Control concept

ABS Control

During the ABS control procedure, the front wheels are controlled individually while the rear wheels are controlled together. An ABS control cycle consists of the three phases: Pressure retention, pressure reduction and pressure build-up.

If a speed sensor signals strong wheel deceleration, i.e. a tendency to lock, initially the brake pressure of this wheel is not increased, i.e. the pressure is retained at the level previously reached.

If the wheel deceleration continues to increase, the brake pressure is reduced so that the wheel is braked to a lesser extent.

The wheel rotates faster again due to the reduced brake pressure. The pressure is increased again when a certain speed limit is reached. Rotation of the wheel is slowed down again by this increase in pressure.

Depending on the condition of the road surface, approx. 5 to 12 control cycles take place per second. The return pump is permanently activated during the control procedure.

With no power applied, the inlet valves are open in their rest position while the outlet valves are closed in their rest position:

ASC control

ASC reduces the high torques at the drive wheels by correspondingly influencing the engine management and by braking the drive wheels.

This is achieved by following control interventions:

• Reduction in choke position:
  The choke adjustment represents the first stage of engine intervention. In this system, the ABS/ASC control unit activates a throttle actuator which is driven by an electric motor and which adjusts the choke in the direction of a smaller throttle opening by means of an operating (Bowden) cable. The throttle actuator is activated by clocking the positive and negative supply.
  The current position of the choke is detected by a throttle potentiometer and signalled back to the ABS/ASC control unit.
  
• Ignition timing in "retard" direction
  Ignition timing adjustment is a supplement to the relatively slow reduction in the choke position. The ignition intervention is controlled by means of the digital motor electronics. Ignition timing adjustment represents a fast yet smoother engine torque reduction than blanking out ignition and injection pulses.
  
• Blanking out of ignition and injection pulses
  Blanking out the ignition serves the purpose of quickly reducing the drive slip. The delay time up to blanking out and therefore to reducing the engine torque is very short. Blanking out the injection which is activated simultaneously prevents uncombusted mixture entering the exhaust system in the case of the ignition signals being blanked out over a longer period of time. Blanking out of the ignition and injection pulses is also controlled by the digital motor electronics.
  
• Brake intervention at drive wheels:
  Brake intervention at the drive wheels takes place analogously to the ABS control based on the three control phases: Pressure build-up, pressure retention and pressure reduction. The pressure for brake intervention during an ASC control phase is applied directly via the intake valve and the self-priming pump element to the corresponding wheel brake. The changeover valve prevents the brake fluid flowing back into the supply reservoir during control cycles.
  

The automatic transmission control is also influenced.