Voltage
drop is one of the most neglected and least understood test procedures.
Ironically, it is also the most useful test procedure for locating
faults and potential faults.
Simply,
voltage drop is the measure of the voltage required to cause the
current flow through a component.
If
you have a voltmeter, take some time to do a little playing, which
will both make you more familiar with the use of the meter and
maybe prevent some future electrical problems. Set your meter
onto a scale, which is just above 12 volts DC, and prepare to
do some testing.
Refer
to your wiring diagram (either from shop manual or downloaded)
and locate the horn circuit. You should be able to trace backwards
from the horn to the horn switch, ignition switch to battery positive.
It's not easy to find and trace the wires but becomes easier with
practice. Now let's trace the circuit the easy way! Put the negative
(black) voltmeter lead onto the horn Brown wire terminal and connect
the positive (red) voltmeter lead onto the battery positive. What
does the meter read? Well, the meter "sees" the voltage
difference between the battery positive and negative because there
is no current flow so no work is being done and so there will
be no voltage drop between the two points being monitored. OK,
so now let's do some work! Switch the ignition on and hit the
horn....the voltage measured now becomes the voltage necessary
to make current flow through the resistance of the circuit between
the meter leads. In other words the voltage now displayed is the
voltage required to make the horn circuit current flow through
the battery + cable, wires, fuse, ignition switch, horn button
and up to (but not including) the horn. Record the voltage. This
is the voltage drop across the positive side of the horn circuit.
Now
move the negative (black) voltmeter lead to the ground side (black
wire) connector on the horn. Honk and measure the voltage drop
across the horn. This is the only useful voltage drop in the circuit
but more on that later.
Finally,
connect the voltmeter positive (red) lead to the ground side (black
wire) connector on the horn and the negative (black) voltmeter
lead to the ground side of the battery. Honk and measure the voltage
drop across the ground side of the horn circuit.
We
have three voltage drops; the Vd in the circuit from battery to
horn; Vd across the horn; and Vd in the circuit from horn to battery.
The total of the three voltage drops will equal the battery voltage
during the honk.
Now
let's consider the meanings of the three voltage drops...which
one is the Vd across the useful work being done? Of course! The
Vd across the horn is the one indicating the work we want to be
done so the others must be unwanted. Since we have (in round numbers)
12 volts available to push current through our horn, we want to
use the whole 12 volts across the horn to make the maximum current
flow through the horn but if any voltage is required to make current
flow in any other part of the circuit then that much less voltage
is available to make current flow through the horn.
So
what we want to do is to use the measure of voltage drop to find
unwanted voltage drops so that we can decide if they are a problem,
which needs to be removed.
**Some
Ohm's Law and Watts.**
The
horn is rated at 2.5 amps, 12 volts according to the wiring diagram.
What does this translate to?
Work
being done in an electrical circuit will usually be expressed
in Watts. 60-Watt bulb for example.
Watts
can be calculated Amps x Volts = Watts. Thus the 60 Watt /12 volt
bulb requires 5 amps while a 60 watt/120 volt bulb requires 0.5
amp. Different volts and amps but the same work being done, the
same watts.
We
will over simplify by disregarding the dynamic loads but the pattern
will be true.
The
horn 2.5 amps x 12 volts = 30 watts. OK, but what happens if we
don't see a Vd across the horn of 12 volts? Hmmm, here comes Ohm's
Law to the rescue! Ohm's Law tells us that one volt is required
to force one amp through one ohm of resistance. So the horn 12
volts divided by 2.5 amps = 4.8 ohms.
Let's
say that we have a Vd of 1 volt in the horn positive circuit.
That leaves 11 volts, which we will measure across the horn, right?
Right. Since the horn has 4.8 ohms of resistance with 11 volts,
11 volts divided by 4.8 ohms = 2.9 amps. How much work (honk)
will be done? 2.3 amps x 11 volts = 25.3 watts, not as loud! Not
as much work done.
To
turn the problem around, the voltage drop will show the work being
done. There should be next to no voltage drop where we want no
work to be done. Do we want work to be done to force current through
the horn button? No, we don't. That will simply result in heating
of the button.
What
will happen if we install a louder horn? Unless the horn is louder
because it is more efficient then it must be louder because it
does more work, has less resistance so more current flow results
in more watts of work at the same voltage. OK but if there is
more current flow through the same circuit, there will be more
voltage drop in that circuit because there is more current flow
through the same resistance.
Ok,
say we install the new horn and it isn't a loud as we hoped? (Is
it ever?) We can do our voltage drop measurements to see how much
Vd is occurring in the circuits. There should, ideally, be no
drop except across the horn. If we find a drop elsewhere we can
use the voltmeter to find it and eliminate it. Voltage drops can
be done across parts of the positive circuit such as across the
horn button itself. If we find more than (as a rule of thumb)
0.2 volts or more across any wire or component (other than the
one we want doing the work) we should find a way to eliminate
it.
One
easy way to eliminate the voltage drop across part of the horn
circuit will be to install a relay. The relay will use the horn
button and the other positive side wiring to operate the relay,
which requires a very small current, which results in a small
voltage drop. This means that full battery voltage will be available
to operate the relay. The relay will have low resistance (low
Vd) in the switched circuit so full battery voltage will be available
to the horn. This will result in a loud honk and long life for
the horn button.
Try
similar measures to the headlight, taillight, starter and other
circuits. Any Vd of 0.2 volts or more except across the component
intended to do the work can be located by moving the voltmeter
leads along the circuit in question until we have the Vd between
the leads. Now we can decide how to eliminate the drop.
Brighter
lights and longer component life can be had with a little voltmeter
work. The more you do, the easier you will find it to be. The
voltmeter is a window to what is happening in most circuits.
Norm |