Body control unit (Fog lamps): Description and operation

Fog lamps

The BCU operates the rear fog lamps and the front fog lamps (where fitted) via the IDM. The BCU front fog lamp operation can be programmed to operate under one of three set conditions. The BCU will ensure the logical conditions are satisfied for the lamps to operate under the set conditions.

Front fog lamps Front fog lamp operation is monitored by the BCU, which allows only the front fog lamps to operate when the side lamps or headlamps are on. When the side lamps, headlamps or ignition switch is turned off, the BCU also switches off the fog lamps. When the side or headlamps are switched on again, the front fog lamps will remain off unless the front fog lamp switch is pressed to resume operation. If the rear fog guard lamps are selected on, switching off the front fog lamps will also switch off the rear fog guard lamps.

When the fog lamp switch is operated, an earth path is completed and the BCU allows the fog lights to be switched on providing the logical preconditions have been satisfied. The BCU then supplies a voltage supply to the fog lamp relay, to illuminate the fog lamps.

The front fog lamps option can be programmed in one of three states dependent on market/ customer requirements, these are: l Option 1– not fitted. l Option 2 – main beam no effect. l Option 3 – off with main beam.

The BCU checks the status of the following inputs to determine the logic action for providing an output to the front fog lamp relay: l Ignition state. l Main beam state. l Side lamps. l Dipped beam. l Front fogs selected (press button, not latched).

If the ignition state is crank the state of the front 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 front fog relay state is restored. If the ignition is turned off, the front fog relay is turned off.

For option 3 configuration, if the main beam is turned on the state of the front fog relay is memorised and the relay is switched off. Pressing the front fog switch while main beam is on will not be recognised. When the Main beam state returns to OFF, the memorised front fog relay state is restored.

In the event of a communications link failure while the front fog relay is on, the front fog relay will be switched off.

Rear fog lamps The rear fog lamps operation is monitored by the BCU, which only allows the rear fog lamps to operate when the side lamps or the headlamps are on. When the side lamps, headlamps or ignition is switched off, the rear fog lamps are also switched off. When the side lamps or headlamps are switched on again, the rear fog lamps will not switch on again unless reselected by operating the rear fog lamps switch. If front fog lamps are fitted, the rear fog lamps will be switched off if the front fog lamps are switched off.

A supply voltage to the rear fog lamps relay is provided from a fuse in the passenger compartment fuse box, then through two electronic switches in the IDM. With the lighting switch in the side lamp or headlamp position, an earth path from the coil of the rear fog lamps relay completes the circuit through the two switches in the IDM to switch the rear fog lamps on when the BCU receives a request signal from the rear fog lamps switch to turn the circuit on.