Operation

Heated Front Screen Operation
When the HFS switch is operated, an earth path is completed from the HFS switch to earth header C0017 LHD/C0018 RHD on a B wire, via header C0760.

The completion of the earth path from the HFS switch, completes an earth path from the BCU to the switch on a KO wire. The earth path completes a circuit within the BCU for an internal electronic switch which receives its feed from fuse 29.

The internal switch closes, completing a circuit which allows the feed from fuse 13 to flow through the internal switch, then to the HFS switch on a PS wire. The feed illuminates the HFS 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 HFS switch is operated, the completed earth path closes the internal switch. This completes the connection from the heated screen relay coil to the BCU on a KN wire. The earth path from the BCU is on a B wire to earth header C0551.

The completed earth path energises the heated screen relay, closing the relay contacts, allowing the feed from fusible link 1 to flow through the relay contacts to fuses 7 and 8.

The feed from fuse 7 is connected to the LH front screen element on a PS wire. The LH front screen element is connected to earth header C0018 LHD/C0017 RHD on a B wire.

The feed from fuse 8 is connected to the RH front screen element on a PK wire. The RH front screen element is connected to earth header C0017 LHD/C0018 RHD on a B wire.

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

Air Temperature Control (ATC) ECU operation
On vehicles fitted with air conditioning, the HFS 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 HFS switch into the PS wire from the BCU to the switch. This illuminates the HFS switch indicator light.

Simultaneously, the ATC ECU also provides an earth path into the KO wire from the BCU to the HFS switch. This earth path allows the internal electronic switches within the BCU to close, powering the front screen elements as described previously.