNCTION: If the ABS system detects any fault that may adversely affect its performance it will switch to a fail-safe shut down mode. When this happens the electricity supply to the solenoid valves is deactivated, stopping control signal output, this causes the valves to switch to their default state. (Open for inlet valves / Closed for Outlet valves) The System will light the relevant warning lamp at the same time to inform the driver the vehicle has either an ABS or an ABS and EBD failure. In this condition, the conventional hydraulic brake circuit will still function, allowing brake pressure to reach all wheels. It should be noted that with an ABS fault, it will be possible to lock the wheels and in the Case of an EBD failure the brake force proportioning between front and rear will no longer function and hence rear wheel lock is possible before front.
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