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 will as extensive damage to the wiring. This can be caused by an electrical component drawing to 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 that is for the body controls that is located under the dash. 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 a 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 electicity 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.
If one of the wires become 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 elecrical 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 that 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 the larger in size than the other wire's 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 is 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.
boyet on November 04, 2019:
I have a 2008 Nissan Altima that keep blows the 30 amp fuse
Zack on August 15, 2019:
I replaced the 5 amp instrument cluster fuse in my truck the gauge lights start working but only for about an hour then it blows. What could be causing it?
rasak bajito on June 24, 2019:
I like all this that is hear
David on March 12, 2019:
My alternator wire touch metal and now there is no like Electron is
working in my car and
john kemp on November 15, 2018:
battery going flat
Kevin Dolphin on August 14, 2018:
My aygo keeps blowing the 30 amp fuse when trying to start engine
Craig Graf on July 16, 2017:
I keep blowing fuse on me head and tail light, I can have on all day trying to find something wrong, but the minute I take it out on the road I blow it. It on Dune Buggy so the body is fiberglass and I can not find any hot wires
Pedro Velazquez on June 21, 2017:
Very good article. I am blowing fuses and have yet to figure out why but this article did put more in perspective. Can the author come down here to Puerto Rico and help me fix it. It has been giving us trouble for two months now and I have been a mechanic all of my life. But my thought is, if man made it I can fix it.
David Edwards on June 09, 2017:
My wipers keep on blowing after i put new fuses in they work once then they blow on a mk1 vauxhall zafira
Ryan on February 23, 2017:
The best explanation I've ever read after all others I have read
Ldrjr from Alabama on December 29, 2011:
Very good. The malibu- when you juice to the lines. I take it as you have a positve and negative voltage at the fan motor? If that's so then its the motor. Take a jump wire to the dark blue wire on high speed relay and ground it see if it energizes the 2 high speed relays. Fan cont2 should have a ground b6 it goes to the ground on the fan and ties to a strap that goes to top left frontvof radiatorIt has 3 fan relays. From the schematic neither fan will run without relay 2 low or high. Or if you want i can send you wiring schematic. Let me know
Isaac on October 29, 2011:
I have a question. My cooling fans won't turn on, I have a 2001 Chevy Malibu. I have replaced the fan motor, relays, cooling sensor, thermostat, and water pump on my car, the cooling fans wont' turn on. The temp. gauge is working, and once it gets up to right below the redline, I turn the motor off so the engine won't get too hot, but the fan still does not turn on. Radiator is fine, thermostate is working, coolant is flowing, but the fan won't turn on. I have the wires bundled together, so I can't check each line, but there juice in the lines to the motor. Any ideas?
Peter Enmore on June 08, 2010:
This is a great explanation with a diagram. Everyone make sure you listen to the directions and most importantly disconnect the components before running the diagnostic test!
meansdavid (author) from Gig Harbor Washington on June 06, 2008:
Thanks Bob for the comment. I liked your hub on cherry smoothie's, I'll have to try one.
Bob Ewing from New Brunswick on June 05, 2008:
I used to work on my own cars, some years back, this hub brought back some good memories as well as being helpful, thanks.