DIAGNOSIS AND TESTING - HEADLAMP
When a vehicle experiences problems with the headlamp system, verify the condition of the battery connections, fuses, charging system, headlamp bulbs, wire connectors, relay, high beam switch, dimmer switch, and headlamp switch. Refer to the appropriate wiring information.
Each vehicle is equipped with various lamp assemblies. A good ground is necessary for proper lighting operation. Grounding is provided by the lamp socket when it comes in contact with the metal body, or through a separate ground wire.
When changing lamp bulbs check the socket for corrosion. If corrosion is present, clean it with a wire brush.
When it is necessary to remove components to service another, it should not be necessary to apply excessive force or bend a component to remove it. Before damaging a trim component, verify hidden fasteners or captured edges are not holding the component in place. Don't replace parts unless you test an your sure that is the problem. Check the basic's first ! Then find a wiring diagram an see what all is involved , connectors , grounds , power an do a voltage drop test on the circuit to find the problem !An a scan tool would come in handy as well ,to do bi-directional tests !
The configuration of the headlamp system of headlamps, park lamps and fog lamps is determined by the BCM. The BCM -body control module determines the lighting configuration as a result of the inputs from the ignition switch, headlamp switch and multi-function switch. A PCI bus-serial data communication network is transmitted from the BCM to the FCM to enable the necessary drivers to set the illumination configuration. Four wires are connected between the headlamp switch and the BCM. The first wire contains information regarding the position of the headlamp switch (Off, Automatic Headlamps, Automatic Headlamp switch fog, Park with Fog, Head, or Head with Fog Lamps). The second wire contains information regarding the position of the dimmer switch (Dome Lamp, Daytime Brightness, Dimming Level or Off). The third wire is a dedicated signal return (ground) wire. The fourth wire provides power to the front fog lamp indicator. You should take this to someone who knows what's up rather then just replacing parts !