Confound it the befuddling nature of vintage headlight switches.

In classic cars, you pull the knob straight out and you get parking lamps at the first detent, then, headlights with the knob all the way out. Headlight switches are great when they work properly. One summer evening, you’re out cruising with a friend and the road goes dark—which is a very intimidating and dangerous experience especially in total darkness.

I will cite you an example.

Long ago in my youth during a visit to New York City, it was raining and I was on the GW Bridge headed to New Jersey in a classic Mustang. The headlights began flashing off and on. They were never off long enough to be dangerous—but very unsettling when you’re trying to find a motel in the dark in the rain.

(Image/Jim Smart)

How Vintage Headlight Switches Work

Automotive headlight switches are all quite similar in function whether you drive a Ford, Chevy, or Dodge. They are circuit breaker protected, which was why my headlights were flashing off and on. (There was an obvious reason why mine were flashing off and on—either a short circuit or a defective switch.) The reason headlights were no longer fuse protected beginning in the 1960s was a blown fuse would leave you in the dark without a prayer. The circuit breaker protection system prevents a fire by cycling the lights off and on due to a potential short circuit or high resistance and heat, yet keeps the road lit briefly until you can stop and get help.

Learn more on the importance of electrical breakers and fuses here: All About Vehicle Electrical Fuses

Headlight switches are a high-amp load device because headlights consume a lot of electricity and the switch can get quite warm. However, as switch contacts become corroded and dirty, resistance increases and contacts get hotter. The built-in circuit breaker’s contacts also suffer from corrosion and dirt. Resistance increases, contacts get hot, and the circuit breaker cycles lights off and on.

Most of the time when you experience cycling headlights, it isn’t a short circuit, but instead corroded circuit breaker or headlight switch contacts. There’s little point in disassembling the switch to clean contacts when you can order a new headlight switch. However, if you own a classic car where replacement or reproduction parts are not available, you are then faced with finding a good new old stock switch or carefully opening up your headlight switch and cleaning the contacts.

When you open up a vintage headlight switch, you will find both headlights and parking lamps are protected by separate circuit breakers. Headlights are have a higher amp rating than parking/driving lamps. Your task is to carefully open the switch as a fact-finding mission to learn what went wrong and how to correct the problem.

Adding a separate electrical relay system can help reduce the amount of electrical current entering your headlight switch (and give you brighter headlights too). Read more about the popular “headlight relay mod” in this article.

We’re going to look inside a Ford headlight switch, which is very similar to Chrysler’s (same supplier) and quite similar to GM and AMC headlight switches. All are sliding contactor block style where power travels through the contactor to carry power to head, parking, and instrument lights. To get your switch healthy again, clean out the factory lubricant and dress each contact with fine emery paper to remove any corrosion or contaminants. Once you have clean contacts, you can apply the appropriate lubricant to the contactor and contacts and be ready for reassembly.

This is a typical push-pull headlight switch with a slider contactor block inside. (Image/Jim Smart)
Headlight switches are engineered for high-amp operation with a heavy-duty sliding contactor inside with dual circuit breaker protection. Headlight switches are exposed to heat, humidity, and corrosive elements in the air. Contacts become contaminated from use and become corroded and dirty with time. (Image/Jim Smart)
Here’s an old school headlight switch from the 1950s with nothing more than pin fuses as circuit protection. The downside is when you blow fuses you lose lighting on the affected circuit, which makes driving dangerous. (Image/Jim Smart)
Headlight switch basics include the main switch body, which includes switching and circuit breaker protection inside (red arrow) and the variable resistor (black arrow) for instrument light dimming. The instrument lighting dimmer resistor suffers from the same issue as switch contacts and circuit breakers—corrosion and dirt that makes instrument lights flicker and hard to adjust. The coil (looks like a spring) is a very long wire, which creates the resistance to dim lights. It can be cleaned with contact cleaner. (Image/Jim Smart)
This button is used to release the switch shaft for service. (Image/Jim Smart)
Turn the knob all the way left to dome light while depressing this button, which will release the shaft for switch removal. (Image/Jim Smart)
Earlier, we mentioned the instrument lighting variable resistor (also known as a potentiometer or rheostat), which is nothing more than a long rolled up wire coil that rides a contact to the instrument lights. When you’re at the beginning of the coil, there’s no resistance and the lights are bright. Turn the knob (coil) to the other end where there’s a lot of resistance and the lights are dim. Turn the knob far left to the detent and the dome light comes on. In this image, we are all the way left at the dome light. (Image/Jim Smart)

Inside the switch body is a sliding contactor block, which routes power to the headlights and parking lamps. All the way in, there’s no power to the head and parking lights. Here, the contactor is all the way out for headlights. (Image/Jim Smart)
The contactor is also shown here removed from the switch cavity for inspection. (Image/Jim Smart)
The open switch cavity yields some insight into how it works. On the left in yellow are contacts for the instrument lighting resistor coil. Bottom right and mid-right are the circuit breaker contacts. Power comes into both—with the headlight breaker being 18 amp breaker for twinset headlight (four headlamps for high and low beam) and 12 amp for singles (two lamps). The round contacts, which are located beneath the contactor, go to headlights and parking lamps. When amp draw become excessive, the headlights will cycle off and on. Parking lamps and running lights are protected by a 15 amp breaker. (Image/Jim Smart)
These two contacts—yellow and orange arrows—provide current flow through the resistor coil to dim and brighten instrument lighting. Think of the resistor coil like a water faucet where resistance comes from the rate electricity passes through this coil. High electricity flow means low resistance and bright lights. Low electricity flow (high resistance) limits current flow and this dims or darkens the lights.

Variable resistance isn’t just handy for interior lights, that same principle is what controls your vehicle’s HVAC fan speed too. Read more about it here: What is a Blower Motor Resistor? (Image/Jim Smart)

Late model vehicles work on a similar principle as the early sliders. They have built-in circuit protection. (Image/Jim Smart)
In some applications, the variable resistor instrument light dimmer is a separate device tied to the headlight switch. (Image/Jim Smart)

Keywords
electrical, headlights, switches
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Author: Jim Smart

Jim Smart is a veteran automotive journalist, technical editor, and historian with hundreds of how-to and feature articles to his credit. Jim's also an enthusiast, and has owned and restored many classic vehicles, including an impressive mix of vintage Ford Mustangs.