Mustang electrical systems are clever pathways that accomplish a big job whenever we twist the key and hit the road. Yet, they don't always work as they're supposed to because parts wear out, people make mistakes, and connections corrode and fail. What's more, electrical systems sometimes do weird things that we're just not going to understand, even with a degree in electrical engineering. At their best, electrical systems illuminate the lights, vibrate the sound system, ignite the fuel/air mix, and help warm or cool the cabin. Without the electrical system, we would all be walking, and sometimes we do.
Automotive electrical systems work differently than home systems, yet the overall approach for getting electricity to do our work is basically the same: current flow from positive to negative, which explains two-prong outlets in the home and a two-terminal storage battery in our Mustangs. At home, we use 110/115 volts of alternating current. In Mustangs, it's 12-15 volts of direct current.
Did you know you can buy a...
Did you know you can buy a complete new electrical system from Virginia Classic Mustang? This is the main wiring loom for a '66 Mustang. Although the electrical system is intimidating, there's virtually nothing to it. Replacement of the entire electrical system can be performed in a day.
Power for Mustang electrical systems begins and ends at the battery, which stores 12-15 volts of direct current to keep a Mustang operational. The charging system is designed to keep the battery charged by converting the engine's rotary motion to electrical energy by way of an alternator, or a generator in the case of '64 1/2 models.
Cars utilize a single-wire electrical system, meaning current flows through wires to each accessory and back to the battery's negative terminal through the car's body. In homes, electricity flows through two wires with an extra ground wire on hand to help prevent electrical shock. Automobiles don't need the return wire because the steel or aluminum body is used as a conductor to complete the circuit. This is where the term negative ground comes from.
Sometimes a ground wire is needed to carry electricity when the body can't, such as between the engine and firewall. Why? Because the engine rests on rubber mounts that insulate it from the body and chassis. The alternator or generator's ground connection helps ground the engine to some degree, but it's not enough.
In '65-'66, Ford used firewall...
In '65-'66, Ford used firewall bulkhead connectors to get power to and from the engine compartment. The bulkhead connector behind the engine handles the engine's senders and ignition power. The bulkhead connector near the master cylinder carries power from the battery to the electrical system, plus power to the headlights, parking lights, and horns. Beginning in '67, Ford moved these vulnerable connections under the dashboard to protect them from the elements.
Weak and faulty grounds explain many of the electrical gremlins that are difficult to troubleshoot. When electrical components start acting weird, such as dim headlights, a stereo that mysteriously quits, an ignition system that leaves you stranded, turn signals that suddenly blink rapidly or not at all, and other unexplainable surprises, the problem is a weak ground somewhere. This is why there are no unimportant grounds. They all serve a purpose.
There's no real mystery to electricity. It is simply the flow of electrons, tiny subatomic particles of matter smaller than an atom. These energetic guys are tiny, but they do a lot of work when they become a team on a mission. This takes us back to the importance of proper grounding. When electricity can't follow a proper path, it finds alternative paths. If you don't ground the engine, for example, the ignition system will find an alternative path to ground. When it can't, the engine quits. Weak headlight or taillight grounding sends electricity searching for alternative paths to ground, sometimes bringing the radio to life when the ignition is turned off. That's what we mean by electrical gremlins that cannot be explained. Most of the time, it's a faulty ground.
Think of your Mustang's electrical system like you would a home plumbing system. Electricity flows through wiring like water flows through pipes. When there is resistance to the flow of water, we get less water. Electricity works the same way. When there's resistance to the flow of electricity, there's less of it. However, when we create resistance to the flow of electricity, we get something we don't get from water: heat. Heat and resistance are what make light bulbs glow. Heat and resistance are also what make cigarette lighters get hot. They can also start fires when resistance comes from a weak connection.
Resistance is created with switches, resistors, resistance wire, filaments, and other controls. We use it to control fan-motor speed, light intensity, sound-system volume, ignition-coil voltage, and more. When we create resistance, we impede the flow of electricity.
Fan Speed Control: A Lesson in Resistance
There are a lot of misconceptions about how heater-fan controls work. There is but one truth. Heater-fan switches are not variable resisters. Resistance is the same in both (two-speed) or all three (three-speed) positions. Resistance comes from a resistor package installed in the heater plenum. The resister package consists of two lengths of resistor wire, which control resistance not to the fan motor but to ground. When the fan switch is in Low, resistance to ground is high, which impedes the flow of electricity to ground. This lowers fan speed. Move the switch to Medium and resistance to ground becomes less, raising fan speed. Move the fan switch to High and bypass the resistance wires completely, going directly to ground. The fan then runs at full speed.
This is a typical Mustang...
This is a typical Mustang three-speed fan switch. It is logical to assume it is a variable resistor because it offers three positions, but it is actually an electrical traffic cop that routes power to a variable resistor in the heater box.
Three wires route power from...
Three wires route power from the fan switch to the resistor package and fan motor. Power passes through the fan switch to the motor, then to the appropriate resistor to ground.
This is the variable resistance...
This is the variable resistance package, located on the heater box to aid resistor cooling.
LED Sequential Turn Signals
By now, most of you have seen Mustang Project's groovy LED taillights and parking lights for classic Mustangs. These easy-to-install lighting upgrades command more attention because they're brighter. What's more, they never burn out. Also, because they incorporate solid, reliable electronics, they will last virtually forever. Opt for the sequencing LEDs in back and LED bulbs in front. For more information, contact www.mustangproject.com.
No Unimportant Grounds
Grounding terminals exist throughout your Mustang's electrical system. All of them are important to proper electrical-system function. For best results, always ensure each of them is tied to the body/chassis as shown. Too many of us overlook the engine-to-firewall ground lead. Don't forget this one. If you do, expect all kinds of ignition and charging system woes.
Don't Cut That Wire!
We have seen all kinds of wiring molestation through the years, none of it pretty or necessary. Do you know how many sources for power there are in a classic Mustang? At least four that we can think of, which means you need never cut a wire to get power. There are also two triplex plugs for accessories and instrument lighting. All you need are bullet connectors to get power.
This is the instrument-lighting...
This is the instrument-lighting triplex plug for auxiliary lighting. Look for the yellow plug for auxiliary power to run accessories.
Power is available at the...
Power is available at the ignition switch from '65-'67.
A Word or Two About Lighting
Automotive lighting has come a long way since 1965. Today's lighting technology places greater demands on an old Mustang's electrical system. Halogen and xenon headlamps require greater amounts of power, which places quite a load on headlight switches. This means shorter service life and the need for as much alternator as you can muster. The most alternator you can expect with externally-regulated systems is 65 amps. Go to a single-wire alternator and you can count on upwards of 150 amps.
Halogen taillight bulbs are much brighter, which is a great safety feature. However, Halogen lamps run much hotter, which mandates special taillight lenses and venting of your taillight dishes. LED taillights run cool as long as you opt for ones that are visible from a distance.
The fuse box is the first...
The fuse box is the first point of entry for most of the electrical-system power. Fuses handle light-duty electrical loads like the heater fan, instrument lights, some accessories, radio, courtesy lights, and more. Headlights and parking lamps are protected through a circuit breaker in the headlight switch.
When you order your electrical...
When you order your electrical system from Virginia Classic Mustang, don't forget this fuse kit, which includes every fuse required. It's a good idea to purchase one or two additional kits just in case.
Do your headlights cycle off...
Do your headlights cycle off and on when they have been on a while? If so, the circuit breaker is faulty and the switch needs to be replaced. Virginia Classic Mustang can help with everything you need to replace the switch.
We suggest using the best...
We suggest using the best headlights possible for nighttime safety and driving pleasure. Xenon lamps seem to be the best headlight out there now. Make sure they are properly aimed.
Headlight switches are multi-taskers...
Headlight switches are multi-taskers because they do many things, including dimming the instrument-panel lights through the variable resistor shown here. Turn it counterclockwise to the limit and it illuminates the courtesy lights. As its name implies, the variable resistor impedes the flow of electricity, dimming the lights.
Brake-light switches are found...
Brake-light switches are found behind the instrument panel in all Mustang applications since August of 1964. Prior to that, the brake-light switch was a pressure switch located on the master cylinder.
Tail and parking-lamp bulbs...
Tail and parking-lamp bulbs employ two filaments, a dim one for parking lights and a bright one for turn signals and braking. The shorter, thicker filament is the brighter one because there's more resistance. The taller, thinner filament is dimmer for parking lights. We see a few of these wired backwards, meaning bright for parking lamps, which leads people to believe you are braking.
Brighter bulbs are available...
Brighter bulbs are available for poorly lit classic-Mustang instrument panels. However, be cautious about bulb heat. While you're at it, replace the instrument-voltage limiter, also known as a voltage regulator. When instruments don't work or max out, the voltage limiter is faulty and must be replaced.
Whenever you replace the turn-signal...
Whenever you replace the turn-signal switch...
...don't forget to keep the...
...don't forget to keep the original turn-signal plug on the switch harness. Throw it away and you will have to find a replacement.
Detailing the engine compartment...
Detailing the engine compartment is the best time to replace all underhood wiring, which is exposed to extremes of heat, cold, moisture, and dryness. Play it safe and refresh all underhood wiring whenever you detail. Don't be afraid to protect it with flexible conduit while you're at it.
Virginia Classic Mustang has...
Virginia Classic Mustang has virtually everything you need for exterior lighting. This is the taillight harness and everything that goes with it. New taillight dishes, back-up lamps, and parking lamps make a big difference in safety and the quality of your restoration.