Jim Smart
December 22, 2006

Electricity has been doing our work for more than 100 years thanks to the efforts of inquiring minds who have paved the way for us. Thomas Edison, for example, understood that resistance to the flow of electricity created heat, and with enough current flow, light.In time, he learned if the filament were placed in a perfect vacuum void of oxygen, it would glow brightly without burning up. The electric light was born.

The electric light, however, is only a small part of what electricity does in our classic Fords. Electrical systems channel electricity to work for us. Think of your Ford's electrical system like an interstate highway system for busy electrons. In any electrical system, there are conductors, which carry electricity, and insulators, which don't. Insulators vary in resistance value, just like conductors. We want great resistance to the flow of electricity with an insulator just like we want low resistance in our conductor.

Although electrical systems are intimidating, they are really quite simple once you become familiar with them. Each wire is color coded for easy identification. Gang plugs exist for effortless service and component removal.

Copper and aluminum are typical conductors in automotive electrical systems. If extraordinary conductionof electricity is required, rare metals-like silver, gold, and platinum-make even better conductors than copper and aluminum. Copper and aluminum are the norm because they're more affordable and will work in just about any application. Automotive electrical systems use a combination of aluminum and copper depending on the application and function. Most of the time, you're going to see copper wiring.

The flow of electricity through copper or aluminum wiring is controlled by how much electricity you're trying to move and how much resistance there is to flow. Resistance creates heat regardless of how much electricity you're moving. When you flip on a heater or drop a piece of bread into the toaster, the heating elements in your toaster or heater create impedance or resistance to the flow of electricity, which makes them red-hot

Resistance is important in just about anything you do with an automotive electrical system. A wire that's too small for the load yields resistance and heat.

Every enthusiast should have a digital volt/ohmmeter as well as a test light to help determine power and resistance in electrical circuits. A voltmeter measures voltage-whether you have power or not-and how much. An ohmmeter measures resistance to the flow of electricity, measured in ohms. These are the two most important functions. A test light tells us at a glance if we have power.

What Is Electricity?
Electricity operates much like thermodynamics. Heat energy (mole-cules in motion) travels toward a colder medium (molecules in slower motion). An excess of electrons will flow toward an area having a shortage of electrons. This flow of electrons is known as electric current.

Electric motors, for example, function from the flow of electric current through windings. The electrification of these windings creates a magnetic field or fields. As you know from high-school physics, magnets attract or repel. That attraction or repulsion, through electrification, runs the rotor or armature in a circle to create rotary motion around an axis. When we energize an electric motor, we're creating a circular magnetic field where a motor virtually chases itself in a circle. The whirring we hear is the continuous attraction of magnetic fields, which runs the armature in a circle. A generator or alternator uses a similar idea to get electricity in motion via the rotary creation of magnetic fields. In this instance, we move electrons instead of them moving us.

So How Does It Work?
Electric current flow through conductors is how we make electricity work for us. We start and stop the flow of electric current through the use of switches or variable resistors. Switches are electric-current traffic cops. When we close a switch, we make contact and complete a circuit (green light), allowing the flow of electricity. When we open the circuit and break the contact (red light), the flow of electricity stops. A variable resistor, as its name implies, varies the flow of electric current. When we have a lot of resistance, we slow down the flow of electricity. When we reduce resistance, increasing current flow, we allow more current to flow. Variable resistors exist in many forms. A dimmer switch is a variable resistor-like we find with a headlight switch or radio volume control. We vary current flow to control brightness, motor speed, and a host of other things.

For electrical work, repair, and troubleshooting, you need cool tools of the trade: wire strippers, razor knife, electrical tape, butt connectors, plugs, and more. It's a good idea to have all kinds of connectors on hand for repairs and installation. Don't forget heat-shrink and zip-ties while you're at it. Good wire crimpers make all the difference in your connections. Take it a step further by soldering all of your connections once crimped.

Learning how to fix electrical problems is only a matter of knowing how electricity flows through a Ford's electrical system. There's no magic here, only the controlled routing of electrons. Electricity begins with the storage battery at the positive post.It flows through the system and back to the negative post, which is grounded to the car body and chassis, hence the term "negative ground." Your Ford's engine block, body, and chassis are an important part of the electrical system because they channel electricity back to the battery's negative post.

Power from the battery's positive terminal flows to either the fuse box or ignition switch first, depending on destination. Then, it flows to the switch, accessory, or lamp where it travels to negative ground. Power for lighting comes via a switch or variable resistor. Lighting circuit protection comes via a fuse or circuit breaker. Because headlights are high-current-draw safety devices, they are protected by a circuit breaker in the headlight switch instead of a fuse to keep the lights working in the event of a short circuit. The circuit breaker cycles headlights off and on so we can see the road and bring our Ford to a safe stop before getting help. Circuit breakers also cycle headlights off and on when switch contacts become corroded and resistance to the flow of electricity becomes high-creating heat at the breaker.

Automotive electrical-system wiring is color coded for easy identification. This makes short work of finding lead origins and destinations. When creating your own wiring, say for a stereo or electric-fan install, use multiple wire colors and draw a diagram. It will help in the future for repairs or wiring changes.

Trouble Shooting Electrical Problems
Although a lot of us are mystified by electrical-system woes, most are easy to troubleshoot and understand. Marvin McAfee at MCE Engines has a straightforward approach to trouble-shooting just about anything. He suggests starting with the most obvious stuff first. If the bulb won't light, is it burned out? Is there power to the socket? Check the bulb first. Check the fuse. Don't always assume the worst when something quits. Most of the time, it is something like a dead battery, a burned-out bulb, or a failed switch. Rarely are electrical system failures wiring related-most of the time, it's component failure.

When something electrical quits, determine what else is working and what isn't. If everything in the vehicle is dead, begin your troubleshooting at the battery. Never assume because you just came off the road or because the battery is brand-new that it is fully charged. Examine battery cables for corrosion or loose connections. Check the battery's negative ground at the engine block. Check the ground strap between the engine and firewall. This is all common sense stuff, but you'd be surprised how many miss it, including us.

Headlight switches and circuits are protected with a circuit breaker inside the switch. Circuit breakers are used for safety because they don't completely break the circuit should a short occur. They cycle the headlamps off and on. Also in the headlamp switch is a variable resistor to control instrument lighting.

Classic {{{Ford}}} electrical systems are all similar. Wiring color codes are virtually the same across all models and years. For example, instrument lights are blue with a red stripe on Mustangs, Galaxies, Falcons, Fairlanes, and F-series trucks. The same can be said for left and right turn-signal indicator lamps, high beams, taillights, backup lights, headlamps, heater-fan wiring, and the rest of it. All have the same color-coding regardless of carline type. This makes short work of troubleshooting when you know what it all means.

Ford electrical systems have two basic sources: power direct from the battery and switched-power from the ignition switch. The latter is live only with the ignition switch in the "on" or "acc" position. Items like brake lights, emergency flashers, horns, courtesy lights, headlights, instrument lights, and the cigarette lighter are live all the time, protected by fuses or circuit breakers. Turn signals, the heater fan, the radio, the transmission indicator light, the windshield wipers, and other accessories get power from the ignition switch, also protected by fuses. This is important to know when any of these items quits.

This is a variable resistor-a coiled stretch of resistance wire designed to increase impedance (resistance) as we dim the lights. The solid contact turns on the courtesy lights.

Any time electrical components quit operating, you need to determine if they are getting power to begin with. Check the fuse first. When in doubt, replace the fuse before doing anything else. Sometimes a fuse can be blown at either end of the filament and appear to be good. The same is true for light bulbs that don't look burned out. Sometimes there's only a tiny break in the filament not visible to the naked eye. Some offshore light bulbs can suffer meltdowns of the filament towers, causing internal bulb shorts and fried wiring. You can also check the fuse or the bulb's integrity with an ohmmeter. If there's no continuity through the fuse or bulb, it is bad.





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Here's the variable resistor, located in the heater plenum.

Controlling The Flow Of Power
Electrical component operation depends on how much power we route through the component to ground. It is easy to assume we control the flow of power before the component, but we can also control the flow of power to ground. Heater-fan speed is controlled that way. We control the flow of current to ground after the component instead of at the switch by having a two- or three-way switch that routes power to ground via a two- or three-wire resistor in the heater box. The resistor is in the heater box in order to run cooler, keeping heat away from the switch. This is also the case with in-dash air-conditioning systems.

When you flip on the fan switch, power is routed from the fan motor to ground via the switch. In other words, we already have power at the motor. Without a ground, the motor does not function. When we complete the ground, the motor functions. If we create resistance to the flow of electricity to ground, we control fan speed. The more resistance there is, the more slowly the fan operates. The less resistance the faster the blower wheel spins.

The fan switch routes power to ground through this variable resistor.

Heating and air conditioning fan switches are not variable resistors. They are contact switches that route electricity from the motor to a variable resistor to ground.

Air-conditioning systems function via a couple of traffic-cop switches.The compressor clutch is an electromagnet that engages the pulley and compressor shaft when it is energized. We energize and engage the clutch via the fan and thermostatic switches with under-dash air conditioning. With in-dash air conditioning, we have to jump three hurdles to get power to the clutch. This means if your compressor clutch isn't engaging, you need to check all of the switches.

With under-dash air conditioning, bypass switches one at a time to determine fault. If both switches check out good, test the compressor clutch by running a jumper lead from the positive terminal to the compressor-clutch lead. If the clutch doesn't engage, it's faulty. Don't forget to check grounds while you're at it. A faulty ground will stop clutch engagement as quickly as disconnecting the battery.

This is a typical three-speed fan switch. It is not a variable resistor as many believe. Resistance takes place at the variable resistor at the heater plenum.

In-dash air-conditioning compressor clutch engagement requires that three switches work properly-the fan switch, the thermostatic switch, and the vent-door switch. All must make contact in order to get power to the compressor clutch. On newer Fords, you must have a pressurized air-conditioning system (serviced with refrigerant) or the pressure switch won't allow operation either.

Reliability depends on the switch and how often it is used. Windshield-wiper switches are normally reliable. Headlight switches seem to have a lifespan of five to seven years in regular use. Some last five years-others last 40. Much depends on climate, conditions, and use. Headlight switches fail when circuit breakers cycle headlamps off and on. Corrosion gets the best of the contacts inside.

It is easy to improperly install an alternator wiring harness during replacement. Snap photographs of your original alternator before removing the harness. Checking your Ford Shop Manual is also a good idea. This is something you don't want to get wrong.

Charging And Starting System Problems
Likely the largest electrical system struggles for most of us are charging and starting systems. Defective voltage regulators-especially mechanical types, cause dead batteries and no-charge conditions. Mechanical regulators suffer from burned contacts, which hamper charging. Rarely will it be the generator or alternator.

Dead batteries call for a slow trickle charge, known as deep cycling, over an eight-hour period, then a load test. If the battery doesn't survive a load test after deep-cycle charging, it's defective. This can be tested at any reputable auto parts store. Because these folks sell a lot of batteries, alternators, and starters, they're equipped to check your battery. They are also set up to test alternators and generators.


Starter power begins at the battery and solenoid. Solenoids are nothing more than electromagnets-heavy-duty switches (relays) we energize with the ignition switch-that get power from the battery to the starter and ignition coil. When you turn the key to start, the "S" lead fires the solenoid, which makes contact between the battery and starter. At the same time, power passes through the "I" lead to the ignition coil. Solenoids fail for two basic reasons: They can stick in the "on" position and continue cranking our engines, or they just don't work at all due to internal failure. They cannot be serviced, only replaced.

Getting Started
Starters fail primarily because they work in a hot and dirty environment. Brushes become contaminated. Breaks develop in the copper windings. Extreme header heat, especially after shutdown, can cause a starter to seize up short term until things cool off. Starter drives can also fail, making it impossible for the starter to engage the flywheel or flexplate. Sometimes all your starter needs is clean-up and lubrication. White grease and WD-40 applied inside the Bendix starter drive makes all the difference. Clean slippers and brushes can also make an old starter like new again.

Starters get blamed for solenoid woes, too. It isn't always the starter. Check the solenoid for continuity by disconnecting the starter and checking for contact at the terminal with a test light. Listen for the "click" when someone turns the key to start and watch the test light. If the test light doesn't illuminate or you don't hear the click, further investigation is necessary. When solenoids don't respond, it isn't always the solenoid. Make sure the "S" lead is getting power to the solenoid from the ignition switch. Sometimes the ignition switch has burned and pitted contacts. The best solenoid function test is bridging the path between the battery terminal and the "S" terminal with a screwdriver, taking care not to ground the screwdriver (or better yet, a remote starter switch). If the solenoid clicks and your test light illuminates (starter terminal), the solenoid is good.

How Do Senders Work?
Instrument senders complete a circuit to ground. There are two basic types of senders: on/off and variable resistor. On/off types complete the circuit to ground to illuminate the light and get your attention. On/off oil-pressure senders complete the circuit to ground when oil pressure drops below a predetermined point. On/off coolant temperature senders complete the circuit to ground when coolant temperature gets too high.

Variable resistor senders vary the resistance to ground, which controls instrument reading. When there is no resistance to the flow of electrons to ground, the gauge maxes out. When there's a lot of resistance to the flow of electricity to ground, the instrument reads low. Instrument senders control the flow of electricity to ground to either illuminate a warning light or move an instrument needle.

The fuel-sending unit is another example of a variable resistor, which works off a float in the fuel tank. When the tank is empty, resistance is high and the needle rests at "Empty." When the tank is full, resistance to current flow is low and the needle races to "Full." A quick test for gauge function and wiring integrity is to ground the sender's wire lead. With the key on, the light on the dash will illuminate or the mechanical gauge will peg to the high side (full, hot, and so on).

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