Overview
This document contains information on:
General
Function requirements
Function
General
Maximum full-stop braking with locked wheels leads to total loss of lateral stability, steerability and stability.
The anti-lock braking system (ABS) prevents the wheels from locking up when braking and as a result maintains steerability and driving stability during the braking procedure. The braking distance is reduced with ABS.
If ABS engages, the driver will observe slight vibrating on the brake pedal.
Function requirements
Drive train operational
Engine in operation
Brake and traction system functional
Function
A rpm sensor is located at each of the four wheels.
The brake lines routed to the individual wheel brake cylinders from the master brake cylinder are divided into two separate channels. Each of the two channels can be locked with a quick-acting solenoid valve. There is an intake and exhaust solenoid valve for each wheel.
Slip has occurred if the actual distance covered per wheel revolution deviates from the actual tire circumference. The optimal slip and so the best braking effect is achieved not by maximum brake pressure, but rather by exactly dispensed brake pressure.
If a wheel is braked so sharply that it exceeds the range of the optimal slip, ABS regulation engages.
ABS regulation comprises the following three phases:
1 Hold pressure: The brake pedal is operated, the wheel brake pressure increases and the circumferential speed indicates a tendency to lock. The intake solenoid valve closes on the wheel where there is a tendency to lock. The wheel brake pressure remains the same even if the pedal pressure increases.
2 Pressure reduction: If the wheel slip increases despite constant wheel brake pressure, the exhaust solenoid valve is opened on the wheel where there is a tendency to lock. The wheel brake pressure decreases.
3 Pressure buildup: If the optimal wheel slip is undershot, wheel brake pressure is built up again in stages. For this, the exhaust solenoid valve is closed and the intake solenoid valve is briefly opened several times.
These three phases repeat several times per second until the slip remains in the optimal range.