Voltage drop testing is the best way to find electrical faults, at
least in my opinion. So what is voltage drop?
First of all a couple basic
things about electrical circuits. Circuit means circle, electricity must
complete a loop in order to have electron flow. If a switch is open, or
a break in the wire there is not a flow of electrons, and electron flow
is needed to perform work.
The pressure needed to
get the electrons to flow is voltage. A volt meter reads the difference
in pressure between the two leads, that is all it does. For example if
you connect your volt meter to a fully charged 12 volt battery the
difference in pressure between the positive and negative posts should be
about 12.66 volts. But if you touch the leads to each other there will
be no pressure difference between them so your reading will be 0 volts.
The same if you hooked up your leads to each end of an isolated wire, 0
volts. There will be a slight drop in voltage though, because perfect
conductors don't exist.
So if we have some
kind of restriction in our conductor, maybe a bad connection, a loose
crimp, corrosion, or some broken strands in our wire, these restrictions
will cause a drop in the pressure of the electrons flowing through the
wire. And we can measure this loss of pressure with our volt meter or
dmm. (digital multi meter)
But the circuit we're
working on has to be operating with current flowing in order to perform
voltage drop tests. And the voltage reading you get will be proportional
to how bad the connection is, or corrosion, or broken wires, or
whatever the problem is causing the unwanted drop in voltage.
When a load, say a light bulb
is in the circuit we should have source voltage coming into the load, in
this case 12.6 volts. The positive lead on the dmm would go to the
point of highest voltage in this simple circuit. That would be the
positive battery post, NOT the battery connector or clamp but the post
itself. Then place the negative lead on the dmm onto the positive wire
or input of the bulb. This will check the entire positive side of the
circuit from the battery to the bulb. There will be a slight drop in
voltage compared to our 12.66 across the posts. There is a very slight
voltage drop at every connection and switch in the path, and the longer
the wire, the more voltage drop there will be.
Say we had a bad switch on the positive side of our circuit, it would
drop some of our voltage, say 2 volts. Our bulb would still light but it
would be dimmer
than it should be. So you can work your way back to isolate the voltage
drop, You can even put the positive and negative leads on the in and out
terminals of the switch. If you have 12.5 volts or so going into the
switch and only 10.5 coming out, you have identified the problem, the
switch could be bad, or the connections could be loose or corroded. If
you test between the input and output sides of the switch, there should
be 0.0 or just slightly more difference in the reading. If it is much
more, the switch or connections are bad.
But you must always check the negative side of the circuit also,
it is just as important as the positive side.
When the circuit is working properly, the
load uses all the voltage. (Or almost all, there will still be a small
amount) So if our simple light bulb
circuit was working properly, we could check from the positive post of
the battery. (I say the post and not the connector or battery terminal
clamp because this will test the connection of the terminal to the post
as well) to the input or positive side of the bulb. Generally the
voltage drop should be 0.1 volt or less.
Then we need to check the negative side of the circuit
as well. Place the positive lead of the dmm on the negative terminal of
the bulb, and the negative lead to the negative battery post on the
battery. If the reading on the dmm is not 0 or 0.1 or very close to it
you have a problem on the negative side, possibly a poor connection or
corrosion is causing the unwanted resistance.
When testing I
like to run a ground wire directly back to the negative post of the
battery, this will eliminate any chance of grounding your dmm to a poor
ground and getting faulty readings.
Some people prefer to
disconnect wires and test using the ohm setting on their dmm, you can do
this but say you have a wire with corrosion on the inside of the
insulation, or all but a couple strands of wire are broken. You would
still get a good reading with an ohm meter continuity test, but the
circuit would not operate correctly under a load. Or if you take out a
dim bulb and check the voltage on the positive side of the bulb, you
will get a good reading of 12.6 volts on our circuit but it won't light
up nice and bright with the bulb in. But you would assume the wire to
the bulb socket
to be good when it is not.
corrosion is hidden inside the wire's insulation and you don't see it by
looking, this corrosion reduces the wires ability to carry the amount
of electron flow needed. Some guys poke holes in the wire insulation
when testing, I never do this, because that is just inviting corrosion
into the wire. But if you insist on doing this, at least seal the hole
you made with some of your wife's fingernail polish or something. Don't
use silicone though it creates ammonia which causes corrosion.
Again I think the voltage drop test is the best way to find electrical
problems. If I'm helping a friend or someone with his jeep or car even
if the problem is not electrical I do a couple basic tests, like
checking engine and body grounds. Connect the dmm between the negative
post of the battery and the engine , and with the fuel pump disabled
crank the engine a few turns and check the volt reading it should not be
more than 0.5 volts at the most, lower numbers are better. Then check
between the negative battery post and the firewall with the engine
running and all the electrical loads turned on, again the lower number
the better. If anyone is interested I will post some other tests to