Ford Pick-Ups/Bronco 1976-1986 Repair Guide



For any electrical system to operate, it must make a complete circuit. This simply means that the power flow from the battery must make a complete circle. When an electrical component is operating, power flows from the battery to the component, passes through the component causing it to perform its function (lighting a light bulb, for example) and then returns to the battery through the ground of the circuit. This ground is usually (but not always) the metal part of the vehicle on which the electrical component is mounted.

Perhaps the easiest way to visualize this is to think of connecting a light bulb with two wires attached to it to your vehicle battery. The battery in your vehicle has two posts (negative and positive). If one of the two wires attached to the light bulb was attached to the negative post of the battery and the other wire was attached to the positive post of the battery, you would have a complete circuit. Current from the battery would flow out one post, through the wire attached to it and then to the light bulb, causing it to light. It would then leave the light bulb, travel through the other wire, and return to the other post of the battery.

The normal automotive circuit differs from this simple example in two ways. First, instead of having a return wire from the bulb to the battery, the light bulb returns the current to the battery through the chassis of the vehicle. Since the negative battery cable is attached to the chassis and the chassis is made of electrically conductive metal, the chassis of the vehicle can serve as a ground wire to complete the circuit. Secondly, most automotive circuits contain switches to turn components on and off as required.

There are many types of switches, but the most common simply serves to prevent the passage of current when it is turned off. Since the switch is a part of the circle necessary for a complete circuit, it operates to leave an opening in the circuit, and thus an incomplete or open circuit, when it is turned off.

Some electrical components which require a large amount of current to operate also have a relay in their circuit. Since these circuits carry a large amount of current, the thickness of the wire (gauge size) in the circuit is also greater. If this large wire were connected from the component to the control switch on the instrument panel, and then back to the component, a voltage drop would occur in the circuit. To prevent this potential drop in voltage, an electromagnetic switch (relay) is used. The large wires in the circuit are connected from the vehicle battery to one side of the relay, and from the opposite side of the relay to the component. The relay is normally open, preventing current from passing through the circuit. An additional, smaller, wire is connected from the relay to the control switch to the circuit. When the control switch is turned on, it completes the circuit. This closes the relay and allows current to flow from the battery to the component. The horn, headlight, and starter circuits are three which use relays.

You have probably noticed how the vehicle's instrument panel lights get brighter the faster you rev the engine. This happens because you alternator (which supplies the battery) puts out more current at speeds above idle. This is normal. However, it is possible for larger surges of current to pass through the electrical system of your car. If this surge of current were to reach an electrical component, it could burn the component out. To prevent this from happening, fuses are connected into the current supply wires of most of the major electrical systems of your vehicle. The fuse serves to head off the surge at the pass. When an electrical current of excessive power passes through the component's fuse, the fuse blows out and breaks the circuit, saving it from destruction.

The fuse also protects the component from damage if the power supply wire to the component is grounded before the current reaches the component.

There is another important rule to the complete circle circuit. Every complete circuit from a power source must include a component which is using the power from the power source. If you were to disconnect the light bulb (from the previous example of a light bulb being connected to the battery by two wires together (take our word for it-don't try it) the result would literally be shocking. A similar thing happens (on a smaller scale) when the power supply wire to a component or the electrical component itself becomes grounded before the normal ground connection for the circuit. To prevent damage to the system, the fuse for the circuit blows to interrupt the circuit, protecting the components from damage. Because grounding a wire from a power source makes a complete circuit, less the required component to use the power, this phenomenon is called a short circuit. The most common causes of short circuits are: the rubber insulation on a wire breaking or rubbing through to expose the current carrying core of the wire to a metal part of the vehicle, or a short switch.

Some electrical systems on the vehicle are protected by a circuit breaker which is, basically, a self-repairing fuse. The circuit breaker opens the circuit the same way a fuse does. However, when either the short is removed from the circuit or the surge subsides, the circuit breaker resets itself and does not have to be replaced as a fuse does.

The final protective device in the chassis electrical system is a fuse link. A fuse link is a wire that acts as a fuse. It is connected between the starter relay and the main wiring harness for the car. This connection is under the hood, very near a similar fuse link which protects all the chassis electrical components. It is the probable cause of trouble when none of the electrical components function, unless the battery is disconnected or dead.

Electrical problems generally fall into one of three areas:

  1. The component that is not functioning is not receiving current.
  3. The component itself is not functioning.
  5. The component is not properly grounded.

Problems that fall into the first category are by far the most complicated. It is the current supply system to the component which contains all the switches, relays, fuses, etc.

The electrical system can be checked with a test light and a jumper wire. A test light is a device that looks like a pointed screwdriver with a wire attached to it. It has a light bulb in its handle. A jumper wire is a piece of insulated wire with an alligator clip attached to each end.

If a light bulb is not working, you must follow a systematic plan to determine which of the three causes is the villain.

  1. Turn on the switch that controls the inoperable bulb.
  3. Disconnect the power supply wire from the bulb.
  5. Attach the ground wire on the test light to a good metal ground.
  7. Touch the probe end of the test light to the end of the power supply wire that was disconnected from the bulb. If the bulb is receiving current, the test light will go on.

If the bulb is one which works only when the ignition key is turned on (like the turn signal), make sure the key is turned on.

If the test light does not go on, then the problem is in the circuit between the battery and the bulb. As mentioned before, this includes all the switches, fuses, and relays in the system. The problem is an open circuit between the battery and the bulb. If the fuse is blown and, when replaced, immediately blows again, there is a short circuit in the system which must be located and repaired. If there is a switch in the system, bypass it with a jumper wire. This is done by connecting one end of the jumper wire to the power supply wire into the switch, and the other end of the jumper wire to the wire coming out of the switch. If the test light lights with the jumper wire installed, the switch or whatever was bypassed is defective.

Never substitute the jumper wire for the bulb, as the bulb is the component required to use the power from the power source.

  1. If the bulb in the test light goes on, then the current is getting to the bulb that is not working in the vehicle. This eliminates the first of the three possible causes. Connect the power supply wire and connect a jumper wire from the bulb to a good metal ground. Do this with the switch which controls the bulb turned on, and also the ignition switch turned on if it is required for the light to work. If the bulb works with the jumper wire installed, then it has a bad ground. This is usually caused by the metal area on which the bulb mounts to the car being coated with some type of foreign matter or rust.
  3. If neither test located the source of the trouble, then the light bulb itself is defective.

The above test procedure can be applied to any of the components of the chassis electrical system by substituting the component that is not working for the light bulb. Remember that for any electrical system to work, all connections must be clean and tight.

At the rate which both import and domestic manufacturers are incorporating electronic control systems into their production lines, it won't be long before every new vehicle is equipped with one or more on-board computer, like the EEC-IV unit installed on the truck. These electronic components (with no moving parts) should theoretically last the life of the vehicle, provided nothing external happens to damage the circuits or memory chips.

While it is true that electronic components should never wear out, in the real world malfunctions do occur. It is also true that any computer-based system is extremely sensitive to electrical voltages and cannot tolerate careless or haphazard testing or service procedures. An inexperienced individual can literally do major damage looking for a minor problem by using the wrong kind of test equipment or connecting test leads or connectors with the ignition switch ON. When selecting test equipment, make sure the manufacturers instructions state that the tester is compatible with whatever type of electronic control system is being serviced. Read all instructions carefully and double check all test points before installing probes or making any test connections.

The following section outlines basic diagnosis techniques for dealing with computerized automotive control systems. Along with a general explanation of the various types of test equipment available to aid in servicing modern electronic automotive systems, basic repair techniques for wiring harnesses and connectors is given. Read the basic information before attempting any repairs or testing on any computerized system, to provide the background of information necessary to avoid the most common and obvious mistakes that can cost both time and money. Although the replacement and testing procedures are simple in themselves, the systems are not, and unless one has a thorough understanding of all components and their function within a particular computerized control system, the logical test sequence these systems demand cannot be followed. Minor malfunctions can make a big difference, so it is important to know how each component affects the operation of the overall electronic system to find the ultimate cause of a problem without replacing good components unnecessarily. It is not enough to use the correct test equipment; the test equipment must be used correctly.