The headlight switch may be located either on the dash by the instrument panel or on the steering column. It controls most of the vehicle’s lighting systems. The most common style of headlight switch is the three-position type with OFF, PARK, and HEADLIGHT positions. The headlight switch will generally receive direct battery voltage to two terminals of the switch. This allows the light circuits to be operated without having the ignition switch in the RUN or ACC (accessory) position.
When the headlight switch is in the OFF position, the open contacts prevent battery voltage from continuing to the lamps. When the switch is in the PARK position, battery voltage that is present at terminal 5 is able to be applied through the closed contacts to the side marker, taillight, license plate, and instrument cluster lights. This circuit is usually protected by a 15- to 20-ampere fuse that is separate from the headlight circuit.
When the switch is located in the HEADLIGHT position, battery voltage that is present at terminal 1 is able to be applied through the circuit breaker and the closed contacts to light the headlights. Battery voltage from terminal 5 continues to light the lights that were on in the PARK position. The circuit breaker is used to prevent temporary overloads to the system from totally disabling the headlights.
FIGURE. (A) Instrument panel-mounted headlight switch. (B) Steering column-mounted headlight swith.
FIGURE. Operation with the headlight switch in the PARK position.
FIGURE. Operation with the headlight switch in the HEADLIGHT position.
The rheostat is a variable resistor that the driver uses to control the instrument cluster illumination lamp brightness. As the driver turns the light switch knob, the resistance in the rheostat is changed. The greater the resistance, the dimmer the instrument panel illumination lights glow. In vehicles that have the headlight switch located in the steering column, the rheostat may be a separate unit located on the dash near the instrument panel.
The dimmer switch provides the means for the driver to select either high- or low-beam operation, and to switch between the two. The dimmer switch is connected in series within the headlight circuit and controls the current path for high and low beams. In the past, the most common location of the dimmer switch was on the floor board next to the left kick panel. The driver operates this switch by pressing on it with a foot. Positioning the switch on the floor board made the switch subject to damage because of rust, dirt, and so forth. Most newer vehicles locate the dimmer switch on the steering column to prevent early failure and to increase driver accessibility. This switch is activated by the driver pulling the stock switch (turn signal lever) rearward.
FIGURE. A steering column—mounted dimmer switch.
FIGURE. A headlight circuit indicating current flow with the dimmer switch in the LOW-BEAM position.
The complete headlight circuit consists of the headlight switch, dimmer switch, high-beam indicator, and the headlights. When the headlight switch is pulled to the HEADLIGHT position, current flows to the dimmer switch through the closed contacts. If the dimmer switch is in the LOW position, current flows through the low-beam filament of the headlights. When the dimmer switch is placed on the HIGH position, current flows through the high-beam filaments of the headlights.
FIGURE. A headlight circuit indicating current flow with the dimmer switch in the HIGH-BEAM position.
The headlight circuits just discussed are designed with insulated side switches and grounded bulbs. In this system, battery voltage is present to the headlight switch. Hie switch must be closed in order for current to flow through the filaments and to ground. The circuit is complete because the headlights are grounded to the vehicle body or chassis. Many import manufacturers use a system design that has insulated bulbs and ground side switches. In this system, when the headlight switch is located in the HEADLIGHT position, the contacts are closed to complete the circuit path to ground. The headlight switch is located after the headlight lamps in the circuit. Battery voltage is applied directly to the headlights when the relays are closed. But the headlights will not light until the switch completes the ground side of the relay circuits. In this system, both the headlight and dimmer switches complete the circuits to ground.
No matter if the headlights use insulated side switches or ground side switches, each system is wired in parallel. This prevents total headlight failure if one filament burns out.