When you place the headlamp switch in the HEAD and LO positions, the internal contacts of the switch close. This provides a path to ground the coil of the lighting relay through CKT 717 (RED/BLK). Circuit breaker #16 applies voltage at all times to the other side of the lighting relay coil. When ground is provided, the relay energizes, switching the relay contacts. A ground path is now provided to the low beam filaments through CKT 691 or CKT 697 (RED/BLK), through the now closed contacts of the relay, through CKT 716 (YEL) to the closed contacts of the low/high lighting relay to G105. With ground provided, the low beam filaments illuminate.

When the headlamp switch is in the HI or PASS position, a ground path to the low/high lighting relay is provided through CKT 721 (RED/YEL). Voltage is applied at all times through circuit breaker #16 and CKT 719 (ORN) to the coil of the low/high lighting relay, causing the relay to energize. With the relay contacts switched, a ground path is provided to the high beam filaments through CKT 690, CKT 692, CKT 696 or CKT 697 (BLK/WHT) through the now closed contacts of the low/high lighting relay to G105. With ground provided, the high beam filaments illuminate.

When the brake pedal is depressed, the Stoplamp Switch contacts close. The Stoplamp Switch has voltage available at all times from Circuit Breaker 25 (Circuit Breaker 23 on vehicles with Hydraulic Brakes) through the CKT 103 or CKT 110 (ORN) [CKT 470 (GRN/WHT) on vehicles with Hydraulic Brakes]. When the brake pedal is depressed and the switch contacts close, this voltage energizes CKT 104 or CKT 105 (WHT) [CKT 471 (WHT) on vehicles with Hydraulic brakes] to the Turn/Hazard Flasher. The Turn/Hazard Flasher is a solid state logic device which controls the operation of the Stop, Turn and Hazard lamp functions. the Turn/Hazard Flasher distributes the voltage to the Stoplamp bulbs and to the Right and Left Trailer Turn Relays, which provide dedicated stop lamp signals to the Trailer Wiring Provision Connector.

These circuits are used with a dual filament, single bulb turn/stoplamp system. The Turn/Hazard Flasher logic enables one filament in a dual filament bulb to operate as a turn signal lamp even though the brake pedal is depressed. Both the Left and Right Tail Lamps contain dual filament bulbs as well as single filament back-up lamp bulbs. Both bulbs in the Left and Right Tail Lamps are grounded by CKT 150 (BLK) to Ground G 404.

The Park, Side Marker and Clearance Lamps are energized whenever the Headlamp Switch is in the PARK or HEAD position. This causes a ground to be passed through the Headlamp Switch, which energizes the Taillamp Relay. When the Taillamp Relay is energized, voltage is distributed on CKT 94 (BRN) to the Front and Rear Park Lamps, the Right and Left Side Marker Lamps, the Right and Left Side Mirror Lamps, the Interior Body Builder Connector, and the Trailer Connector.

Grounding for the Front Park Lamps is through CKT 150 (BLK) to ground G 107. The Clearance Lamps, the Left and Right Front Side Marker Lamps, and the Lighted Mirror Lamps are grounded through CKT 550 or CKT 650 (BLK) to Ground G 105. The Right and Left Taillamps are grounded through CKT 150 (BLK) to G 404.

The Marker Lamps are located on the top front portion of the cab. The Marker Lamps are controlled by the IP mounted Marker Lamp Switch. When the switch is pressed, current is passed through the switch on CKT 112 (ORN) to the Marker Lamp Relay. The Marker Lamp Relay switches power from CKT 178 (GRN) to CKT 177 (BRN) when it is energized by the Marker Lamp Switch. Power that is passed into CKT 177 (BRN) is also supplied to the Trailer Connector on CKT 53 (BLU) and through connector C 208 to the Roof Harness, where it supplies the front ID Lamps and the Clearance lamps.

The Turn Signal Lamps and Hazard Lamps operate from the same lamps. When operating as turn signal lamps, ground is provided from the Turn Signal Switch through either CKT 132 (BLU/BLK) or CKT 133 (BLU/ORN) to the Turn/Hazard Flasher. This causes the Turn/Hazard Flasher to operate only the front and rear bulbs on the selected side of the vehicle. When operated as Hazard Lamps, ground is provided to the Turn/Hazard Flasher by the Turn Signal Switch on both the Left CKT 132 (BLU/BLK) and the Right CKT 133 (BLU/ORN) at the same time and all four bulbs are flashed together.