BODY CONTROL UNIT: DESCRIPTION AND OPERATION (page 6 of 16)

BODY CONTROL UNIT

86-3-24 DESCRIPTION AND OPERATION

The heated rear window 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 rear window switch is operated, it completes a path to earth, signalling a 
request for heated rear window operation to the BCU. The BCU then supplies a feed to the heated rear window switch 
to light the switch indicator, and earths the supply from the coil of the heated rear window relay via the IDM to turn the 
heated rear window elements on. The BCU maintains a supply for a predetermined time (15 minutes), then opens the 
earth path after the time-out period to turn the heated rear window operation off. If the heated rear window 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 rear window 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 rear window circuit on 
or, if it is already on, to re-start the time out period. 
 

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 AIR CONDITIONING, DESCRIPTION AND OPERATION, Description.

In the event of a communications link failure occurring while the heated rear window enable is on, the relay and lamp 
will be de-activated.

Wipers and washers
Front and rear wash/ wipe operations are controlled by the BCU via the IDM.

Front wipers and washers
When front intermittent wipe is selected using the wiper switch on the column stalk, the BCU signals the IDM to 
operate the wiper motor. The IDM then supplies a voltage to the normal speed winding of the wiper motor for a period 
of no longer than 500 ms. The timed feed ensures the wiper motor operates, but does not complete more than a single 
wipe of the screen. The BCU checks the position of the delay switch to determine the delay period which should be 
utilised for the intermittent wipe operation. The delay period of the front intermittent wipe is maintained until the wiper 
switch is moved to the off, normal or fast speed position, or the delay period is changed.

When front intermittent wipe is selected, a delay period of 500 ms will be incurred before the wipers operate. This 
delay prevents the occurrence of a wipe action when the wiper switch is changed to the off position from the normal 
or fast modes of operation.

To enable the front intermittent wipe facility, the BCU checks the state of the following inputs:

l

Ignition state.

l

Front intermittent wiper switch.

l

Front intermittent wiper rate.

If the ignition switch is in the off or crank position, the front wipers are not active and the front wiper enable output is 
switched off.

If the option to initiate a wipe action in addition to a front screen wash action has been configured in the BCU, the 
wipers will operate 400 ms after the washer pump becomes active. If the washer switch is released within 400 ms, 
the wipers will not be activated. If a washer switch is pressed for longer than 400 ms, the BCU will operate the wipers 
after an initial delay of 400 ms, and will continue to operate for as long as the washer switch is depressed plus an 
additional 4 seconds.

The front wash/ wipe programme can be configured to operate in one of two ways:

l

Option 1 – normal.

l

Option 2 – no wipe.

The BCU checks for the following inputs:

l

Ignition state.

l

Front washer pump switch.

The BCU can provide the following wash / wipe outputs depending on the logic states prevailing:

l

Front wiper enable.

l

Front wash/ wipe in progress flag.

In the event of a communications link failure occurring while the front wash/ wipe programme is in operation, the front 
wiper enable output is switched off.