Few things are more intimidating than sudden darkness on an isolated stretch of road. If you've been driving a classic Mustang for any time at all, you've probably lost your headlights at least once. Most of the time, the headlights cycle off and on, then mysteriously return to normal for no apparent reason. While the mysteries of this phenomenon might set your nerves on edge, the explanation is quite simple.
When headlights or taillights cycle off and on, the culprit is usually a faulty headlight switch. Because headlights are high-load electrical components, they draw a lot of power (amps) and switches can get warm. With new headlight switches, heat isn't an issue because the circuit breaker and switch contacts are clean with good continuity. When switches get older, corrosion takes its toll on connections (switch contacts) and things get toasty because of high resistance. Heat causes the circuit breaker to do what it's designed to do-cycle the headlights off and on. In fact, it cycles the same way your turn signal flasher does and for the same reason-heat and its effect on bimetallic contact points.
Headlight switches contain two separate circuit breakers for the headlights and taillights. Instrument lamps are protected with a 2.5-amp fuse, while dome lights get their protection from a 7.5-amp fuse. Those fuses are located in the fuse box under the dash. When a fuse blows, it indicates a short circuit in the instrument or dome lamp circuits. Tail and parking lights usually short out due to wiring problems (short to ground). They can also short out due to defective bulbs. This has become more common with off-shore replacement bulbs because their filament support towers can distort, touch, and short to ground. When a parking or taillight fuse blows, check the bulbs first.
Headlight switches were never intended to be serviceable components. When they go bad from age and use, they should be replaced. We visited Mustangs Etc. for a close look at headlight switches and how they work. Function is quite simple, employing sliding contacts to close and open circuits to the headlights, parking lights, and taillights, along with a rotating rheostat for instrument and dome lights.