Jim Smart
February 18, 2015

Mustang electrical systems are viewed by a lot of us as bizarre black magic and something nearly impossible to understand. Most of us flip a switch or turn the key without giving much thought to how the result happens. However, if you’re restoring a classic Mustang or building a cool restomod from scratch, you should have a basic understanding of automotive electricity and a goal based on what your Mustang’s electrical needs are.

When you’re planning a Mustang project, be it stock or modified, you need to plan for the kind of electrical system demands expected. Stock restorations have modest electrical demands, which means you don’t need any more than a stock wiring harness. Virtually every classic Mustang electrical system item is available from Mustangs Plus. Your needs will include the main wiring loom, underhood harness, taillight harness, and any smaller harnesses, such as engine gauge feed, Rallye Pac, air conditioning, two-speed wiper/washer, and the like. Plan on replacing all electrical switches in the interest of safety and reliability. As long as your existing wiring’s insulation is flexible and not broken into the copper wire, it does not need to be replaced.

There are popular misconceptions when it comes to switches. Switches generally complete the path to ground—flip the switch, close contacts, and complete the circuit to ground back to the battery. Whether you are turning lights on, spinning a blower motor, or awakening a sound system, it is all basically the same approach. The same is true of instruments and warning lights, which carry the flow of electricity to ground via a sending unit (on/off or variable resistance).

Heater fan switches are not variable resistors. They are on/off multi-position switches that route current to ground through a variable resistor at the heater box. On low speed, you have high resistance; on medium, you have medium resistance to ground; and on high, you have minimum resistance to ground and full fan speed. Ditto for windshield wiper motors and other variable speed devices. Sometimes you are dealing with a multi-speed motor instead of a separate resistor package.

01 Plan out your Mustang’s electrical system before tearing everything apart. Few things are more frustrating than ordering the wrong harness and switches only to have to wait for returns and replacements. Unless you are doing a concours restoration, plan on using wiring protection such as Painless PowerBraid or ClassicBraid wire wrap.

02 If you’re restoring a stocker, the factory electrical system sports enough amperage capacity to support accessories and the like.

03 However, if you’re opting for high-power headlights, a subwoofer, navigation system, electronic fuel injection, and more, you’re going to need an external harness or a complete high-amp aftermarket electrical system. The stock main wiring loom is easy to install and connect. Follow the wiring schematics in your Ford Shop Manual and take pictures of the old system before removal.

04 This is the firewall to headlamp wiring loom, which connects to the main wiring loom underdash (‘67-’78) or at the firewall (‘65-’66). Great authenticity, by the way, with easy to identify and install connections.

05 The taillight wiring loom has a protective web covering much as it did from the factory.

06 Where it gets challenging is the taillight socket connections, which have to be swapped and crimped along with the fresh spring provided.

07 Mustang ignition switches with the center accessory power post (‘65-’67), which is live anytime the ignition switch is in the run or accessory position, presents an electrical hazard. If anything grounds against this post, it will short and start a fire.

08 There is no circuit protection if this happens. Always cover this post with an insulator cap. Any accessories you power off this post must have a fuse or circuit breaker.

09 In ‘68-’69, Ford went to a better multiplex ignition switch connector. One issue with these plugs is the female connectors within the plug that work loose, creating resistance and heat. When you plug yours in, make sure all the pins are secure.

10 This is a ‘68-’69 ignition switch and replacement connector. If you have to replace the connector, seriously consider soldering the wires for good continuity. If you’re unable to solder, use a high-quality wire crimper. Ford offered these, as did the aftermarket, which tell us these connectors and switches have been problematic.

11 This is the main power lead, alternator connection, and start circuit connections all located at the starter solenoid. The only issue you can expect here are “I” and “S” connectors that don’t always stay connected. You can go with eyelet connectors if these guys prove stubborn. Just remember to crimp the connections tight.

12 Headlight switches and connectors can prove problematic because individual connectors can and do come loose. When you connect the headlight switch, make sure all connectors are firmly secured to the headlight switch. Headlight switches fail when circuit breaker contacts become dirty and corroded inside the switch, causing headlights to cycle off and on. Dirty contacts create electrical resistance and heat, which pops the breaker, thrusting you into darkness. Replace the switch.

13 This is a three-speed fan switch for a ’68 Mustang with air conditioning. It is not a variable resistor but instead a multi-position switch. The device located in the upper righthand corner is the variable resistor, which is located in the heating and air-conditioning plenum where it is cooled because resistance generates heat. Resistance to ground determines fan speed.

14 Although there are plenty of reproduction and aftermarket wiring packages, there’s nothing quite like new old stock if you’re performing a concours restoration or want the fierce reliability of a factory wiring harness. This is one example of what Mustangs Etc. in Southern California has in its extensive new old stock inventory.

15 There are many types of Ford plugs and connectors used in classic Mustangs and other vintage Fords. This table enables you to understand what you have and what to look for if you have a damaged plug that cannot be salvaged. Mustangs Etc., as well as other stockers of new old stock, can help if you know the part number and configuration.

16 This one gets us in trouble more times than not. The pink resistor wire to your Mustang’s ignition coil is designed to work with the stock Autolite or Motorcraft ignition coil and points. If you are running an aftermarket ignition coil of any kind, ascertain what the required resistance is. PerTronix’s Ignitor module, as one example, is not happy if it gets its power via this pink resistor wire, which limits power. You must bypass this pink lead (behind the instrument cluster) and get power straight from the green and red lead. This is switched ignition power to the ignition coil.

17 PerTronix, always innovative in its approach to products people need, introduces the quick-change wire crimping tool (T-3001), available from Summit Racing Equipment for roughly $105.97. This tool gives you mechanical advantage over most crimpers because it locks down tight. What’s more, it offers interchangeable die sets that you can swap on the fly as you’re working.

18 Mustangs Plus stocks aftermarket electrical systems and accessories from American Autowire and Painless Performance. Which system you choose depends on needs and individual tastes. Each system has its advantages and both have served enthusiasts well over the years. The nice thing about each system is the abundance of circuits and circuit protection. What’s more, these systems plug into your factory switches and accessories.

19 Painless Performance, available from Mustangs Plus, offers you PowerBraid and ClassicBraid wiring protection wraps that improve appearance and protect wiring from chaffing. PowerBraid has a hard-core demeanor popular with racers and performance enthusiasts. ClassicBraid yields a more subtle look while keeping with a more stock appearance. You can wrap PowerBraid or ClassicBraid around your factory original wiring harness for protection and good looks.

20 These are factory-style battery and starter cables. Though Ford engineers believed they were enough, you need a heavier gauge cable to get the job done, especially if you have high demand or a stubborn engine that doesn’t like to start in hot or cold weather. You can do a lot of cranking and melt the insulation off stock battery cables.

21 Don’t kid yourself when selecting fuses and circuit breakers. Always follow a manufacturer’s recommendations for circuit protection. If the manufacturer suggests a 20-amp fuse or circuit breaker, follow that advice to the letter. If you’re protecting a vital system, such as night lighting, horns, or any other system involving vehicle safety, use a circuit breaker. Circuit breakers interrupt power momentarily, but they do not terminate power like a blown fuse will.

22 Those building restomods must reconsider the factory 35- to 65-amp alternator with external regulator. These will not keep a battery charged with demanding accessories. This is the Mr. Amp 100- to 130-amp alternator and circuit protector from Performance Distributors. It bypasses your Mustang’s factory regulator completely, making it acceptable to keep the regulator there for appearance purposes. And if you are married to the original alternator, opt for a solid-state voltage regulator.


Aftermarket Wiring Systems

If you’re building a restomod long on accessories and cool function, a stock electrical system isn’t going to be able to handle the extreme demands that go with today’s distracted driver rides. Sound systems, climate control, power accessories, high-intensity headlights, special effects lighting, and more will smoke a stock wiring harness quickly. Mustangs Plus offers two terrific aftermarket electrical systems from American Autowire and Painless Performance. Which system you choose depends on your needs and tastes. The beauty of an aftermarket electrical system is capacity—virtually limitless in its execution because the sky is the limit where you can add just about anything you want. You can even break your Mustang’s electrical system up into sub systems as necessary to simplify your overall electrical package.

When you’re planning out your Mustang’s electrical system, it is always wise to plan-in excess capacity. Not enough of us do that. We always perceive we have enough, then find ourselves empty-handed when it’s time to add accessories. While the factory battery cables look sharp and keep show judges happy, they do not offer enough capacity even with a stock electrical system. It is suggested you build-in capacity and go with heavier cables, including the starter cable, when you wire up. And if you’re going with a trunk mount battery, you want welding cable and generous grounding while you’re at it.