Editor's note: Lance Morgan specializes in electrical diagnosis and repair for '65-'73 Mustangs at his shop in Edmond, Oklahoma. With more than 30 years of experience, we figured he was the right guy to tell us about the problems he sees again and again. Lance operates the Mustang Grabber Registry Web site at www.1970mgr.org as well. The Web site also includes electrical tips, such as installing headlight relays.
Few endeavors can intimidate or frighten '65-'73 Mustang owners more than diagnosing an electrical problem. To most, Mustang wiring and harnesses are complex mysteries. In actuality, the vintage Mustang's wiring system is an easy area to diagnose and repair.
The electrical arrangement of your Mustang is its nervous system. Each component, bulb, and switch receives an electrical charge, then sends that charge to ground. Each wire's function is to make an electrical component do its job. Some components perform several jobs at once, so they may have more than one wire. It still all goes back to the idea that current flows from the battery to a component, and then to ground, which is a flow of current called a circuit. Each circuit in the system is designed to operate something, whether it's a horn, a brake light, a gauge, or the radio. If a circuit is interrupted or goes the wrong way, then those components won't work properly.
I've worked on '65-'73 Mustang electrical systems since the late '70s. Some problems seem to be common to all Mustangs, especially after the passage of time. Nothing lasts forever, of course, but if the Mustang wiring isn't abused or exposed to the elements, it will last for years.
Ford's wiring system is a pleasure to work with and very efficient in design. The company used simple techniques and color codes to easily follow how the system functions. The wiring schematics, available from Mustang parts vendors, are also easy to follow once you get used to reading them.
The wiring harnesses are about as modular as you can get. Each section plugs into the others to handle all electrical functions. Rarely does a section need to be replaced. If it does, installing a new one is a plug-and-play situation. New replacement harnesses are the only way I recommend rewiring a stock or mostly stock '65-'73 Mustang.
The most common problems typically involve component failures from age and use. Here, we cover some of the most common problems I encounter with '65-'73 Mustangs. Each year has its own distinctive issues, but this should cover most of them. It's also an excellent idea to purchase factory manuals and wiring schematics for your Mustang. Always be sure your wiring is safely wrapped and protected from the elements.
The best tools you can have when diagnosing or repairing electrical problems are patience, confidence, and intuitive thinking. Remember, whatever the problem, it's usually something very simple.
Turn Signal Switch
The turn signal switch is by far the most common source of problems. Most take it for granted, but it does a lot more than you think. The switch is important for horns, turn signals, brake lights, and flashers.
The function of the horns is performed by a pair of small spring-loaded brass pins, or shafts, that contact the inner and outer ring of the horn contacts on the backside of the steering wheel. They complete the circuit to the horns when you press the ring. At the ends of the pins are larger-diameter heads that hold the springs in place, keeping them in contact with the horn rings. One pin is hot all the time, which is why you can honk the horn whether or not the ignition is on. Over time, the heads wear down, and the springs no longer hold the pins. That's when it's time to change the switch. It doesn't hurt to put a thin film of white lithium grease on the horn contact rings.
The function of the turn signals is integral with the function of the brake lights. When the turn signal switch is in its center or neutral position, the circuit for both brake lights is complete; when you press the brake pedal, both brake lights will light. This is also important for the flashers.