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Diagnosing open and short circuits

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In the last article I laid out the basics of electrical diagnosis. In this article I’ll provide a brief process that technicians can use to diagnose common open circuits, and less common short circuits. The diagnostic tool that will be used for both types of diagnosis is voltage drop.

Click HERE for a refresher on what voltage drop testing is.

In both cases, a good quality DVOM is required, as well as some diligence and the ability to read a wiring diagram. Remember also, the difference between short circuits and open circuits. Open circuits cause an interruption in the electrical circuit resulting in the electrical load (bulb, motor, computer, etc.) not to work, while short circuits cause too much current in a circuit resulting in a blown fuse (which is of course now an open circuit!).

Open Circuits

Open circuits are by far the most common of electrical faults. Open circuits are broken wires, open connectors, inoperable switches, etc. Anything that forms resistance so high that current can’t pass through it will become an open circuit. Broken wires can be caused by repetitive bending of wires such as in a door harness, rodents chewing through the harness, or significant chafing of the wire against another component. Switches can become gummed up with road grime, or corroded from water intrusion.

Often, technicians confuse open circuits with short circuits. Remember, an open circuit is one where the circuit has actually had current flow stopped by extremely high resistance. A short circuit is too much current flow due to very low resistance.

To test for open circuits, voltage drop testing is your best tool, but you’ll start with voltage available.

Using the wiring diagram, identify some easily accessible areas to measure for voltage available. With one probe on the wire in the circuit, place the second probe on a KGG (known good ground).

If you measure source voltage, that means that all of the circuit voltage is being dropped between your meter leads. Move the positive lead (the lead in the circuit) to another test location closer to the ground point while keeping the other lead on the KGG.

As you move down the circuit you will eventually find a spot where the meter will read less than 0.1 volts. When this happens you know that the voltage is being dropped somewhere between your last test point and ground.

Short Circuits

Although not as common as open circuits, short circuits do occur and can be troublesome to diagnose. A short circuit occurs when current flows directly to ground instead of through a load. Because a path directly to ground has much less resistance than a load, electricity will not only choose this path, but it will do so with increased current, usually resulting in a blown fuse.

A short circuit will blow a fuse, melt a fuseable link, or cause a circuit breaker to trip.

Diagnosing a short circuit involves replacing one of these circuit protection devices with something that can handle the excessive current flowing in the circuit. Many different forms of “short finders” are available on the commercial market. If you don’t own one of these you can make a useable tool by wiring a sealed beam headlight bulb with two leads and some male spade terminals.

Note: Power Distribution
Power distribution descriptions will vary from wiring diagram to wiring diagram. Some diagrams will provide a chart showing each fuse circuit down the left hand side, with each system circuit across the top. Where there is a solid black dot, the system circuit can be found on that fuse. Find the fuse that keeps blowing and move across the page from left to right noting each system circuit that you come across. Make a list and disconnect one system at a time.

Other wiring diagrams will only provide a list of system circuits by fuse, or an actual wiring diagram of the fuse box. If you only have a wiring diagram to go by, you will have to be diligent in making sure you don’t miss a system circuit connected to a particular fuse.

Back to finding a short circuit: Plug the headlight bulb into the fuse holder in place of the blown fuse. When you turn the circuit on, the headlight should light as it consumes the excess current. Next, identify what components are on the circuit by using the wiring diagram.

Once you have a list of components on that particular fuse circuit, begin to disconnect each component, one at a time, until the headlight goes out. The component you disconnected to make the headlight go out is the circuit you need to investigate.
To find the actual short, with the headlight bulb still in the circuit, and the component plugged in, disconnect the circuit at various locations until you find a location where the bulb does not go out.

You have now narrowed your trouble spot to somewhere between the last connector you disconnected (the one that DID NOT make the bulb go out) and the power source. Now a visual inspection or a continuity test will help you make a final diagnosis.

As always feel free to contact me with article ideas, questions, comments, or complaints! Don’t forget to subscribe to this column to be notified every time a new article gets posted!