Lance Morgan
August 20, 2007

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.

A new headlight switch can cure a lot of ills, including nonworking instrument panel lights.

When you turn on a turn signal-let's say the left one-two things happen. First, the circuit for the left brake light is interrupted, and second, the circuit of that brake light is connected to the turn signal flasher and the flashing sequence begins. This is why when you're stopped with the left turn signal on, the left brake light flashes but the right brake light stays on. The nighttime dimmer taillights are unaffected.

Sometimes a brake light won't work because the switch has failed internally or has become weak. The weight of the turn signal lever can take it just past center, thus interrupting the brake light circuit but not quite starting the flashing sequence. It can be a real puzzler. In this case, the switch must be replaced.

Dash Lights Don't Work
The dash lights get their current from the headlight switch. Current flows from the battery to a headlight-switch contact; through a rheostat on a white, ceramic turn wheel; then to the bulbs. The rheostat is a small circular spring installed in the ceramic wheel that dims or brightens the dash lights as you turn the headlight-switch knob. The spring can get rusty or faulty and cause dead spots or not light the bulbs at all. In this case, the switch needs to be replaced.

In '65-'68 Mustangs, the instrument-cluster housing grounds to the car when it's installed, so keep that in mind when testing. Sometimes you need to attach a ground wire from the housing to the dash frame to perform a good test of the bulbs. The '69-'73 Mustang dash lights ground through the printed circuit board.

A bad constant voltage regulator, located on the back of the instrument cluster, will prevent the gauges from reading correctly or at all. It can be inspected with a test light.

Fuel, Temp, or Oil Gauges Don't Work
Your gauges are really very simple. Current flows from the ignition switch to a device called a constant voltage regulator, to the gauge, and then to a sending unit. The voltage regulator changes the 12-volt current to a pulsing lower-voltage signal and then sends it to the gauge. The sending unit of that gauge acts as the ground to complete the circuit. It increases or decreases the quality of the ground, and the gauge reads the average voltage of the circuit through the pulses. If a gauge isn't working and it's definitely a good gauge, it either isn't getting the pulse from the voltage regulator or the sending unit is faulty.

Check the voltage regulator with a test light by checking for 12 volts or a pulsing signal. If all is well, examine the sending unit by removing its boot terminal and testing it for the pulsing signal. If you have a pulsing signal, then your sending unit is bad. If you have no signal at all, then the gauge is at fault. You can test your gauge by grounding the sending unit's boot terminal directly to ground to quickly see if the gauge reads all the way. If it does, it's okay. Don't let it stay grounded for long or you could damage it.

Another problem is that gauges may not be perfectly centered in the cluster housing, allowing one or both of the posts to touch the edge of the opening. If the hot post touches, it can short the gauge and it won't read properly. If the sending-unit post touches, the gauge will jump to full when you turn the key. The gauges must be perfectly centered with the posts not touching the metal cluster housing.

A test light checks for a pulsing signal from the voltage regulator. If you get the pulse, the sending unit-in this case, for oil pressure-is probably bad.

Won't Start
It's frustrating to hop into your Mustang, turn the key, and nothing happens. There are a number of things that could be the problem, but nothing that can't be diagnosed and fixed.

If nothing is happening or there is a clicking sound from underhood, the first thing to do is get a test light, remove the boot terminal from the starter solenoid terminal marked "S"- usually a red-with-blue-stripe wire-and have someone turn the key while you test the terminal to see if the light turns on. If it doesn't, the start circuit from the ignition switch is at fault. In that case, either the ignition switch is bad or the circuit in that red and blue wire needs to be checked. If your Mustang has an automatic transmission, the neutral safety switch could also be bad.

If your Mustang won't start, check the condition of the battery terminals first. Many times cleaning the posts and cable ends will fix it.

If the test bulb does light, the problem is either the starter solenoid, the battery, the starter, or any of the connections. You'd be surprised how many times I've solved this problem by removing the battery cables to clean its posts and cable ends. Above all else, the battery cables and posts need to be clean and tight. The best tool for this is a post cleaner you can buy at any auto parts store. It has a wire brush for cleaning the inside of the cables and a circular wire brush to spin on top of the battery posts. Install the cables and make sure they're tight. Now try to start your Mustang.

If that doesn't cure the problem, check the battery. Measure the current from the positive side to the negative side with a voltometer. If the voltage is below 12V, the battery needs to be charged or replaced. With a charged battery, the car should start.

Battery Not Charging
Checking your charging system is an easy task. All you need are the tools to remove the battery cables and a voltmeter. The first thing I check when a battery isn't charging is the current draw. To do this, remove the positive cable from the battery, then connect a voltmeter with the red positive clip touching the battery positive post and the black negative clip touching the removed positive battery cable. The voltmeter should read 0 volts. If you get anything close to 12 volts, something is turned on or shorted, draining the battery as the car sits.

Next, make sure the battery is receiving a charge from the alternator. Start the car and connect the voltmeter with the red clip touching the positive battery post and the black clip touching the negative battery post. You should see from 13.5 to 15 volts. If it's less than 13 volts, the alternator or the regulator is at fault.

One thing to remember about the charging system: never remove battery cables from the battery posts while the engine is running. This can blow a diode in your alternator or cause other problems. Some people use this method to see if the alternator is working. Don't do this. If you get the 13.5 or so volts at the battery while the engine is running, the alternator and regulator are working.

Bad Connections
Bad connections can often cause the signs that indicate a component problem. Over time, the fuse panel can get rusty or the tabs that hold them fatigue and the fuses don't make good contact. If the holding tabs are rusty, they need to be wire-brushed, then treated with a cleaner such as WD-40. I've solved a number of problems by popping the fuses out of the holders, spraying the fuse panel, and reinstalling them. If the holding tabs don't hold the fuse tight, they can be gently bent closer together to hold better. When spraying the panel, I coat the backside as well. Be sure the battery is disconnected.

Every now and then I come across a fuse wrapped with a chewing gum wrapper or aluminum foil. This isn't a solution. If a fuse is blowing, there's a reason. Without the protection of the fuse, a wire or wires in that circuit can get too hot and melt, taking other wires with it. Find the problem and fix it or leave the fuse out until it's fixed.

Connectors themselves can become loose and not make good contact. Sometimes I take a thin knife blade and slightly bend the terminal to make it tighter on the component. Other times, I use a pair of needle-nose pliers to gently squeeze the terminals together.

Bad Grounds
This sort of goes along with bad connections. As I mentioned earlier, a circuit is the flow of current from the battery to a component, and then to ground. If all goes well up to the component, a bad ground can make the circuit incomplete. Some things ground through the wiring harnesses. The harnesses have ground wires and ring terminals strategically placed throughout the car. Some things, such as taillight-bulb sockets, ground to the metal where they install. Just make sure the surface is clean and free from rust or paint.

Fuse-panel tabs need to be clean and tight to make good contact with the fuses.

Flickering Dash Lights or Headlights
Other than faulty connections or a bad alternator, sometimes this can be caused by a low battery. If a battery isn't holding a charge, the system keeps trying to charge it. Check the battery first and you may find the weak point.

Sometimes the headlight switch can be the cause. It contains an internal breaker that can get too hot and fail, causing headlights to flicker or go out. The lighting system is the hardest draw, designed for the headlights of the day. Today's halogen headlights are brighter, pulling more power and stressing the system. You may want to consider installing relays on the headlight circuits.