Jim Smart
April 25, 2006

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 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 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.

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.

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.