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REVISITING
ELECTRIC
AVENUE
BY SAM BELL
A haphazard electrical diagnosis can take
you down plenty of one-way and dead-end
streets. Learning and remembering a few
important concepts will greatly reduce the
number of detours you’ll encounter.
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Photos: Thinkstock
L
ast month I left you
in the middle of describing the Statue of
Liberty Play. If you
didn’t do the experiment, you may want
to do it now. After washing your
hands, take your DMM and walk
out to the vehicle. Set your DMM
to DC Volts and attach the black
lead to the COM (negative) port and
the red lead to the VΩ port. Clip
the red probe on the positive battery terminal and the black probe on
the negative. Your reading should be
about 12.6V. Now remove the red
probe from the positive battery terminal and lick your fingers on both
hands. With your left hand, touch
the positive terminal of the battery;
with your other hand, touch the red
probe and read your DMM. Most
techs are surprised to find their
DMM showing 12+V, as seen in the
left photo on page 30.
Wow! Is Lady Liberty’s tablet
really a battery in disguise? Could
that explain the light in her other
hand? The answer is no, not really.
To see why, we need to conduct
one more experiment. You’ll need
a 12V test light, or at least a 12V
bulb and couple of pieces of wire to
make things easier.
We’re going to set up exactly as
we did for the Statue of Liberty
Play. Take your DMM and walk
out to the vehicle. Set your DMM
to DC Volts and attach the black
lead to the COM (or negative) port
and the red lead to the VΩ port. As
before, clip the red probe on the
positive battery terminal and the
black probe on the negative. Your
reading should be about 12.6V.
Now remove the red probe from
the positive battery terminal and lick
your fingers on both hands. With
one hand, touch the positive terminal; with the other hand, touch the
red probe and read your DMM. This
time it should be no surprise that
your meter is reading somewhere in
the neighborhood of 12V.
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Revisiting Electric Avenue
Left: As “Lady Liberty” illustrates in the Statue of Liberty Play, full electrical voltage
can pass right through your body! The voltage shown, 12.64V, is the result of a basic
principle of electricity. Electrical potential, or voltage, passes through even major resistances undiminished until the circuit is subjected to a load. Put another way, there
is no voltage drop without current flow. Right: Once asked to perform some actual
work—i.e., once subjected to a load by the introduction of a test light—voltages
which looked just fine may drop precipitously—to 2.37V, as shown here.
But what does that mean? Can
you use that voltage to do some useful work? Clip your test light lead
to the battery negative (where the
DMM COM lead
is) and pick up
the probe end of
your light with
the same hand
that holds the red
lead from your
meter. What has
changed?
Suddenly, our
“hand voltage”
has dropped to
just a couple of
volts, as seen in
the right photo
above. And yet
our test light is not lit. How do we
reconcile this with the readings we
had before?
Once again, there’s no voltage drop
when there’s no current flow. But
wait! There is a voltage drop; we just
saw it drop. Does that mean there is
scale. Leave the COM (black lead)
attached to battery negative, as before. Attach the red lead to the test
light probe tip. As before, grab the
red lead with one hand while your
other hand squeezes the battery
positive post. Read the meter. You
should see something along the
lines of a few millivolts. That reading is the amount of voltage being
dropped through (by) the test light.
All the rest of the battery’s 12V or
so are being dropped through your
body. By the time the positive flavor of current reaches the red lead,
there’s simply not enough of it left
to illuminate the test light.
Bottom line: Voltage tests of an
open circuit may be misleading.
Proper circuit analysis requires
knowing where and under what
conditions any voltage readings
have been taken. Tip: Voltage drop
readings are always taken under
load, and are always taken on the
same side (positive or negative) of
the circuit.
Is Resistance Futile?
Thus far, although we’ve talked a little about resistance, we haven’t actually tried to measure it directly. Primarily, this is because resistance tests
via a DMM use
very small voltages and currents.
These voltages
are supplied by
the DMM’s internal battery.
Static resistance
readings obtained in this
way may fail to
indicate the true
state of affairs.
For example,
let’s suppose
that someone
has put on one of those universal
temporary emergency battery cable ends a few years ago. Inside
Voltage tests of an open circuit may
be misleading. Proper circuit analysis
requires knowing where and under
what conditions any voltage readings
have been taken.
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current flow? And if there’s current
flow, why isn’t the test light lit?
Turn your DMM to the mVdc
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the mass of corrosion that has
built up over time there are only
five strands of wire making resistance-free contact between the battery post and the auxiliary lead to
the underhood power distribution
center (PDC). If we do a resistance
test, we’ll see essentially zero ohms
on our meter, indicating that those
few strands are intact. But if we turn
on the headlights, the rear window
defogger and the high blower fan (all
fed via the PDC) and measure the
voltage drop from the battery post
itself to the PDC positive feed stud,
we may see a drop of several volts.
This is because the dynamic resistance of the circuit is altogether different from its static resistance. Dynamic resistance refers to the cause
of voltage drops due to planned
or unplanned resistance in actual
use, as opposed to static resistance,
which is measured in an isolated circuit under no-load conditions.
For some components—usually
sensors—static resistance measurements may be sufficient, but for most
components such measurements
merely confirm continuity. To judge
actual dynamic resistance, both voltage and current measurements are
required. In effect, this makes voltage drop testing the gold standard
for most troubleshooting purposes.
battery positive post. Connect the
black lead to the positive side of
the load connector, as close to the
load as you can get without piercing any wires. Make sure the load
is switched on. Record your results.
To test the ground side, connect
the black lead of your DMM (set
up for Vdc) to the battery negative
post. Connect the red lead to the
negative side of the load connector,
as close to the load as you can get
without piercing any wires. Make
sure the load is switched on. As you
See how you can
finish the job right
www.hunter.com/motor
Voltage Drops
We’ve all heard of voltage drop tests,
time and again, in class after class, yet
somehow most of us still find them
confusing. What are voltage drops,
and how do we do these darned tests?
Remember the Statue of Liberty Play? There’s no voltage drop
without current flow. That means
you do the test with the circuit
turned on. There are two sides to
every circuit—power and ground.
Proper circuit testing requires that
you examine each side. To test the
power side, connect the red lead of
your DMM (set up for Vdc) to the
Circle #16
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did before, record your results.
Now evaluate your results. In
a good circuit, both power and
ground voltage drops will be relatively small. For large power consumers, like a starter motor or
rear defroster grid, a good rule of
thumb is that voltage should drop
no more than .1V per connector
whether on the power or ground
side. For low-current electronic
circuits, voltage drops should be
closer to .01V per connector.
If the load does not operate but
your voltage drops are normal, substitute an appropriate dummy load.
(By substitute, I mean unplug the
original device and plug in your
dummy load at the original connector, using appropriate connector
pins and jumper wires as needed.)
What makes a substitute appropriate or inappropriate is the magnitude of its current draw. For example, you could use a high-beam
headlamp (around 4A) to substitute
for a small electric motor like a fuel
pump, while you’d want something
heftier, like a wiper motor, to judge
a starter circuit.
For most computer-controlled
circuits, dummy loads should be
limited to something under .1A,
like a #18 or #2174 lamp. (Computer-controlled ignition coils or injectors may draw a full amp or more.
Most other computer-controlled
circuits consume far less current.)
When in doubt, look at the fuse rating for the circuit you’re testing. You
don’t want or need to pop the fuse; if
you substitute a load that draws about
a third to half the fuse rating, you
should be okay for further testing.
Just because your test load now
“works”—the light shines, or the
motor turns—don’t jump to the
conclusion that the wiring is fine.
Repeat your voltage drop tests with
the dummy load on. (It’s hard to
judge whether a light is fully bright
or only 90% as bright as it should
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Fig. 1 Lady Liberty obligingly reveals her secrets. The voltage drops shown are proportional
to the resistances of the conductive paths connecting them. The total is equal to the available
battery voltage, while the individual drops on both the power and ground sides of the circuit
add up accordingly. Bottom line: Voltage drop measurements must be taken under load.
This photo was taken KOEO on a Toyota, but it could have been almost any vehicle. Each of
the two voltmeters has its black (COM) lead connected to the battery’s ground terminal. Each
positive lead is connected to a back-pin lead inserted into a different terminal of the injector
harness. Note that both sides of the harness show the same 12.05V. All we know so far is that
the injector has continuity and that, somewhere, the power is on.
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Revisiting Electric Avenue
Sometimes, installing a bypass around an inaccessible voltage drop (this one was deeply
buried beneath this Prius’ corroded main fuse panel) may be the most practical and economical solution. Be sure to document your bypass by attaching an indelible label and by
providing your customer with a modified schematic to keep in the glovebox.
Fig. 2 This schematic indicates the proper procedure for voltage drop testing. The load is
turned on and a voltmeter measures the voltage drop on each side of the load, both power and ground. High current consumers may tolerate drops on the order of .1V per connection, while electronic circuitry may be adversely affected by drops barely in excess of .01V
per connection. Field tolerances may vary. Checking known-good vehicles helps you gain
familiarity with what constitutes “normal.”
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be on the basis of sight alone. Your
voltage drop test will reveal the difference, if there is one.) If your
voltage drop readings are okay,
odds are the original device or
component has failed; replace it
with confidence. If the readings are
not normal, repair the circuit wiring as needed.
In a pinch, for testing purposes, you can usually bypass suspect
wiring by unplugging the affected component and independently
supplying it with both power and
ground. Polarity (which wire gets
power and which gets ground) is
important in many cases. Reversing
power and ground may damage or
destroy a perfectly good component, so make it a habit to maintain
correct polarity whenever you bypass the original wiring.
For bypass testing only, when in
doubt as to which wire is which, always consult the wiring diagram. If
no diagram is available, unplug the
load, then turn the circuit on and
measure voltage at each pin of the
harness (vehicle side) plug. Since
the circuit is now open, you should
expect to find voltage on one pin
and ground on the other. It’s usually not advisable to use this technique on components with more
than two wires, especially if you’re
relatively inexperienced in electrical troubleshooting.
Some sensor circuits may be engineered to operate at a reduced
voltage. This is usually a 5V reference, but may be a different value,
such as 7.0 or 9.0V. In these cases,
first unplug the sensor and verify the correct open-circuit voltage
supply level. You may also need to
check the circuit diagram and the
principles of operation. In some
cases, such as with temperature
sensors, it’s normal to see a drop of
several volts at operating temp.
My readings are too high! Now
what? Leave whichever test lead
December 2016
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1.67A
1.67A
ammeter
ammeter
1.67A
ammeter
(-) 1.5V (+)
0.0V
(-) 1.5V (+)
1.5V
3.0V
1.41A
1.41A
ammeter
ammeter
0.0V
1.41A
ammeter
(-) 1.25V (+)
0.0V
(-) 1.25V (+)
1.25V
2.5V
1.41A
1.41A
ammeter
ammeter
0.0V
1.41A
ammeter
(-) 1.5V (+)
0.5V
(-) 1.5V (+)
1.5V
3.0V
0.0V
Fig. 3 Top: Our famous flashlight has gone Napoleonic! Yep, it’s a “Blownapart.” Here we’ve
put an ammeter in series at various points in our flashlight. The reading of 1.67A would be the
same no matter where we took it, and is correct for a 3V, 5W lamp. Center: Here we’ve put
an ammeter in series at various points in the flashlight. The 1.41A reading would be the same
no matter where we took it, and reflects the same lamp but a reduced voltage supply due to
weak batteries. Above: Here we’ve again put an ammeter in series at various points in the
flashlight. The reading of 1.41A would be the same no matter where we took it, and reflects
the same lamp but a reduced voltage supply due to a corroded ground connection.
was connected to the battery where
it was when you found the excessive voltage drop. Move the probe
closest to the load to the other side
of the load connector (to the harness end of the plug) in the same
circuit. As always, use appropriate backprobe pins rather than risk
future damage from piercing the
wire’s insulation.
Is your reading now okay? If that's
the case, try disconnecting and reconnecting the connector to clean
off any corrosion. Repair as needed,
then retest, making sure that the circuit is turned on. If the reading did
not improve when you changed sides
at the last connector, consult your
wiring diagram to find the next one
closer to the battery. Take the same
readings on each side of the connector cavity. In some circumstances, it may be more practical for you
to test at more accessible connectors, remembering any you skipped
in case it becomes truly necessary
to test them. In any event, remember that the load must be switched
on throughout.
Still no joy? Keep working your
way back towards the battery,
whether you’re on the power side
or the ground side. When working
on the ground side of the circuit,
remember that while many devices
ground directly via their mounting hardware, others may plug into
harnesses featuring multiple connections in series, frequently including in-line splices which may
become problematic after years
of exposure to water, salt or high
humidity. In some cases, even the
chassis itself may have become
unusable as a ground path. Test,
don’t assume!
Sometimes it may become necessary to install a bypass around
a severe but inaccessible voltage
drop. Best practice here is to tag
your repair. You can (and should)
even print out the original circuit
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Revisiting Electric Avenue
schematic, draw in your modification and include it with the bill.
Ask your customer to keep it in the
glovebox for future reference. This
makes everyone’s life easier.
Measuring Current
KOER voltage levels in a vehicle with a good charging system are lowest at the alternator or generator case, highest at its positive terminal. The closer meter’s positive lead is
connected to the B+ terminal of the alternator, while its negative lead is connected to the
battery’s positive terminal. It reads 00.11V. The far meter has its positive lead connected
to the battery’s negative terminal and its negative lead connected to the alternator’s case.
It reads 00.003V. Note that both DMMs show positive values.
pulsating
14.4V
12.6V
direct
AC riding
on DC
square
5.0V
variable
0.0V
AC
Fig. 4 This drawing illustrates several types of voltages and currents. The magnitude of
the voltage is indicated by its distance from the 0V line as shown on the vertical axis on
the left side. Time flows from left to right on the horizontal axis. The red trace shows an
example of an alternating current with both the direction of flow and voltage changing
over time. The black trace, labeled variable, is an example of a typical sensor output,
such as a throttle position sensor or MAP sensor. The yellow trace, labeled direct, is typical of constant battery output. The blue trace, labeled pulsating, depicts typical alternator charging voltages, and is sometimes characterized as “rectified AC riding on DC.”
The dotted line intersecting the bottom of each blue hump indicates the DC voltage
level. The purple line depicts a superimposed three-channel scope trace of rectified AC
voltage “riding on” DC voltage—approximately 1V AC riding on 9V DC in this example.
The green line shows a typical DC square wave. Such square waves are what you would
expect to see used in sensor, control or communications circuits.
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So far, we’ve stuck mostly to measuring voltages. Let’s look briefly at
amps, or current. Standard DMMs
are set up to measure current directly, by being placed in-line with the
circuit being analyzed. Let’s look at
our familiar flashlight once again, but
this time we’ll have to disassemble it,
at least partially, in order to hook up
our DMM.
Here, the red lead is plugged into
the Amps DC socket. The black lead
remains plugged into the COM port.
The meter dial selects the Amps
function. Your meter is equipped
with a special internal fuse, usually
10 or 20A. Many technicians install
a lower amperage fuse in-line to protect the meter and its remarkably
expensive fuse. You can splice in an
in-line fuse holder from your local
supplier, or you can buy a commercial version such as Electronics Specialties’ Part No. 134.
The vast majority of blown meter fuses occur when technicians
hook up their test leads in parallel, like across the load, without
first moving the red lead back to its
voltage measuring port and without returning the meter selection
dial to voltage. This oops! moment
essentially uses the meter to short
circuit power to ground. If only
the internal fuse is damaged, count
yourself lucky for having bought a
well-engineered DMM.
With the flashlight on, we measure the current flow through the
meter from the left end (negative
post) of the left battery to the contact spring (Fig. 3 top on page 35).
We use a jumper wire to complete
the circuit from the base of the
spring back to the negative conduc-
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SERIES AND PARALLEL CIRCUITS
C
urrent limiting helps protect circuits from overload.
This is because the current
flow in any series circuit is
the same throughout the entire
circuit. You could think of it this
way: If all the cars on a two-lane
highway are traveling bumper to
bumper as fast as they can go,
then two more lanes are added
before the next interchange, the
earlier two-lane restriction continues to limit maximum possible traffic flow. At most, only the
same number of cars per minute
tor. Our reading, 1.67A, is correct
for a 5W bulb in a 3V circuit. (Of
course, if we turn the flashlight off,
there will be no current flow.) This
gives us a calculated resistance of
1.8 ohms for the bulb. (We have to
use a calculated value because a direct resistance measurement won’t
apply enough current to cause the
bulb’s filament to incandesce. Filament temperature is a major factor
affecting dynamic resistance of an
incandescent bulb.)
What amperage would we expect
to find if we measured the current
to the same bulb after the battery
voltage had dropped to 2.5V? If the
bulb’s dynamic resistance remains
constant at 1.8 ohms, our current
reading will drop proportionately to
1.39A, and the bulb would appear
noticeably dimmer. In the real world,
however, that dimness also signals a
cooler bulb filament. This, in turn,
reduces the actual dynamic resistance of the filament, just enough
that amperage may remain ever so
slightly higher than expected, like
the 1.41A shown in Fig. 3 center.
What amperage would we expect if we measured the current
flow in our corroded flashlight? We
start with the same battery voltage
of 3.0V. Our same 5W bulb has a
calculated resistance value of 1.8
ohms. But the 3V we start out with
can pass a given signpost by the
highway.
This is analogous to a current-limiting resistor. If our highway ends with an Exit Only multilane ramp, it was part of a series
circuit. But if only some of the
lanes must exit (and there are no
new entrance ramps), then our
roadway was part of a parallel
circuit. By definition, a parallel
circuit offers multiple travel paths
to get from power to ground via
different loads, while a series circuit offers only one path.
soon drops to 2.5V as the current
passes through the extra resistance
of the corrosion. Seems like we’re
losing one-sixth of the voltage before we get to the intended load.
That means current should be re-
duced proportionately from 1.67 to
1.41A (Fig. 3 bottom).
Although in this instance the corrosion is an unintended resistance,
it illustrates an important point—
namely that current limitation can
be achieved via resistance. This is
the reasoning behind current-limiting resistors such as those found in
many early electronic fuel injection
systems and in so-called pull-up or
pull-down sensor circuits, including
many Vref (reference voltage) sensor circuits. Additionally, current
limiting may help protect power
switching transistors from overload
(see “Series and Parallel Circuits”
above left).
This article can be found online
at www.motormagazine.com.
December 2016
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