Anatomy of a Short
and how to repair it using a test light.
by Mr. Tidy
You turn your ignition key on and your front running lamps suddenly extinguish and remain
dark. You try your turn signals and have no lights still. After pulling off the left hand side cover
you find that the "Signal Fuse" is blown.  You put a new fuse in and the same situation occurs.

          How does a simple electrical lighting circuit work ?

          Why does the fuse keep blowing ?

          How can I fix my lights without wasting a dozen new fuses ?

I'll try and answer these questions and give you a procedure for using a "test lamp" to troubleshoot
a short circuit in your bike wiring.
 

Before we even start let me say, "if a short suddenly occurs after doing a modification to your bike,
what did you just finish doing ?". More often than not, it's usually something we did that created
the problem.  Always backtrack as to what you did last, as a logical first step.


 
     Below is a simple circuit consisting of a battery, switch, fuse, and red lamp.
With the switch in the open position there is no current flowing through the circuit.
With the closing of the switch , current flows through the switch, fuse, and through the lamp and
ultimately the higher resistance filament starts to glow and produces a light, the current proceeds
back to the battery.

    We have now seen how a simple basic lighting circuit works.


 
Now lets see what happens when this simple circuit develops a short in the wiring.


You will notice now that the wire between the fuse and the lamp is touching the bottom wire that runs
from the battery to the lamp. 
     "Current flows through the path of the least resistance" 
As the switch closes, current flows through the switch through the fuse then as it touches the
other wire, the path of least resistance is back to the battery itself.  The result is a direct short 
circuit across the battery, or an almost 0 ohms resistance allowing current draw to be maximum.
The weakest link in the circuit is the "fuse" after the rated amount of current is reached in the fuse
it melts and opens up as it is designed, thus protecting the wiring and other components of the circuit.

We now know why a fuse blows.

Lets build a test light:
 

We will use a simple homemade test instrument called a test light. 
It is 2 pin socket base that accepts a single filament bulb e.g.. #1156, 
attach 2 leads approx. 3' in length to the socket pigtails.
Attach 2 alligator clips to the leads so you can attach the test light to various components 
of the bike for testing.

OR
 

John Cosentini of Motorcycle Enhancements had a good idea to take a blown fuse and solder the wires onto a blown fuses spade lugs where they are exposed at the top of a fuse. 
  
 

Now, how do we proceed to correct the situation without buying several boxes of fuses before we find the problem:

In the following animation you will see that the initial image is without a fuse and the switch is open.
As the "Test Light" is applied across the fuse block terminals and the switch is closed the light
that is across the terminals of the fuse block will glow brightly.

Upon removing the wire that is touching the bottom wire the short is cleared, 
Notice that bright yellow light automatically dims and the tail light starts to glow at the same 
time, but is dim itself. The short has been cleared and the taillight is working.
But why are both bulbs dim?

What we have now is an extra lamp added to the circuit. in series with the taillight. 
In a series circuit, the voltage, is divided between the loads.

The lamps are 12 volts bulbs but, there is only have 6 volts across each bulb, 
this is why they are both dimmer than usual.

Did we actually fix the problem or is there one still present?

Yes :
As soon as we remove the test light and replace it with the correct fuse back in the circuit,
the resistance of the circuit goes back down and 12 volts is then again placed across the
tail light and it glows brightly again.

The reason for this explanation is so that you will realize that just because you cleared the
short in the circuit does not mean the test light will go out when the problem is fixed.
It is a common misconception and if you don't realize it, then you might assume that the problem
persist and keep on looking for a problem that doesn't exist.

Common wiring shorts are from pinched wires within a harness or through openings of
a engine case, component, or insulation has rubbed off and exposes a bare wire that touches the
frame or engine case. Retracing your steps after a bike modification or moving a wiring harness
around will sometimes quickly expose where a shorted wire exist and clear the problem.

Your test light can also be connected to ground or the frame of the bike to feeder wires to
the bikes various components to determine if you are getting power to said items.

This is how your shorted wiring was repaired using a test light without the needless
waste of blowing multiple fuses.