Nothing can be more frustrating or confusing than trying to find a
short in your 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 let’s get started...
First, let’s
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 let’s 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.
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By not breaking the connection, the computer will
maintain it’s 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.
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.
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 supposed 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 disconnecting 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 alternator or appropriate
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?
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 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. To make our short detector, we'll need the following items:
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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.
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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 its 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. 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 connector, 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.
I am one of the technical advisers on Just Answer
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