Voltage is available at all times to the headlamp switch from the LIGHTING fuse through CKT 42 (RED). A 17A circuit breaker which resets automatically is within the headlamp switch. The circuit breaker opens heating due to an overcurrent condition. With the circuit breaker open, the current flow stops. The circuit breaker's thermal element cools. The circuit breaker re-closes.

The headlamp switch has the following 3 positions:

When the switch is in the headlamps position, voltage is available in CKT 10 (YEL) to the daytime running lamp (DRL) control module. The DRL control module uses this voltage signal to inhibit DRL operation when the headlamps operate normally.

The headlamp dimmer switch is part of the multifunction lever on the steering column. By pulling the multi-function switch lever up the axis of the steering column, the driver changes the switch selections. Repeated switch operations alternate between the high beam selection and the low beam selection.

With the selection of the low beams, voltage is available through CKT 12 (TAN) to the low beam connector pins in sealed beam headlamps, or to the low beam headlamps in composite headlamps. Because the headlamps are grounded, current flows through the low beam filaments and the headlamps emit light. With the selection of the high beams, voltage is available through CKT 11 (LT GRN) to the high beam connector pins in sealed beam headlamps, or to the high beam headlamps in composite headlamps. Because the headlamps are grounded, the headlamps operate.

CKT 11 (LT GRN) also makes available high beam voltage for lighting the blue headlamp high beam indicator lamp in the instrument cluster. Because the instrument cluster's lamp circuits ground at ground G201 through CKT 150 (BLK).

The DRL control module operates the DRL when the vehicle is in operation and the headlamps are not lighted. The DRL control module receives power when the ignition switch is in the RUN position or the START position from GAUGES fuse 4 through CKT 39 (PNK). The DRL control module has ground through CKT 150 (BLK). The DRL control module determines that the vehicle is in operation when the control module receives power from GAUGES fuse 4 and the parking brake is not set.

The parking brake closes the contacts of the park brake warning switch, when the parking brake sets. This grounds CKT 1134 (LT BLU). This ground connection lights the brake indicator lamp at the instrument cluster. The brake indicator lamp also lights when the ignition switch in the START position 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 prevents abnormal conditions in the braking system from affecting the operation of the DRL control module. The grounding input with the ignition switch in the START position is a lamp test design. The duration of the grounding input is a short period of time. The grounding input does not impact operation unless the driver releases the parking brake.

If CKT 1134 (LT BLU) does not ground, CKT 10 (YEL) does not have voltage. Then CKT 39 (PNK) has voltage. Then the DRL control module operates in order to light the daytime running lamps. The module accomplishes this by closing internal contacts (logically equivalent) in order to ground CKT 592 (LT GRN/WHT) from the DRL relay. One side of the DRL relay coil grounds. Because the relay's coil has voltage available from GAUGES fuse 4 through CKT 39 (PNK) when the ignition switch is in the RUN position or in the START position, the coil energizes. The normally-open relay contacts have voltage at all times from DRL fuse 15 through CKT 340 (ORN). The DRL diode module functions in order to reduce the voltage applied to the headlamps. This reduction in voltage decreases the intensity of the headlamps. This voltage is available to the low beam headlamps through CKT 12 (TAN) when the relay energizes. Because the headlamps have ground, the lamps operate.

The grounding of the CKT 592 (LT GRN/BLK) also ground the DRL indicator lamp in the instrument cluster. Because voltage is available to the lamp when the ignition is in the RUN position or the START position through CKT 39 (PNK) from GAUGES fuse 4, the lamp lights.