Find That Short the Easy Way! 


    Nothing can be more frustrating or confusing than trying to find a short in you car's electrical system.  Especially when it's not enough to blow a fuse but still drains your battery!  Obviously, if you find a blown fuse, replace it, turn the key on, and the fuse blows immediately you've found the circuit within your car's electrical system that the short is in.  However, if the draw does not create enough current to blow a fuse, than we have to find out which circuit it's in to fix it.   Before I get started, my intention in this Tech Tip is not to teach Electrical Systems and Electronics.  Instead, I just hope that this article will give anybody a systematic approach to finding a short.  So lets get started...
First, lets make sure that you have a short!  Electrical systems have a specific route for electricity to travel along and any change from this route causes problems.  If electricity can find an easier route it will always take it.  So lets find out if the electricity in your car is finding a ground sooner than it's suppose to...  What we're going to do is take a test light and attach the clip end to the negative battery cable and put the tip on the negative battery post.   Then we're going to remove the

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negative battery cable making sure we don't touch the test light.  Now we haven't broken the electrical connection here.  All we're doing is letting the current, if any, flow out of the negative battery post, through our test light to see if it illuminates, and continue on along the negative battery cable to the short.  Word of caution, If you're attempting this on a computer controlled car it is very important not to break the connection of the test light to the negative battery post.

    By not breaking the connection, the computer will maintain its learned driving strategy in the Keep Alive Memory (KAM).  It will also ensure your computer will not have to go to factory baseline and relearn controlling your car's engine for maximum fuel economy and performance.  While we're on the subjects of computer control, module equipped cars have a small parasitic drain caused by the modules' KAM.  This is normal and module or modules can draw 10-21 milliamps (mA) with the key off.  However...   

    If you have a parasitic draw that is in the milliamp range and is beyond normal specs, you will have to use an Amp Probe for a Digital Volt Ohm Meter (DVOM) to diagnose this and the method below may not work due to the very low current draw.  But let's get back to those major shorts that give us all those headaches...

    Oh! by the way, a DVOM can be used in place of a test light.  Just ensure your leads are in the proper connections to measure Amps.   Using this tool will not only tell you if you have a current draw when you're not suppose to, but will also tell you how many Amps are being drawn.

    Okay, you've confirmed you have a short, now how do you find it?  The first and easiest test is disconnect the alternator and see if the test light goes out or the DVOM display changes.  Often old or worn out alternators ground themselves internally causing a short.  If it does you've found your short and repair or replace the the alternator or appropiate components.  If the test light remains lit or the amperage on the DVOM does not change, you have to try and start isolating which circuit within your car's wiring has the short.  This can be done by pulling fuses one at a time.   A fuse is nothing more than system protection.  All the current that flows through a circuit must go through the fuse.  By pulling a fuse we've created an open and current can no longer flow.  Continue doing this until the light goes out or the DVOM display changes showing no current draw.  Once you've found the circuit that has the short you must find where in that circuit the short is located.  It's always best to take the least intrusive method to diagnose.  Besides, who wants to tear out the dash or remove the interior just to trace a wire.  So this is where a simple tool you can make comes in very handy...

    When electricity flows through a wire it creates magnetism.  We're going to use this magnetism to find our short!  Many tool vendors and auto parts stores sell short detectors.  The one in my tool box is from MAC.  It's convenient for me because it also has an audible alarm which I can hear in a noisy shop environment.  I'm pretty sure NAPA sells one also.  The tool to the right consists of a 10 Amp circuit breaker with buzzer, wire leads, clips, and a compass.  But for our purposes we're going to make our own which I believe is better than the ones you can buy outright.

    To make our short detector, we'll need the following items:

  • 1 x 20 Amp circuit breaker
  • 2 x alligator clips
  • 2 x spade connectors
  • 2 x 2 foot pieces of 18ga wire
  • 4 x 1 foot  pieces of 18ga wire
  • 1 x magnetic compass of any type
    *modify length of wires to meet your needs*

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MAC's ET251 SHORT CIRCUIT FINDER.  Quickly locates short circuits in all 12v automotive electrical systems.  Eliminates the need to remove upholstery and floor coverings.  Operates through metal posts, panels, and other obstructions.   Simple hooking to affected fuse posts; induction-type needle indicator pinpoints shorts.  Packaged in high impact plastic box; schematic included

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    Connect the pieces together as shown.  This should allow you to hook your tool up in place of a fuse no matter what type of fuses your car uses.  The reason I recommend a 20 Amp circuit breaker instead of a 10 Amp one is it allows current to flow longer and thus building more magnetism.   This is important especially if the wire that the short is in is buried under items like the carpet, rocker cover, door panel, pillars, etc.  The more magnetism we can build the easier it will be to pick it up with our compass.

    At this point it helps to know how the wiring loom or wires run for the circuit that has the short.  A schematic is very helpful here especially if it has component locations.  If you cannot find one or your vehicle is older you will have to trace the wire(s) from the fuse on...but this is not that hard and you'll see why.  The reason we want to know which way the wire is going from the fuse box is because we're going to use the magnetism that is built buy the electricity along that wire to locate the short.  Place your tool into your fuse box or where you pulled the burnt fuse from instead of the fuse and turn the key on to the RUN position.  Now what's going to happen is electricity is going to flow to the short.  Because we have a circuit breaker in there instead of a fuse we're allowing magnetism to build along the wire.  As the current flows through the circuit breaker it starts to heat up (careful the tool may get very hot).  The bi-metal strip inside the circuit breaker will eventually bend away from the contact breaking the circuit and creating and open before any damage can be done.  This is always associated with an audible "CLICK" from the circuit breaker.  With an open, no current can flow through the circuit breaker and the shorted circuit.  With no current flowing, the bi-metal strips cools very quickly and reforms itself back to it's original shape and touches the contact allowing current to flow and once again creating magnetism.

    We're going to use this magnetism and our compass to locate the short.   Here is how the this works...  A wire (conductor) will build magnetism.   A short will not.  So we put our compass along the wire or wire loom and the current flowing through the wire is building magnetism.  This magnetism will draw the north seeking arrow on the compass towards it (one way or another).  We leave our compass in place until we hear the "CLICK" from the circuit breaker.  When the circuit breaker opens the magnetism will be lost and the north seeking arrow of the compass will deflect back towards the magnetic pole of the earth momentarily.  This deflection is important!   Because once we get to our short there will be very little or no deflection at all.   So after each click and deflection we move the compass about 6-12 inches along the wire, wait, and watch for the needle to deflect as the circuit breaker opens and closes.  We continue doing this until we come to a portion of the wire where the needle is not being drawn towards the wire (weak or no magnetism) and there's no deflection of the compass needle when the circuit breaker opens.   Congratulations! you've found the area where the short is located.  However, you still have not found the short, but at least you know where to dig in.  Expose that portion of the wire and locate the short in the wire.  Here's a hint - if there's a bunch of wires there, shorts create heat.  So look for the hot wire, hot metal, or hot component.  Then look for any wire or component that may be broken, cracked, smashed, exposed, or punctured.

    Now that you've found the short, repair it by splicing in a new piece of wire, fixing, or replacing the shorted component.  You can cut & solder, use a butt contector,  or electrical tape if you have nothing else.  At one time automobile manufactures only recommend solder, however, the quality of butt connectors are so good today that many allow this type of repair.  Now that you have the short out of your system verify your repair, replace the fuse if necessary, and pat yourself on the back for a job well done.

    That's all there's to it folks.  Go for it!

- 6NHeat