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.
New (lower) versus old (top)...
New (lower) versus old (top) headlight switches. Headlights fail because they get old and dirty. Rarely do they fail due to poor design or a failed part. Contacts inside become dirty and/or corroded, causing poor continuity that forces circuit breakers to cycle off and on.
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.
Mustang headlight switches...
Mustang headlight switches from '65-'78 are basically the same except for the application (mounting brackets).....
....The switch itself, including...
....The switch itself, including multiplex plug, is the same.
To remove a headlight switch,...
To remove a headlight switch, turn the knob all the way counterclockwise (past the interior light turn-on point) and reach behind the dash to press this release button. Then pull the shaft straight out.
The rotary instrument light...
The rotary instrument light dimmer and interior light work via this variable resistor, also known as a rheostat or potentiometer. Instrument power passes through this resistor. When the resistor and contacts become dirty and corroded, instrument lights will flicker, especially during adjustment. You can clean these contacts with tuner cleaner available at Radio Shack.
Headlight, parking light,...
Headlight, parking light, and taillight power is routed through this contact block to contacts at the plug pins. Your contact block, which is attached to the shaft and knob, slides back and forth to three positions-off, parking, and headlights. Rarely do switch contacts go bad even under the worst of conditions.
This is the headlight circuit...
This is the headlight circuit breaker. Note the contact points and a bimetallic strip, which work together for circuit protection. When contact points become dirty or corroded, it creates resistance to current flow and the resulting heat, which causes the bimetallic strip to bend, opening the points (lights cycle off). The circuit breaker cools quickly and the contact points close (lights cycle on). This starts a process of lights off and on.
At the back of the switch...
At the back of the switch is the taillight circuit breaker, which can cycle the taillights off and on. However, this circuit breaker rarely causes a problem because it carries less electrical load.
A close-up of the headlight...
A close-up of the headlight circuit breaker demonstrates how it works. Contact points carry electrical load. When there's a short or poor contact that causes high resistance, the connection gets hot, causing the bimetallic strip to bend/distort, which opens contacts (lights off). When it cools (quickly), contact is made and lights come back on. If there's a short, it will cycle wildly.
This is an external circuit...
This is an external circuit breaker like we see for air conditioning, fog lamps, and convertible power tops. It works the same way via heat and bimetallic contact point protection.
Older Ford headlight switches...
Older Ford headlight switches didn't have a circuit breaker. Note the more compact size. These were fuse protected, which would put unsuspecting souls in the dark given a short or high resistance. Circuit breaker protection was a safety feature that came in the 1960s to keep drivers from being completely left in the dark.
This is a '65-'68 C5ZZ-11654-B...
This is a '65-'68 C5ZZ-11654-B headlight switch with knob and escutcheon.
Here are two C5ZZ-11654-B...
Here are two C5ZZ-11654-B headlight switches side by side. On the left is a one-size-fits-all offshore switch for virtually every '65-'78 Mustang application. On the right is a Motorcraft C5ZZ-11654-B (SW-441) switch. For safety, we suggest the Motorcraft #SW-441 switch, even though it costs more.
This is a '69-'70 C9ZZ-11654-A...
This is a '69-'70 C9ZZ-11654-A headlight switch (SW-817) from Motorcraft. Note the different bracket.
Here's the D3AZ-11654-A Motorcraft...
Here's the D3AZ-11654-A Motorcraft switch (SW-1226) for '71-'73 Mustangs. Note the absence of a bracket.
The headlight switch shafts...
The headlight switch shafts and knobs insert into the headlight switches. Push all the way in until the shaft bottoms and clicks. Gently tug on the knob to make sure it's seated. If the shaft comes out, it didn't seat properly.
We wanted to show you this...
We wanted to show you this Ford headlight switch with Cougar vacuum switching. A sliding vacuum switch directs negative pressure to the Cougar headlight door actuators. Switch the headlight off and the doors close. Pull the knob out and the headlight doors open.