ABS/ASC 5
ABS/ASC 5: Changes compared with 1993 model year:- The hydraulic unit has 2/2-way control valves, as in the ABS 5 system.
- The plunger hydraulics with reservoir used on the E31 and E34/M60 with reservoir as an independent component between the hydraulic unit and the rear-wheel brakes are now deleted. Omission of the hydraulic servo circuit simplifies the filling and bleeding procedures.
- On the E34/M50, intake air is regulated at an additional throttle assembly (E31, E341M60: already used on production cars).
Components:
Hydraulic Unit:
Since on cars with ASC the driven (rear) wheel brakes have to be controlled separately, this hydraulic unit is provided with additional inlet and outlet valves (4-channel system).
The changeover valve with integrated pressure limiting valve is used for the changeover the ABS to ASC operation, and disconnects the master cylinder from the wheel cylinder. The suction valve makes the connection between the return flow pump and the master cylinder.
If an antilock brake regulating action commences, the return flow pump, which is self-priming, delivers brake fluid at zero pressure through the open suction valve to the appropriate wheel brake circuit (pressure build-up). In the pressure retention phase, the inlet valve is also closed. The outlet valve is energized if pressure has to be reduced.
The reservoir and damping chambers are of the same pattern as in the ABS hydraulic unit.
The pipe unions have threads of different sizes to prevent confusion.
Control module:
Basically similar in construction and function to the ABS control module, with suitable adjustments to the regulating logic. An additional processor controls and monitors the throttle butterfly positioning motor.
The 83-pin ABS/ASC control module can be encoded by way of the MoDiC to suit a specific model, engine and transmission. A pre-coded control module can also be obtained if the central code key is specified.
Throttle butterfly motor:
This is a DC motor with flange-mounted gears.
One type of motor is installed for all cars with double throttle butterfly. It is connected to the throttle butterfly by a wire cable.
Function:
The speeds of the four road wheels are registered by the ABS sensors. If slip at a driven wheel exceeds a predetermined amount, so that the cars traction or dynamic stability are affected, the following control process commences:
a) Repositioning the throttle butterflies:
The outer throttle butterfly, located directly in front of the main throttle butterfly, is held open during regular operation by a spiral spring. When ASC is taking effect, it is turned by the electro-mechanical positioning motor to a preset value, so that engine torque is reduced. Since the main throttle butterfly is not affected, there is no feedback to the accelerator pedal, which does not move under the driver's foot.
b) Alterations to ignition timing:
If the system has to alter the ignition timing, the engine management (DME) control module retards it so that engine torque is reduced. If wheelslip is severe, the ignition and fuel injection are gradually phased out as well.
c) Brake applications:
Unlike engine torque modifications, the brakes are only applied up to a maximum road speed of 100 km/h.
If the wheelslip thresholds are exceeded, the affected driven wheel is braked to the necessary extent.
In the road speed range up to 40 km/h, the individual-wheel brake application has the same locking effect as a limited-slip differential, and therefore increases traction.
When moving away from a standstill when both driven wheels have the same level of traction, or in all circumstances above 40 km/h, the brake applications have the effect of increasing stability.
Braking pressure is built up by energizing the solenoid valves and the pump as appropriate until the nominal slip value is no longer reached.
The regulating logic takes account of cornering, aquaplaning, differences in tire circumference and engine drag moment when coasting.
Switch and Warning Light:
The ASC is activated and in the operating mode whenever the ignition is switched on.
If driving conditions make it desirable for more wheelslip to occur (for instance, rocking the car out of deep snow), ASC can be turned off at its switch.
Regardless of the position of the switch, however, any faults which occur are registered and memorized by the system.
The ASC warning light on the instrument panel indicates the ASC's operating mode:
Light off: ASC ready to operate
Light flashing: ASC in action
Light permanently on: ASC passive or defective
When the ignition is switched on, both the ABS and the ASC warning lights come on briefly while the system's internal test is in progress (about 2 seconds).
Diagnosis
The diagnosis concept is similar to that for ABS 5, but with the additional ASC functions included.
ABS 5 and ABS/ASC 5 diagnosis takes place by way of the DIS/BMW Service Tester/MoDiC, from Diagnostic Software Version 13.0 on. During diagnosis,
the relevant repair and testing instructions must always be complied with.
The ABS/ASC control module is codeable (must be encoded to the vehicle).
After Repairs
After repairs have been made to electrical or hydraulic brake components, a functional test must be performed. Service Information 34 05 94 (3692) summarizes the extent of diagnosis capability of previous wheelslip control systems.