Discovery 2. Manual - part 744

ALARM SYSTEM AND HORN

DESCRIPTION AND OPERATION

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1 Theft deterrent LED
2 Receiver
3 Volumetric sensors
4 Central door locking switch
5 Body Control Unit (BCU)
6 Bonnet activated alarm switch
7 Vehicle horn
8 Alarm sounder
9 Fuel cut off switch

10 Fuel flap release switch
11 Door latch switches, drivers door key lock/ 

unlock switches

12 Battery Backed Up Sounder (BBUS)
13 Passive remobilisation exciter coil

ALARM SYSTEM AND HORN

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DESCRIPTION AND OPERATION

Alarm system block diagram

1 Body Control Unit (BCU)
2 Remote handset
3 Receiver
4 Fuel cut off switch
5 Ignition switch
6 Central door locking switch 
7 Volumetric sensors
8 Bonnet activated alarm switch 
9 Drivers door key lock/unlock switches

10 Door latch switches 

11 Fuel flap release switch
12 Intelligent Driver Module (IDM)
13 Battery Backed Up Sounder (BBUS)
14 Alarm sounder
15 Vehicle horns
16 Direction indicators
17 Door lock actuators
18 Engine Control Module (ECM)
19 Starter motor 

ALARM SYSTEM AND HORN

DESCRIPTION AND OPERATION

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Description

General
The anti-theft system deters the theft of the vehicle with both active and passive systems.  The active systems monitor 
the vehicle perimeter and, in some markets the vehicle interior, for intrusion.  If the system detects intrusion, an alarm 
sounds.  The passive systems include a vehicle immobiliser as well as protection against intrusion through super 
locking the doors.  The vehicle is immobilised whenever the ignition key is removed from the ignition switch.  Super 
locking prevents the interior door handles from opening the doors.  In addition, there are many features and functions 
that enhance system operation.

It is possible to perform the various functions associated with the anti-theft system remotely. To accomplish this, the 
handset generates a radio frequency when the lock button or unlock button is depressed. This signal is transmitted 
to the Body Control Unit (BCU) via a receiver located in front of rear sunroof beneath the headlining. Precise alignment 
of the handset to the receiver is not necessary. The approximate operating range between the handset and the 
receiver is 10 metres (33 feet).

It is important to understand the configuration of the system on each particular vehicle.  There are many 
different combinations of features with dedicated operating modes.  System configuration for each vehicle 
can be determined using TestBook.

The anti-theft system is controlled centrally by the BCU, which is located below and behind the passenger glovebox. 
A serial communication link communicates between the Intelligent Driver Module (IDM) located within the fusebox, 
the BCU and the LED located in the instrument panel.

The state of the alarm is stored in the BCU.  If the battery is disconnected while the alarm is armed, the alarm will be 
triggered when the battery is reconnected.

NOTE: Disconnecting the battery while the alarm is armed triggers the alarm.  Always disarm the alarm system before 
disconnecting the battery.

The BCU also receives inputs from the following components:

l

Ignition key in sensor.

l

Ignition switch.

l

Fuel cut off switch.

l

Fuel flap release switch.

The BCU also interacts with the following components:

l

Starter relay.

l

Vehicle horns.

l

Direction indicators.

l

Fuel flap release actuator.

l

Courtesy lamps.

ALARM SYSTEM AND HORN

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DESCRIPTION AND OPERATION

Door latch switches

The BCU uses the door latch switches to indicate if a door is open or closed.  There is a door latch switch within every 
door latch assembly, including the tail door latch assembly.

Input/Output
The input from the door latch switches to the BCU is either zero volts or an open circuit.  Zero volts indicates the door 
is open.  An open circuit indicates that the door is closed.  When the BCU sees an open circuit, it pulls the input high 
internally.

The driver's door latch switch has a dedicated signal input to the BCU.  This allows the BCU to identify the driver's 
door position.

The BCU use a common signal input for the remaining door latch switches.

TestBook provides the ability to monitor the real time state of the door latch switches.  Remember that the driver's 
door is always in a known state.  The remaining doors are combined into a single state.  The BCU cannot distinguish 
the state of the individual doors, other than the driver's door.