ABS 5/ABD (4-Channel System)
NoteThe ABD (Automatic Brake Differential) complements the ABS system and is used as a traction aid at speeds up to 45 mph. For this purpose, an active input into the brake system is made if one driven wheel starts to spin. Since ABD control has to trigger the driven wheels individually, the ABS/ABD system is designed as a four-channel system. With ABS control of the rear axle, the inlet and outlet solenoid valves of both rear wheels are triggered in parallel (common control as with a ABS 5 3-channel system).
Functional description of ABD
If a driven wheel starts to spin when the vehicle accelerates, this is sensed by speed sensors in the control unit and ABS control is started. The wheel speeds of both Driven wheels are compared with each other and with the speeds of the non-driven wheels. If a specified wheel speed difference is exceeded, a brake pressure is built up at the spinning wheel. Adding residual drive torque and braking torque thus allows the wheel that does not spin to transmit a higher drive torque.
To ensure driving stability, the ABD system is switched off if any wheel spin tendency is sensed at the second driven wheel and the ABD control pressure in the brake is reduced. A wheel spin tendency of the second driven wheel means: Both driven wheels wheels (rear wheels) rotate at a higher speed than the non-driven wheels (front wheels).
At road speeds above approx. 45 mph (front - axle speed signal), ABD control is terminated or not reactivated anymore. To avoid thermal overloads on the brake system, ABS control is not enabled by the control unit anymore if a boundary temperature was computed from brake load and time factors. Operation of the ABD function is displayed to the driver by a green information lamp. A red warning lamp lights up if the ABD is inoperative.
Schematic diagram - ABS 5 / ABD
1 - ABS/ABD control unit
2 - Tandem brake master cylinder
3 - ABS/ABD hydraulic unit (4 hydraulic outputs)
4 - ABS sensors
5 - Central Information System
6 - Proportioning valve (2x)
7 - Brake light
8 - Stop light switch
9 - ABS warning lamp
10 - ABD warning lamp
11 - ABD operation lamp
12 - DME control unit