Operation

Heated Rear Window Switch Operation
When the HRW switch is operated, an earth path is completed from the HRW switch to earth header C0017 LHD/C0018 RHD on a B wire, via header C0760.

The completion of the earth path from the HRW switch, completes an earth path from the BCU to the switch on an NB wire. The earth path completes a circuit within the BCU for an internal electronic switch which receives its feed from fuse 29 in the passenger compartment fusebox.

The internal switch closes, completing a circuit which allows the feed from fuse 13 to flow through the internal switch, then to the HRW switch on an NP wire. The feed illuminates the HRW switch indicator light and is earthed from the switch to earth header C0017 LHD/C0018 RHD on a B wire, via header C0760.

The feed from fuse 29 is connected to a second internal switch within the BCU. When the HRW switch is operated, the completed earth path closes the internal switch. This completes an earth path from the coil of the heated rear screen relay, through the Intelligent Driver's Module (IDM) and from the passenger compartment fusebox to the BCU on an SK wire, via header C0293 LHD/C0292 RHD. This connection is the serial data bus between the IDM and the BCU.

The BCU earth is connected from the BCU to earth header C0551 on a B wire. The IDM is also connected to earth header C0551 on a B wire from the passenger compartment fusebox.

The completed earth path energises the heated rear screen relay, closing the contacts and allowing the feed from fuse 8 in the passenger compartment fusebox to flow through the contacts.

The feed flows from the passenger compartment fusebox to the rear screen heater element on an NP wire.

The rear screen heater element is connected to earth header C0706 on a B wire.

The momentary operation of the non-latching HRW switch, signals internal electronic switches within the BCU to close. Internal circuitry within the BCU holds the switches closed for a pre-determined period or until the HRW switch is pushed a second time.

Air Temperature Control (ATC) ECU Operation
On vehicles fitted with air conditioning, the HRW can be operated when 'DEF', 'feet/screen' or 'screen' is selected on the ATC control panel.

When one of the above selections is made, the ATC ECU provides a feed to the HRW switch into the NP wire from the BCU to the HRW switch. This illuminates the switch indicator light

Simultaneously, the ATC ECU also provides an earth path into the NB wire from the BCU to the HRW switch. This earth path allows the internal electronic switches within the BCU to close, powering the rear screen heater element as described previously.