Discovery II (LT) V8-4.6L (2003)
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Body Control Module: Description and Operation
Description
General
The Body Control Unit (BCU) is located behind the passenger glovebox and is connected to the main harness by four connectors on its bottom edge
and an additional connector located on the side of the BCU casing. Mounting the BCU behind the fascia makes it reasonably inaccessible for intruders
to disable the anti-theft system.
The BCU uses solid-state microprocessor control to perform logical operations and timing functions for a variety of the vehicle's electrically operated
systems, these include:
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Door locking.
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Anti-theft alarm and immobilisation system.
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Exterior lighting including direction indicators and hazard warning lamps.
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Courtesy lighting.
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Wipers and washers.
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Electric windows and sunroof.
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Heated windows.
The BCU also communicates with several other electronically controlled systems such as the EAT ECU and SLABS ECU and also has a datalink
between the Intelligent Driver Module (IDM) and the instrument pack. The datalink is a low speed bus capable of transmitting and receiving messages
at a data rate of 10,400 bits per second. Additional inputs and outputs to peripheral devices are included which are necessary for determining vehicle
status for particular logical operations e.g. crank, ignition key inserted, fuel flap enable etc.
The BCU receives its power supply from the engine compartment fuse box, and is protected by a 10 A fuse. The BCU communicates with the IDM to
provide the control signals to perform power switching operations in conjunction with dedicated relays.
IDM
The IDM is integrated into the passenger compartment fuse box, which is mounted behind the fascia below the steering column. There are no
harnesses between the fuse box and the IDM. The IDM performs the power switching operations for several of the vehicle's electrical systems.
The IDM communicates with the BCU and the instrument pack via a serial interface. If the BCU or the IDM is replaced, the communications link
between the two units has to be re-established. This can be done either by switching on the ignition and leaving it on for five minutes, or by using
TestBook. The vehicle immobilisation will remain active until the communications link between the BCU and IDM has been re-established.
Transit mode
To prevent excessive battery drain during transit to overseas markets, the vehicle is placed in a transit mode. The following functions are disabled
when the vehicle is in transit mode:
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Volumetric sensors.
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Passive immobilisation.
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Immobilisation of the vehicle by use of door lock.
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Ignition key interlock.
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Electric seat enable time-out with driver's door open.
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.
