Charging System: Description and Operation

Safety Battery Terminal And Monitoring Of Battery Cable (shielding)

The battery cable is fitted with an electrically conducting shield, which is insulated against the body and against the battery cable. The shielding consists of a low-impedance metal mesh. A cable exits from each end of the shielding, i.e. at the safety battery terminal in the luggage compartment and at the battery earth point in the engine compartment. Both connection leads are connected to the airbag control unit.

Safety battery terminal
The battery safety terminal is installed in the positive lead from the alternator just in front of the positive terminal of the battery. The cable ends in a tapered plug. If the monitoring of the battery cable determines a short circuit, the safety battery terminal is triggered. This switches off the alternator and the fuel pump.
It also deactivates the starter motor load circuit. The engine can no longer be started.
The interrupted alternator line can be replugged provisionally to restart the engine. In this case, however, the battery terminal isolation function is no longer available as the priming cap has already been ignited. The priming cap and alternator line must then be replaced.

Monitoring of battery cable (shielding)
The potential of the battery cable is read in via two resistors of the same size at the input of the airbag control unit. If the battery cable is OK, half of the power supply (U_V = battery voltage) is measured. In the event of a fault, different measurement values result, as shown in the following table.







If an interruption or short circuit occurs, the relevant fault code memory entry is stored in the airbag control unit.

NOTE:
The battery cable is only cut by the safety battery terminal in the event of a short circuit. If, for example, the outer insulation of the battery cable is damaged but the shielding has no earth connection, the following case could occur:
Moisture (e.g. rain) can gradually reduce the voltage. A short circuit to earth is detected, but the safety battery terminal is not triggered. The following defect location is set in the fault code memory entry of the airbag control unit: "Battery cable (monitoring of shielding): implausible test result". The airbag warning lamp in the instrument cluster is also activated.

NOTE: All work on the airbag must only be conducted with the battery disconnected. Connect and disconnect airbag control unit, sensors and gas generators only with the battery disconnected. The airbag control unit contains vehicle-specific data and must therefore be encoded before placing into operation. Removal and installation in other vehicles is prohibited.