Battery Cable Monitoring
The battery cable is diagnosed by a special circuit between the SBSL and SBSR satellites.
The battery cable diagnosis takes place across the low impedance shield of the battery cable (monitoring shield). The monitoring shield monitors the state of the battery cable as follows:
There are connections to the left B-pillar satellite and the right B-pillar satellite at both ends of the shield. This means that there is usually the same voltage at the analog/digital converters in the satellites. If the voltages differ, there is a fault.
The monitoring shield consists of a low-impedance metal mesh. A connection cable exits from each end of the monitoring shield (at the safety battery terminal in the luggage compartment and at the battery earth point in the engine compartment).
The connection at the safety battery terminal in the luggage compartment is connected to the right B-pillar satellite. The second connection cable in the engine compartment is connected to the left B-pillar satellite.
The satellites contain analog/digital converters that are connected to the microprocessor of the satellite. The connection cables of the battery cable diagnosis are connected to the A/D converters.
The right B-pillar satellite contains a pull-up resistor.
The left B-pillar satellite contains a pull-down resistor of the same size.
The voltage supply of the satellite (approx. 10 V) is applied at the pull-up resistor. Ground is applied at the pull-down resistor.
The very low-impedance cable and the resistors of the same size mean that around half the voltage (approx. 5 V) is applied at the A/D converters.
In the following cases, the battery cable is cut off by the safety battery terminal from the battery and the alternator is switched off:
- Short circuit to ground (body)
- Short to battery positive
If the outer insulation is damaged (e.g. due to friction/scuffing), but the monitoring shield has no connection to ground, the following case could occur:
Moisture (rain) would mean that the voltage would gradually fall. A short circuit to ground would be detected, but the safety battery terminal would not be triggered.
The entry "Implausible measured value" is set in the fault code memory. This would be indicated to the driver by the airbag warning lamp.
| State | Measured Value SBSL | Measured Value SBSR |
|---|---|---|
| Battery Cable OK | 5V | 5V |
| Interuption of the diagnostic connection | 0V | 10V |
| Short circuit to ground | 0V | 0V |
| Short to B+ | 12v | 12v |