Headlamp: Description and Operation

Voltage is available at all times to the Headlamp Switch from the LIGHTING Fuse through the RED (42) wire. Within the Headlamp Switch is a 17 A Circuit Breaker which resets automatically. The Circuit Breaker opens upon heating due to an overcurrent condition. With the Circuit Breaker open, current stops flowing and the Circuit Breaker's thermal element cools. When sufficiently cool, the Circuit Breaker re-closes.

The three position Headlamp switch has an OFF position, a park lamps position, and a headlamps position. When the switch is in the headlamp position, voltage is available on the YEL (10) wire to the Daytime Running Lamp Control Module and to the Headlamp Dimmer Switch. (The Daytime Running Lamp Control Module uses this voltage signal to inhibit Daytime Running Lamp operation while the headlamps are on normally.)

The Headlamp Dimmer Switch is part of the Multi-Function Lever on the Steering Column. By pulling the Multi-Function Switch lever up along the axis of the Steering Column, the switch changes selections. Repeated switch operations alternates between the high beam selection and the low beam selection.

With the low beams selected, voltage is available on the TAN (12) wire to the low beam Connector pins (Sealed Beam Head lamps) or to the low beam headlamps (Composite Headlamps). Because the headlamps are grounded, current flows through the low beam filaments and light is emitted. With the high beams selected, voltage is available on the LT GRN (11) wires to the high beam Connector pins (Sealed Beam Headlamps) or to the high beam headlamps (Composite Headlamps). Because the headlamps are grounded, the lamps operate.
^ The LH Sealed Beam Headlamp is grounded at Ground G108 by the BLK (150) wire.
^ The RH Sealed Beam Headlamp is grounded at Ground G109 by the BLK/WHT (151) wire.
^ The LH Low/High Beam Composite Headlamp is grounded at Ground G108 by the BLK (150) wire.
^ The RH Low/High Beam Composite Headlamp is grounded at Ground G109 by the BLK/WHT (151) wire.

The LT GRN (11) wire also makes high beam voltage available for lighting the Blue Headlamp High Beam Indicator Lamp in the Instrument Cluster. Since the Instrument Cluster's lamp circuits are grounded at Ground G201 by the BLK (150) wire, the lamp lights when voltage is available.

The Daytime Running Lamp Control Module operates the Daytime Running Lights when the vehicle is operating and the headlamps are not on. The Daytime Running Lamp Control Module is powered when the Ignition Switch is in Run or Start by GAUGES Fuse 4 through the PNK (39)wire. It is grounded by the BLK (150) wire to Ground G201. The Daytime Running Lamp Control Module determines that the vehicle is operating when it is powered from GAUGES Fuse 4 and the parking brake is released.

The parking brake closes the contacts of the Park Brake Warning Switch when it is engaged. This grounds the LT BLU (1134) wire. This ground connection is used to light the Brake Indicator Lamp at the Instrument Cluster. The Brake Indicator Lamp is also illuminated when the Ignition Switch is in Start and by the operation of either the Brake Pressure Differential Switch or the Electronic Brake Control Module for abnormal conditions in the braking system. Diode D101 is used to prevent abnormal conditions in the braking system from affecting the operation of the Daytime Running Lamp Control Module. The grounded input with the Ignition Switch in Start is a lamp test design which only lasts for a short period of time. It has no impact on operation unless the parking brake is already released.

If the LT BLU (1134) wire is not grounded, the YEL (10) wire has no voltage, and the PNK (39) wire has voltage, then the Daytime Running Lamp Control Module operates to turn on the Daytime Running Lamps. The module does this by closing internal contacts (logically equivalent) to ground the LT GRN/WHT (592) wires from the Daytime Running Lamp Relay. This grounds one side of the Daytime Running Lamp Relay coil. Since the relay's coil has voltage applied from GAUGES Fuse 4 through the PNK (39) wire when the Ignition Switch is in Run or Start. the coil energizes. The normally-open relay contacts are powered at all times from the DRL Fuse 15 through the ORN (340) wire, the Daytime Running Lamp (DRL) Diode Module and the ORN (340) wire. The DRL Diode Module functions to reduce the voltage applied to the headlamps. This reduction in voltage decreases the intensity of the headlamps. This voltage is applied to the low beam headlamps through the TAN (12) wire when the Relay energizes. Because the headlamps are grounded, the lamps operate.
^ The LH Sealed Beam Headlamp is grounded at Ground G108 by the BLK (150) wire.
^ The RH Sealed Beam Headlamp is grounded at Ground G109 by the BLK/WHT (151) wire.
^ The LH Low Beam Composite Headlamp is grounded at Ground G109 by the BLK (150) wire.
^ The RH Low Beam Composite Headlamp is grounded at Ground G109 by the BLK/WHT (151) wire.

The grounding of the LT GRN/BLK (592) wire also grounds the DRL Indicator Lamp in the Instrument Cluster. Since voltage is available to lamp whenever the Ignition Switch is in Run or Start through the PNK (39) wire from GAUGES Fuse 4, the lamp is turned on.