Electrical Issues, Short or Open?

To the uninitiated, one electrical issue looks like the next.  'My lights quit working, can you fix it?' is a common question, or some variant thereof... I think its a short'.  I explain that I can't know until I check it over with test equipment, and ask the client if they did the basics and checked the fuses.  Usually that question gets a blank look for an answer...  

If you have never studied electricity, an automotive electrical system can look insanely complex with wires everywhere, switches in all sorts of places, various black boxes and connectors scattered everywhere, and to top it all off, nothing is labeled, which makes it even less friendly.  

All circuits in a car, motorcycle, RV or boat are all very similar.  You have a source of power- the battery or alternator or both, a fuse to protect the circuit, a switch, loads that are being powered (light, fan, etc...) and then the wires to connect it all up and conduct electricity from the source to the load and back.

All loads need two wires, one for the supply or negative side, one for the positive or return side.  Someone a long time ago screwed things up by saying that the positive side of a battery is the source of current, but that is nonsense and we need to kill that old confusing and useless paradigm.

Here comes the physics part of it... you have been warned.

Electrons form the outer, reactive shell of an atom of stuff, lets say metal in this case.  Those electrons, the source of the word electricity, carry a charge we arbitrarily call 'negative' while the inner portion of the atom carries a 'positive' charge.  It is these charges and the polarity difference that keeps atoms together, the positive and negative charges are pulled together by forces created by that polarity.  The weak electromagnetic force in physics is the direct result of these charges and  defines the interactions between those charged particles.

Extra electrons on the negative plates of a fully charged battery is what creates the negative charge on that terminal of the battery, and the lack of electrons is what creates the positive terminal.  Current physically flows from the negative side with excess electrons to the positive side with not enough electrons.  Voltage is considered to be the strength of the force created by the difference in the number of electrons stripped form the positive and dropped to the negative side during charging.  A discharged battery has the same number of electrons in both plates, therefore no voltage or current flow can happen.

A flow of electrons, bumped from one atom to the next on and on down the wire, is what we call electricity.  The voltage is the pressure driving the electrons down the wire, while current or number of amps is the actual measured number of electrons that are flowing in a given time past a given point.

In a standard circuit, the current flow is controlled by the resistance of the wires and load, and by the voltage pushing it down the line.  A fuse is rated for a specific flow of current through it, after which the wire inside melts and burns out stopping the flow of electrons through it.  If the resistance is too low in the circuit, the current goes up in direct proportion to the reduced resistance.  The same resistance can pass more current if the voltage is raised as well.  Either way, some safety device should be installed to prevent that over-current from damaging other components and wiring.

An open circuit is the simplest and most common form of failure.  It is also the hardest to track down.  An open circuit is simply one that no longer connects, thus current does not flow.  Somewhere in the circuit, something broke and no longer is connected and conducting.  It could be a switch that no longer switches, a burned out bulb or other component, or even as simple as an old fuse that finally blew from years of use near its limit.  Then there are the nightmare scenarios where there is a break in a wire somewhere in that massive bundle of wires and now we need to find it.  It could be broken in any of a thousand places along as much as 50 feet of wire in a bigger vehicle.  Like I said... nightmare scenario.  In these cases, I will often run a new wire with the old ones and power that load directly skipping the old wire entirely.

If two wires melt or rub through and touch metal to metal, the result is a connection that was not intended and usually results in over current.  This is a 'short circuit'  Literally, the circuit is too short to be healthy, it skips the load and has found a shorter path from one side of the battery back to the other side.  Hopefully the overloading of the circuit results in blowing a fuse, but if the circuit is one of the main power distribution wires, the fuse may be so high powered that it never blows, even while wires are smoking and catching fire.  

Replacing a blown fuse with one of a higher rating is always dangerous and stupid even for trained and experienced technicians running diagnostic tests.  Find the cause of the short, find it and fix it.  Then replace the fuse with the correct one and carry on.  Intentionally overloading a circuit by using a higher current fuse than the wire is designed for WILL result in a wiring fire.

A short circuit from improperly routed wires, or over-fused, under-wired circuits causing a fire is the biggest danger in self wired vehicles.  The car owner has the option to install stereo systems, additional lights and power sockets, inverters and appliances nowdays.  All these new and interesting electrical add-ons can be bought in the parts stores  or truckstops easily but have no decent instructions to educate the novice installer how to keep it safe.  

Helping your mechanic diagnose electrical problems starts by properly describing the problem.  If you can tell your mechanic that the windshield wipers have an open circuit and don't work, its very different from saying it has a short circuit in which the fuse blows every time and you smell burning plastic.  Correctly stating the problem is the first step to your mechanic not spending hours to determine what kind of fault is happening.  

Hope that clarifies a few things for ya... til next time...

MW outta here :-)