Body control unit (Power supply): Description and operation

Power supply

Battery supply to the BCU and the IDM is provided through a 10 A fuse located in the engine compartment fuse box.

The BCU unit receives an ignition switched power supply (ignition switch position II) input via a 10 A fuse in the passenger compartment fuse box.

The BCU receives a signal when the ignition switch is turned to the crank position, it then supplies an earth path to the starter relay coil, to enable the crank operation by supplying power through the starter relay contacts to the starter motor.

Battery voltage is monitored and BCU operation will function normally between 8 and 18 volts. Between 5.7 and 8 volts the BCU is in the 'under volts' state. The status of the battery is used to determine which outputs may be driven.

If a voltage supply above 18 volts is experienced, outputs will not normally be driven except for those functions which are required during cranking (robust immobilisation, antenna coil, crank enable relay and feed to gear position switch contacts W, X, Y, Z). In the over voltage state the vehicle can be driven, but all other functions are disabled and outputs are switched off (power windows, heated screen, direction indicators etc.).

All functions are disabled on power up until communications between the BCU and IDM have been established. If communications cannot be established, operation will commence with degraded functionality.

Battery supply to the IDM is provided through the inertia switch and a 10 A fuse in the engine compartment fuse box. If the inertia switch contacts are closed battery voltage is available at the IDM; if the inertia switch contacts are open there is no battery supply to the IDM. The supply condition of the IDM is signalled to the BCU via the serial bus. If the inertia switch is operated (contacts open) the change in state is detected by the BCU which unlocks the doors if the ignition switch is in position II and the alarm is not set.

The BCU is earthed through a hard-wire connection.

Inputs and outputs

The BCU and IDM process inputs and provide the necessary outputs for control and operation of the vehicle's 'body' systems.

BCU inputs The BCU processes signals received from the following components: l Door latch switches. l Driver's door key lock/ unlock switches. l Bonnet activated security system. l Volumetric sensors. l Central Door Locking (CDL) switches. l Remote transmitter (via receiver unit). l Inertia fuel cut-off switch. l Ignition switch. l Fuel flap release switch.

The input voltages (V_in) for BCU digital signals are defined as follows: l Logic 1 when V_in ≥ 6V. l Logic 0 when V_in ≤ 2V.

BCU input voltages between 2 and 6 volts are indeterminate and cannot be guaranteed.

Analogue input voltages are measured as a ratio with respect to battery voltage.