Anti-Lock Braking System & Torque Control

Anti-lock Braking System ABS is a standard feature on the new 325iX. The ABS logic in the control unit is specially adapted to All-Wheel Drive. The actual rate of deceleration is determined using a mercury deceleration sensor. This new device is located on the left inner fender, aft of the shock tower. The ABS control unit monitors the decel sensor and determines the actual deceleration value when braking with either high or low road traction conditions.

The positioned angle of the decel sensor is vital for correct operation:
therefore, the device is attached to the vehicle with two shear-off type fasteners. This type of fastener will prevent unauthorized removal or tampering.

When braking on a high friction/good traction surface, the decel sensor signals the ABS control unit with an open circuit. The control unit then operates the system in the known conventional mode. However, when braking on a low friction/poor traction surface the ABS control unit continues to monitor a closed circuit. In this situation the system will be operated using the new ABS All-Wheel Drive Logic Mode letting the hydraulic valve pulsate at a different rate.

The decel sensor circuitry is included in the ABS system of fault self-checks. Since the decel sensor circuit is a simple NC (Normally Closed) circuit with self-test values, the ABS safety circuits can differentiate a short to ground or an open circuit fault as compared to normal operation. As with any ABS fault, upon monitoring a decel sensor fault, the control unit will switch the entire ABS system off and illuminate the antilock fault light. While in the "failsafe" mode, normal non-ABS brake system operation is available. The ABS system will also be switched off if an open circuit signal exists for more than 20 seconds.

For further details on ABS, refer to TRI 34 04 83 (2035).

Torque Control Engine braking torque is reduced during low friction/poor traction ABS operation and at road speeds above 15 mph. On cue from the ABS control unit, the Motronic control unit will position the idle control valve to limit engine braking to an acceptable level.

The torque control feature is ended when the ABS operation is complete or when the clutch pedal is depressed. A dedicated second clutch switch is used for this purpose.