Discovery 2. Manual - part 741

BODY CONTROL UNIT

DESCRIPTION AND OPERATION 86-3-31

The BCU checks the status of the following inputs to determine the logic action for providing an output to the rear fog
lamp relay:

Ignition state.

Main beam state.

Dipped beam.

Front fogs relay.

Rear fogs selected (press button, not latched).

If the ignition state is crank the state of the rear fog relay is memorised and the relay is switched off. Pressing the front
fog switch during cranking will not be recognised. When the ignition state returns to Ignition after cranking, the
memorised rear fog relay state is restored. If the ignition is turned off, the rear fog relay is turned off.

In the event of a communications link failure occurring while the rear fog relay is on, the rear fog relay will remain on
until the ignition is turned off or the dipped/ main beam is turned off.

Interior courtesy lamps
To determine when to provide an interior courtesy lamps enable output, the BCU checks the following input
conditions:

Ignition state.

Transit mode.

Driver's door open.

Passenger or rear door open.

The courtesy lamps operate at full brightness when they are first switched on. The BCU then gradually dims the lamps
(fade-out) before completely switching them off. Fade-out occurs over a period of two seconds following the logical
control signal to turn off the lights. When a door is opened, the BCU provides earth paths from the interior lamps, the
load space lamp and the ignition switch illumination. The following table lists the courtesy lamps operation and
duration for holding the lamps on that occurs after the BCU has detected a condition that signals the courtesy lamps
should be switched on:

Instrument pack
The BCU communicates with the instrument pack to provide a range of functions.

Control signal

Additional conditions

Duration

Door open

10 minutes

Unlock signal from remote transmitter

All doors closed and ignition switch not in
position III

1 minute

Unlock signal from the driver's door lock

All doors closed and ignition switch not in
position III

1 minute

Ignition switch turned from position II or III
to position 0

All doors closed

1 minute

Door changed from open to closed

All other doors closed and ignition switch
not in position III

15 seconds

Ignition switch in position III

Timer cancelled

BCU receives a lock command

Timer cancelled

Vehicle has transit mode enabled

15 seconds on all occasions

BODY CONTROL UNIT

86-3-32 DESCRIPTION AND OPERATION

Odometer update
The BCU can be programmed for one of two options:

Option 1 – no odometer error warning.

Option 2 – odometer error warning.

In order to provide an LCD flash request to the instrument pack via the communications link, the following inputs are
checked:

Ignition state.

Instrument pack odometer value (via the communications link).

BCU odometer value.

The function is only active when the ignition state is on. The maximum allowed value is 999,999 miles (1,608,999
km).If the instrument pack odometer value is greater than the maximum allowed value, the maximum value is
assumed. The BCU odometer value is stored in EEPROM. If 16 identical values of the instrument pack odometer
reading is received consecutively, the instrument pack odometer value is compared with the BCU odometer value. If
the consecutive readings from the instrument pack differ, the BCU odometer value is incremented accordingly. If the
BCU odometer value is less than the instrument pack odometer value by up to 10 km, the BCU odometer value is set
equal to the instrument pack odometer value.

If the odometer warning option is enabled, and the contents of the instrument pack odometer value buffer is identical
to, or greater than BCU odometer value

10 km, the BCU sends an LED flash request to the instrument pack.

In the event of a communications link failure, this function will be unable to operate.

Gear position indicator illumination
On automatic gearbox models, two variations of illumination for the gear position indicators on the selector lever can
be programmed into the BCU. In option 1, illumination is provided when the ignition is on. In option 2, illumination is
enabled when the ignition is on and the side lamps are off.

Starter relay
The BCU checks the status of the following inputs and internal BCU conditions to determine whether or not to provide
an output to enable the starter relay:

Autostart inhibit (vehicles with automatic gearbox only).

Security start inhibit (immobilisation check).

Engine running (link to instrument pack).

EEPROM locked (internal check).

IDM and BCU matched.

When the BCU receives a crank signal from the ignition switch, an earth path is completed to the starter relay coil,
provided that the security system has been de-activated. If the ECM has not received a valid unlock/ remobilise signal,
the starter relay will be disengaged and the engine stopped. The BCU also receives an engine running signal from
the instrument pack, so that if the ignition key is turned to the crank position while the engine is running, the starter
motor relay will not be engaged.

If the logic conditions are correct to allow starter operation, the completion of the earth path from the starter relay coil
to the BCU energises the coil and the relay contacts close to supply battery power to the starter motor.

When the ignition switch is released from the crank position, the power supply feed from the ignition switch to the
starter relay coil is interrupted and the relay contacts open to prevent further battery feed to the starter motor.

If a communications link failure is experienced, the BCU will be prevented from detecting the 'engine running'
condition and the BCU will default to assume that the engine is not running.

BODY CONTROL UNIT

DESCRIPTION AND OPERATION 86-3-33

Cruise control
The Neutral/ Reverse/ Park/ Brake selected output is used by the cruise control system. The BCU checks for the
following inputs:

Ignition on.

Gearbox state.

Brake switch activated.

For vehicles with manual transmission, Neutral/ Reverse/ Park/ Brake selected output is on if the ignition is on and
brake switch activated is on. Otherwise, the Neutral/ Reverse/ Park/ Brake selected output is off.

For vehicles with automatic transmission, the Neutral/ Reverse/ Park/ Brake selected output is on if the ignition is on
and brake switch activated is on, or if the ignition is on and 'gearbox state' is in Neutral, Park or Reverse. Otherwise
Neutral/ Reverse/ Park/ Brake selected output is off.

Shift interlock (where fitted)
On automatic gearbox models, the BCU and IDM combine to operate a shift interlock solenoid, so that the gear
selector lever can only be moved when certain logical conditions have been satisfied. Operation of the interlock may
be affected, if the battery becomes discharged.

AUTOMATIC GEARBOX - ZF4HP22 - 24, DESCRIPTION AND OPERATION, Description.

When the gear selector lever is in the Park position and the ignition is off, an interlock solenoid is de-energised, locking
the gear selector lever in the Park position.

The gear selector lever can only be moved from the Park position when the ignition is switched on and the brake pedal
is depressed. When these conditions are satisfied, the brake pedal switch signal is received by the BCU which then
signals the IDM to energise the interlock relay. When the interlock relay is energised, the interlock solenoid is
operated, unlocking the gear selector lever.

When the brake pedal is released, the feed to the BCU is removed and the shift interlock solenoid is de-energised.
The locking mechanism controlled by the solenoid will lock the gear selector lever once it is returned to the Park
position.

The shift interlock option can be programmed in one of three states dependent on market/ customer requirements,
these are:

Option 1– not fitted.

Option 2 – normal operation.

Option 3 – inhibit transfer box.

The BCU checks the status of the following inputs to determine the logic action for employing the shift interlock
solenoid:

Brake switch activated.

Ignition state.

Gearbox state.

In the event of a communications link failure, the shift interlock solenoid enable is switched on for 1 minute every time
the ignition is switched on and switched off when the ignition is turned off.

Ignition interlock (where fitted)
On automatic gearbox models, when the ignition key is inserted in the ignition switch, the ignition key-in switch
contacts close to supply a signal to the BCU to indicate the presence of the key in the switch and provide a supply
feed to the ignition interlock solenoid. If the gear selector lever is in a position other than Park, or the 'transfer box
neutral selected' is on, the BCU will supply an earth path to complete the ignition interlock circuit and cause the
interlock solenoid to be engaged. With the interlock solenoid engaged, removal of the ignition key is prevented. The
BCU will open the circuit, to allow removal of the ignition key, only when the gear selector lever is in the Park position
and the transfer box is in either high or low range.

BODY CONTROL UNIT

86-3-34 DESCRIPTION AND OPERATION

The ignition key interlock option can be programmed in one of three states dependent on market/ customer
requirements, these are:

Option 1– not fitted.

Option 2 – normal operation.

Option 3 – inhibit transfer box.

The BCU checks the status of the following inputs to determine the logic action for operating the ignition key interlock
solenoid:

Transfer box neutral selected.

Gearbox state.

Transit mode.

When the transit mode is on, the ignition key interlock solenoid is off.

Transfer box interlock (where fitted)
The transfer box interlock is controlled by the IDM to prevent transfer box shifter operation unless certain
preconditions have been satisfied.

The transfer box interlock prevents the transfer box being shifted from High or Low to neutral with the ignition key
removed from the ignition switch. When the BCU senses that the ignition key is removed from the ignition switch, it
signals the IDM via the serial data bus. The IDM then provides an earth path for the coil of the transfer box relay,
energising the relay coil and closing the relay contacts to provide a voltage supply to the transfer box interlock
solenoid.

A diode is included in the supply line to the solenoid to prevent residual current causing the solenoid to stick in the
energised position.

The transfer box solenoid interlock option can be programmed in one of three states dependent on market/ customer
requirements, these are:

Option 1– not fitted.

Option 2 – normal operation.

Option 3 – inhibit transfer box.

The BCU checks the status of the following inputs to determine the logic action for employing the transfer box interlock
solenoid (transfer box solenoid enable):

Ignition state.

Gearbox state.

In the event of a communications link failure occurring while the transfer box enable is on, the output will be switched
off.

Gear position switch
On automatic gearbox models, the BCU provides an output which supplies power to the automatic gearbox gear
position switch. The BCU checks for the following inputs before it supplies power:

Ignition on.

Auxiliary.

When the ignition is on, the feed to the gear position switch is on. When the ignition is off and auxiliary is off for more
than 30 seconds, feed to the gear position switch is off.

AUTOMATIC GEARBOX - ZF4HP22 - 24, DESCRIPTION AND OPERATION, Description.

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