The Short and Long of It

Thursday, December 8, 2016

Sometimes it takes a long time to fix a short. What is a short? I am about to give you the long and short of it.

 

It is not uncommon for a customer to complain about something not working in their vehicle. In  their description of the problem they may say that component, for example “power window”, has a short in it.

 

A “short” is an electrical term. Electrical energy is used throughout your automobile in many cases to make it easier to do something that would require a physical manipulation. Let us continue with the power window as an example.

 

Many of us old timers have had a vehicle with manual wind up windows. Yes, we used to wind a window up with a crank mounted on the door. In most vehicles now that winding action is performed by an electrical motor connected through a series of gears, etc to perform the opening and closing of our windows. The winding results from a simple push of a button.

 

That electric motor requires electricity. Electricity is a flow of electrons in a conductor (the wires), similar to water flowing through a pipe or hose. The flow of electrons (electrical energy) will be converted to mechanical energy (moving the window) in the electric motor. This flow of electrons is directed in a loop. The loop is from one side of the source (the positive battery terminal) and back (the negative battery terminal). The negative side of the battery is connected to the whole metal structure of the vehicle.

 

The rate at which the electrons flow is determined by the load. The load is determined by the resistance to electron flow. High resistance is a low load. Small resistance is a high load.

 

The electrical loop that operates a power window motor is designed to carry a limited flow of electrons. The limit of electron flow is controlled by a fuse. A short occurs when the electrical loop is essentially shortened. The electrons find an easier path from one side of the battery to the other.

 

The shortcut back to the battery usually occurs when the insulating material over the electrical wire (outside of the water pipe) is broken and the inner conductor (or wire ) makes contact with the metal frame of the vehicle. The electrons flow without having to pass through the resistive load (the motor in this case) through the metal structure of the vehicle.

 

The high electron flow is halted by the fuse blowing (it actually melts). That circuit is shorted. That window will not move.

 

Finding the short can be simple and require only a short investigation. On the other hand the location of the short can require a much more lengthy diagnostic process.

 

Your mechanic will likely look for the obvious causes first. Vehicle modifications like trailer wiring, extra lighting, alarm systems, stereos, and command starts where the installations were not as well engineered as your vehicle was in the first place are typical trouble spots.

 

Unfortunately modifications are not detailed in our service manuals. Your mechanic will have to figure out what all those extra wires do and where they go.

 

Another common cause of shorts is the result of accidental damage. Crumpled sheet metal sometimes pinches and cuts open wire insulation. The inside conductor ends up touching the body metal making shorter electron paths.

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