ABS/ASC Coding

ABS / ASC

Slip control systems are installed to increase active driving safety. They assist the driver in critical driving situations in maintaining the stability and steerability of the vehicle.

ABS/ASC variants
All E46 models are equipped as standard with ABS/ASC. DSC III, known from E38/39, is available as an optional extra on the 2.8 l model. Systems from ITT INDUSTRIES (formerly Teves) are used on the E46.

The following systems are installed together with the individual types of engines:
- 4-cylinder (M43) models: ABS/ASC-EZA system (brake and engine intervention without throttle control)
- 4-cylinder M47 models: ABS/ASC with brake intervention and fuel volume control
- 6-cylinder M52 models: ABS/ASC-EZA system (brake and engine intervention with throttle control)

Technical features
ASC Mk20 EI:
The difference between the ASC MK20 EI and ASC MK4G is in the way the hydraulics and control unit have been combined to form one unit.

The hydraulic unit and the control unit can be replaced individually in case of repair! After replacing the control unit or the complete ABS/ASC unit, the new control unit must be encoded.

Basically, the ABS and ASC functions have remained the same and correspond to those of the ABS/ASC MK4G. The 6-cylinder models feature the actuator-controlled throttle valve (MDK). The actuator-controlled throttle valve combines the throttle and choke in one component.

New functions
Cornering brake control (CBC:) The cornering brake control stabilizes the vehicle when the brakes are applied while cornering.

Electronic brake force distribution (EBV): The EBV function registers the deceleration of the front and rear wheels and compares them to each other. In this way, braking of the front and rear wheels can be optimally adapted to relevant load conditions.

ASC system structure
The hydraulic unit is screwed to the control unit to form one complete unit. The hydraulic units consists of an aluminum block which accepts the valves and the return pump. 9 solenoid valves and one hydraulically switched valve are arranged in the block:
- 4 inlet valves (electric)
- 4 outlet valves (electric)
- 1 changeover valve (electric) with integrated pressure relief valve
- 1 charging valve (hydraulic)

Wheel speed sensors with pulse wheel
The system operates with 4 passive wheel speed sensors. The function and design are the same as on the E36.

Speed signal outputs: The rear left and right speed signals are registered by the corresponding speed sensors, processed in the control unit and output again as a square-wave signal.

The rear left speed signal is used as the driving speed signal in the instrument cluster.

The rear right wheel speed signal serves as the input signal for other control units, e.g. AGS.

Brake light switch (BLS)
The brake light switch (active) is necessary in order to detect operation of the brakes during ASC control and to consequently terminate ASC control.

In ABS mode, the signal from the brake light switch is used as an input variable thus increasing control comfort.

The passive brake light switch is still used on E36 vehicles. The type of brake light switch used (active/passive) is coded in the control unit.

ASC button
This button serves the purpose of cutting out and cutting in the ASC system. If the switch is pressed with the engine running, a 12 Volt signal is switched to the control unit and the system deactivated. The system is reactivated by pressing the ASC button once again.

If the vehicle is turned off (engine off) with the ASC system deactivated, the system is reactivated when terminal 15 is reactivated.

Instrument cluster with ABS/ASC/ABL lamps
Three indicator lamps are provided in the instrument cluster for the purpose of monitoring the various functions:
- ABS lamp
- ASC lamp
- ABL lamp (general brake warning lamp)

ABS lamp
The ABS lamp is activated directly by the control unit via a separate line:
- ABS lamp comes on for approx. 2 seconds at terminal 15 "ON" (lamp check)
- ABS lamp comes on at terminal 15 "ON" and when ABS fault is found
- ABS lamp is off when ABS is OK.

If the ABS system senses a fault, the instrument cluster recognizes this fault status by means of a high level and activates the ABS lamp. A defective or not connected control unit is also detected in this way. The ASC lamp is also activated in the case of an ABS fault.

ASC lamp
The ASC lamp is activated via the CAN bus:
- ASC lamp comes on for approx. 2 seconds at terminal 15 "ON" (lamp check)
- ASC lamp is off when ASC is OK.
- The ASC lamp flashes at 3 Hz when ASC is active.
- ASC lamp comes on at terminal 15 "ON" when the ASC system is not in operation due to a malfunction or when the system has been deactivated with the ASC switch.

ABL lamp
The ABL lamp (general brake warning lamp) monitors three different functions. It comes on when:
- The handbrake is applied
- The brake fluid level in the reservoir is too low
- CBC or EBV control is not in operation (e.g. defective rear axle control)

CAN-bus
The ASC control unit communicates with the DME and AGS (if installed) control units as well as the instrument cluster via the CAN bus. The ASC control unit sends information on activation/deactivation of the ASC and ABL lamps to the instrument cluster.

The DME informs the ABS/ASC control unit how high the relevant engine torque is. When control is active, the ABS/ASC control unit indicates to the DME control unit whether and by how much the torque is to be reduced.

When control is active, the AGS control unit (adaptive transmission control) receives relevant information from the ABS/ASC control unit. In this way, another gearshift characteristic can be realized in order to avoid the automatic transmission constantly shifting up and down.

Operating principle of hydraulic system
All inlet valves and the changeover valve are open when no power is applied. The outlet valves are closed when no power is applied. The charging valve closes hydraulically when the brakes are applied, it is otherwise open.

During ABS control, the pump feeds the brake fluid back into the master brake cylinder while during ASC control with brake intervention it builds up the necessary brake pressure. The changeover valve and charging valve are allocated to the brake circuit of the rear axle. The ABS return pump is capable of building up the necessary brake pressure only in conjunction with these two valves during an ASC control phase with brake intervention.

Torque control of ASC system
The basic principle of the ASC system Mk 20 El is the same on all models. The difference is in the type of engine torque control. On the M43 engine, the control is based on ignition timing adjustment and individual cylinder blank-out. The M52 engine is controlled by means of the throttle control, ignition timing adjustment and individual cylinder blank-out. On the M47 diesel engine, the torque is controlled by varying the injection volume.

Engine drag moment control (MSR)
The engine drag moment control function is also integrated in the Mk 20 El. The MSR intervenes in the case of excessively high wheel slip when the engine is coasting or when the driver shifts into a lower gear. It controls the engine drag moment in order to avoid excessive wheel slip of the driven axle.

ABS/ASC control function
The control unit starts a self-test when terminal 15 is switched on. If no fault is found, the next check takes place at approx. 20 km/h, in which all solenoid valves and the ABS/ASC pump are activated. If this test is also OK, the system is ready for operation.

The control unit detects whether ABS control braking or ASC control is required by means of the wheel speed sensors.

ABS control cuts in in case of a change in the wheel circumference or if a fixed slip threshold is exceeded while braking.

ASC control cuts in if the drive wheels assume excessively high positive slip during an acceleration phase.

ABS-controlled braking takes place on the basis of the well-known control philosophy: Individual brake pressure control at the front wheels and common control of the rear axle.

If the ASC control unit detects excessively high slip of the drive wheels, it can restabilize the drive axle by way of engine and/or brake intervention.