BODY CONTROL UNIT: DESCRIPTION AND OPERATION (page 5 of 16)

BODY CONTROL UNIT

DESCRIPTION AND OPERATION 86-3-23

A timeout function is included which allows the windows and sunroofs to be operated for a period of 44 

±

 1 seconds 

after the ignition key has been removed from the ignition switch. The timeout function will be cancelled if the BCU 
detects the following conditions:

l

The driver's door has been closed after the ignition has been switched off.

l

Any door open after the ignition is switched off.

The preconditions are programmed into the BCU dependent on market specification.

The BCU does not directly control rear window operation, but it can request the IDM to remove the power feed to the 
rear windows. The IDM enables power supply to be provided to the rear lift relay in the passenger fuse box. When the 
relay coil is energised the relay contacts close and power is applied to the rear window motor. If the window lift 
isolation switch located in the centre console is latched out, the earth path from the LH and RH rear door window 
switches is broken and operation of the rear window motors through the use of the rear window switches is prevented. 
Rear window operation using the console switches is not affected by the isolation switch.

In the event of a communications link failure occurring while rear window enable is on, the relay remains on until the 
ignition switch is turned to the off position.

Heated screens
The heated screen time-outs and logical operations are controlled by the BCU.

Heated front screen
The BCU provides an output to the heated front screen relay and a 'heated front screen active signal' for the heated 
front screen warning lamp. The BCU uses the status of the following inputs to determine the appropriate logical 
operations for this feature:

l

Ignition state.

l

Engine running (link from instrument pack).

l

Heated front screen selected (non-latched button pressed).

If the ignition switch is not in position II, or the engine running signal is OFF, the heated front screen will not be active.

The heated front screen operation period and voltage supply is controlled by the BCU. The BCU receives a voltage 
supply feed from the engine compartment fuse box and an ignition switched supply through a fuse in the passenger 
compartment fuse box. When the heated front screen switch is operated, it completes a path to earth, signalling a 
request for heated screen operation to the BCU. The BCU then supplies a feed to the heated front screen switch to 
light the switch indicator, and earths the supply from the coil of the heated screen relay to turn the heated front screen 
on. The BCU maintains a supply for a predetermined time (5 minutes), then opens the earth path after the time out 
period to turn the heated screen off. During the timeout period, if the heated front screen switch is pressed a second 
time, the circuit is also interrupted to turn off the supply to the heated front screen.

On vehicles fitted with air conditioning, the heated front screen is also operated by the ATC ECU when DEF (defrost) 
is selected. In this case the ATC ECU provides the trigger signal to the BCU to turn the heated front screen circuit on 
or, if it is already on, to re-start the time out period.
 

 + 

 AIR CONDITIONING, DESCRIPTION AND OPERATION, Description.

In the event of a communications link failure occurring while the heated front screen relay is on, the relay and lamp 
will remain on until the ignition switch is turned off or the time-out period elapses or the heated front screen switch is 
pressed.

Heated rear window
The heated rear window will only function when the engine is running, and is operated by a non-latching switch on the 
instrument pack cowl.

The BCU provides a 'heated rear window enable output' and a 'heated rear window active warning lamp signal' based 
on the logic status of the following inputs:

l

Ignition state.

l

Engine running (link from instrument pack).

l

Heated rear window selected (non-latched button pressed).

If the ignition switch is not in position II, or the engine running signal is OFF, the heated rear window will not be active.