System: Anti-Lock Braking System (ABS)
The ABS system can prevent wheel lock when braking by comparing the four active wheel speed sensors to the average vehicle speed. If a wheel is locking during braking or has dropped below a speed threshold programmed in the control unit ABS braking will begin.
ABS braking is possible when vehicle speeds are above 12 kph (7mph).
ABS regulation has three phases:
- Pressure Build
- Pressure Hold
- Pressure Release
Pressure Build already occurs during normal braking, so when ABS first intervenes it will start holding pressure by energizing the Inlet Valve. For example, if the right rear wheel is locking up, both Inlet Valves will be energized, regulating both wheels together. This logic is known as Select Low. Front wheels can be regulated individually as needed to prevent lockup.
Energizing the Inlet Valve closes the brake fluid passage to the calipers and traps the fluid at the current pressure, thus not allowing the brake pressure to rise any further.
If the wheel speed does not increase the Pressure Release phase begins.
Pressure Release occurs when the control unit energizes the Outlet Valve while continuing to hold the Inlet Valve closed. The trapped brake fluid is released out of the calipers, reducing braking pressure.
At the same time, the pump is switched on which draws in the released brake fluid and pumps it back into the pressure-build circuit restoring the volume of brake fluid again in front of the Inlet valve.
Depending on conditions the ABS system may cycle between these three phases from 3 to 12 times a second to prevent wheel lock.