I have 20+ years in the automotive industry, working as a technician and a mechanical breakdown analyst for insurance companies.
Diagnosing an Electrical Short
Diagnosing an electrical short can be very difficult and expensive. The actual repair usually consists of an inexpensive wire connector and some tape. The real expense is the time it takes to locate the problem.
The term "electrical short" refers to when a fuse blows because of an overload in the circuit. The purpose of the fuse is to protect the wiring and electrical components on its circuit. Without the fuse, if there was a short in the wiring the wiring would overheat and melt, and possibly cause a fire as well as extensive damage to the wiring. This can be caused by an electrical component drawing too much current or a wire that touches a ground.
The modern vehicle has several fuse boxes. One is usually designated for the engine controls and is located under the hood. Then there is also a fuse box under the dashboard that is for the body controls. A car sometimes will even have a third, depending on how many electrical devices it has. Each fuse will have an appropriate amp rating for the devices it is protecting.
In order to understand this, let's look at a simple cooling fan motor circuit. If you look at the diagram you will see the circuit consists of the battery, relay, temperature sensor, wire, and control, usually the engine control module. When the engineers designed this vehicle they calculated the amount of resistance in the wiring and the amount of current or volts of electricity the cooling motor will use when running, and using a fancy mathematical calculation they determined that this circuit is going to use about 11 amps of current or flow of electricity under normal operating conditions. Because of this, they have installed a 15 amp fuse in the engine fuse box in order to protect this circuit.
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If one of the wires becomes frayed and makes contact with the metal frame or sheet metal, or the coolant motor windings short internally, the circuit will surpass its current flow capacity (amps) and will blow the fuse. By this happening the wiring and related components will be protected. The question is what caused the short and how to find it?
To isolate the problem it is necessary to narrow down the possibilities. Looking at the circuit described and knowing when the fuse blew will help. If a new fuse is installed and it immediately blows this tells us that the short is somewhere between the fuse box and the relay. If the fuse doesn't blow until the cooling fan comes on, then the short is somewhere between the relay and the cooling fan. We know this because before the cooling fan is turned on by the relay there is no electrical current flowing past the relay. By determining this we have just cut our possibilities in half.
Now let's say that the fuse blows after the cooling motor is turned on. A simple way to determine if it is a wiring issue or if the blower motor is shorted is by using an inexpensive test light. Disconnect the fan motor, and find the main power feed wire from the relay to the cooling fan. The main power feed wire will almost always be the same color as the wire connected to the cooling fan, and it will usually be larger in size than the other wires in the connector. With the connector disconnected from the relay take your test light and connect it to the positive battery post. Touch it to a ground on the frame or an engine bolt and it should light up. Take the test light and touch it to the power feed wire in the connector that you disconnected from the relay. If the test light lights up you know that there is a short in the wire between the relay and the cooling fan relay.
Take a good visual examination of all the wires and connectors on the circuit. If a connector is loose-fitting it can generate heat and cause the fuse to blow. If there is a spot on the wire that is dark or swollen, this is a good indication that this is where the short is. Once the short is located, cut the faulty piece of wire out, and using quality connectors and shrink wrap make a wire repair. Make sure you route the wires in order to protect them from a recurring fault.
On more complicated circuits, a circuit diagram and component locator are essential. Almost all fuses have several different components that it feeds current to. Because of this, if a fuse is blowing, you again need to narrow down your search. You can do this by finding what is called a splice. This is where a group of wires are connected together and then each wire branches off to its separate components. All of these wires receive current from the same fuse. The splice must be disconnected; usually, this is done by removing the solder that binds it together. This will enable you to take each wire and test it as was done with the first example of the fan motor and relay. Once you know which wire is shorted you can use the wiring diagram you acquired to follow the wire and locate the short.
Even though a car has hundreds of feet of wire, and numerous components, by isolating the short it can be quickly located. A few things to remember is that the components must be disconnected in order to use this diagnostic technique. If they are left connected the test light will light up because the electrical current from the test light will flow through the component to a ground, giving you a false reading. Anytime you take on a complicated repair, think it through. It is always best to have a plan of attack. By having the proper tools and quality wire schematics you can systematically trace down the most difficult wiring problem.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.