Abs Function
The brake control module (BCM) (1) registers the acceleration and retardation of the wheels while driving. The wheel sensors (3) (4 sensors - one per wheel) provide information on the wheels' rotational speed to the brake control module (BCM), which uses the information to calculate vehicle speed. The brake control module (BCM) transmits the signal on the CAN network to make it accessible to other systems.
During braking, the brake control module (BCM) receives a CAN signal from the engine control module (ECM) and brake light switch (2) to indicate that braking is occurring. The brake control module (BCM) then goes into standby mode for ABS control. The signal from the brake light switch is not necessary for ABS control.
If the brake control module (BCM) detects that a wheel is locked, the hydraulic unit (4) and the hydraulic pressure for that wheel are adapted so as to prevent wheel lock-up.
The pressure in the circuit is thus regulated to maximize braking effect, which is when the wheel rotates with approx. 12-20% slippage against the surface.
The ABS function is not activated until vehicle speed is at least approx. 5 km/h (3.1 mph), which means that the wheels can lock up at speeds below approx. 7 km/h (4.3 mph). This is of no practical significance to system function or to the driver.
ABS control can be divided into three different steps:
- Open mode (pressure build-up). This is the normal status during braking. The intake valve is open and the outlet valve is closed. This allows brake pressure from the master cylinder to pass to the wheel.
- Pressure maintenance phase (pressure holding). When the brake control module (BCM) detects that a wheel is rotating too slowly, it closes the intake valve. This prevents brake pressure to the wheel from increasing, even if the driver depresses the brake pedal harder. If the rotational speed of the wheel continues to drop, control enters the pressure reduction phase.
- Pressure reduction phase (pressure dump). The intake valve remains closed. The brake control module (BCM) opens the outlet valve and starts the pump to lead brake fluid away from the wheel, thereby reducing brake pressure. Once the rotational speed of the wheel has increased sufficiently, the control module closes the outlet valve, opens the intake valve and control resumes open mode.
Steps 1, 2 and 3 are repeated until braking is completed or ABS control ceases.