1133 Chatmoss Dr., Va. Beach, Va. 23464; (757) 495-8229
Originally written: circa 1994
Last update:
March 12, 2006 - Changed title, and email address
Before you can start to troubleshoot or fix a problem of any
kind, you must have a working knowledge of the item that is broken. This
applies to anything from fixing a washing machine to working on the
electrical system of a car. The following is a primer that will give the
reader the fundamentals for working on the electrical system of a car. I
will start with an introduction to electricity and use a simple lighting
circuit to describe how to troubleshoot any electrical circuit.
The Basics
To start, we need a simple understanding of basic electricity.
Electrons are the negatively charged particles that make up the outer
shells of an atom. Electrons travel in orbits around the atoms similar to
the planets around our sun. The only difference is that more than 1
electron is allowed in a given orbit. In materials that are good
conductors (most metals like copper, gold, silver) the electrons in the
outermost orbit can easily be knocked out of the orbit and travel to
another atom (similar to billiard balls getting knocked around on a pool
table). This movement of electrons is called current. The more
electrons that are moving, the more current flow there is. Current is the
measure of electrons flowing through a wire and is measured in
Amperes (which is symbolized by A). Its symbol in
mathematical equations is I. The best analogy for this is water
flowing in a pipe, with the water molecules representing the moving
electrons.
Voltage is the capability to push electrons through a wire and
is measured in Volts (which is symbolized by V. Its symbol
in mathematical equations is E. Voltage can be equated to water
pressure, in that the pressure pushes the water through a pipe. As the
pressure increases, the amount of water flowing will increase. The
voltage in most of the newer American cars (from the late 50's onward) is
supplied by a 12 volt battery while many of the older cars, use 6 volt
batteries. All batteries and generators provide Direct Current or
DC for short [the current only flows in one direction and the voltage is
constant (see figure 1)].
Household current and current supplied by alternators is called
Alternating Current, or AC for short [the current flows out, then in,
or back and forth}. Consequently, the voltage will build up to a maximum
then fall to a minimum, back to a maximum then to the minimum (see figure
2).
The nice thing about a car's electrical system is that there is not enough voltage to push the current through your body and hurt you, unlike working on the wiring in your house which is 120 volts. At 120V there is enough voltage (push) to electrocute you. However, be careful working on an automotive electrical system because if you were to touch a wrench or a screwdriver from any energized or hot electrical connection to any ground (i.e.. the chassis or engine block) there is enough current to act as an arc welder or burn up the wiring system.
Resistance is the opposition to current flow and is
measured in Ohms (
).
Its symbol in mathematical equations is R. To continue with the
water analogy, any bend, kink, nozzle or restriction in a water pipe can be
considered a resistor. A resistor that can exhibit either no resistance or
infinite resistance is called a switch (like a faucet at the sink or a
nozzle on the end of a garden hose). A piece of wire or a closed switch
should have no resistance (0 ohms). A broken wire or open switch will have
infinite resistance (
ohms).
For a DC circuit, there is a relationship between these three
entities as stated by Ohms' law: E = I * R. For those of you that have
forgotten your High School or College algebra, it requires knowing any two
if the values to calculate the other. Therefore, three equations can be
made from the one. Don't worry - This isn't a Physics class, there will
be no exam. Instead of having to memorize the three equations, here is a
magic pie;
Cover any item you want and the remaining 2 show how to calculate the desired item. For example; if you want to know the voltage, cover the E, the remaining symbols are I and R. This means to multiply the current (I) times the resistance (R) and the result is the voltage (V). Covering I gives E/R, voltage divided by resistance.
A simple electrical circuit, consisting of a battery, a piece of
wire connecting the battery to an on/off switch, an on/off switch, another
piece of wire connecting the switch to a light bulb (also known as the
load, and another piece of wire connecting the light bulb back to
the battery is shown in figure 3. Notice that the switch is shown in the
open position. Therefore, there is no complete circuit for the
electrons to flow through and the light bulb is not lit. This is called an
open circuit.
When the switch is closed, as in figure 4, the electrons can flow from the battery through the switch to the light bulb (thus lighting it) and back to the battery. The is called a complete circuit.
If any of the three wires in the complete circuit were cut, it would act as another open switch and no current would flow in the circuit. Again this would be an open circuit.
In most electrical systems, the amount of wire used to make the
complete circuit can be minimized as shown in figure 5. Remember that a
wire was made of a metal. Well, so is the chassis of the car. Instead of
running the last wire (called the ground or the return wire)
from the light bulb all the way back to the battery, the chassis can act
as the return wire. To do this, one side of the battery must also be
connected to the chassis. Most modern cars have the negative side of the
battery connected to the chassis, this is called a negative ground.
However, many cars, like most early (pre 1967) British cars had the
positive side of the battery connected to the chassis. This is called a
positive ground! Be sure to check which side of the battery is
connected to the chassis of your car. If you have a positive ground car,
it can be very dangerous to jump start another car, or install a radio.
You can burn up the electrical system!
The last item to cover in this introduction is series and parallel
circuits. In a series circuit, the items are placed one after the
other; with the hot or positive side of one item connected to the negative
side of the next as shown in figure 6. Batteries can be placed in series
to add the voltages of each battery such as in a flash light or a portable
radio. Four 1 1/2 volt batteries are placed in series to generate 6
volts. The loads can also be placed in series such as Christmas lights.
The problem with a series circuit is that if one of the loads burns out,
the entire circuit is out.
In a parallel circuit, items are placed side by side, all
the positive sides connected together and all the negative sides connected
together as shown in figure 7. If batteries are placed in parallel, their
voltages do not add, the current capabilities add, the batteries can
produce more current! This is used to extend the on time of an item. For
instance, if 1 battery can power a radio for 1 hour, then 4 batteries in
parallel should power the radio for about 4 hours. If the loads are
placed in parallel they will consume more current. If 1 battery can power
1 light bulb for 1 hour, it can only power 4 light bulbs in parallel for
1/4 of an hour (15 min). The advantage to this is that if one light bulb
burns out, the remaining will still light. This is the way a car is wired
(so is a house).
Test equipment
What type of tools do you need to work on an electrical system?
Aside from the usual wrenches and screwdrivers you will need a pair of
wire cutters (dikes), wire strippers (although a knife or dikes can be
used), crimpers, a soldering iron, and some method of testing for voltage.
Most mechanics use a test light. It looks like an ice pick with a little
light bulb in the handle with a wire coming out the handle. This is a
crude voltmeter and is very inexpensive. However, I prefer to use a
multimeter. These meters can be used to check voltage (either AC or DC),
current, and resistance.
A voltmeter is used to tell how much voltage is present at any given point in a circuit. (Safety tip - always start with the voltmeter set to its highest setting to prevent damaging the voltmeter.) Notice that in the minimized lighting circuit of figure 5, the switch is between the battery and light bulb (the device to be powered). In this circuit, we are opening and closing the voltage supplyor the hot wire.
To use a voltmeter to test this circuit, start with the switch in the open position (light off). Connect the voltmeter's negative lead (black lead) to ground (usually any metal part of a car which is connected to the negative side of the battery - for a negative ground car). Then the voltmeter's positive lead (red) can be connected to point A or B. The volt meter will read 12 volts (the value of the battery). If the positive lead were connected to either point C, D or E the voltmeter would read 0 volts. This is because the switch is not letting any current flow to the light bulb.
When the switch is closed the light bulb should light. Connecting
the voltmeter's positive lead to any of the points A, B, C and D, of
figure 8, would read 12 volt on the voltmeter. Since a wire has
practically no resistance, the voltage along a wire can be considered a
constant. Therefore, point A could be any place from the battery to the
connection on the switch (point B). Likewise, point C could be anywhere
from the other connection on the switch to the hot connection at the light
bulb (point D). Point E is said to be the ground or return path.
Therefore, all the voltage (pressure) would be dissipated across the light
bulb and the voltmeter would read 0 volts (assuming a good ground).
Using what you've learned
Now that we have an understanding of the basics, lets use a
multimeter to solve a problem:
Using the typical circuit of figure 8, if the switch is closed and
the light doesn't light, there are five possible causes: a bad battery, a
bad switch, a bad bulb, a bad ground, or a broken wire.
Checking the battery
To check the battery, the switch should be open (turn off the
light) to isolate the battery. The negative lead of the voltmeter is
connected to the negative battery terminal and the positive lead from the
voltmeter is connected to the positive battery terminal. The battery is
marked 12V and is so indicated in figure 8. The voltmeter should read
12V. (Note: a voltmeter draws very little current, therefore, a bad
battery could actually indicate 12V on a voltmeter.) Next, close the
switch (turning on the light). If the battery is bad, the voltmeter will
indicate less than 12 volts. The lower the voltage, the worse the battery
is. If the battery is bad, replace it and try the circuit again. The
light should light if the new battery is good (not always the case) and
there is no other problem in the circuit.
Checking the switch
To check the switch, the positive lead from the voltmeter should
be connected to the battery side or hot wire of the switch (point B). The
negative voltmeter lead is still connected to ground. The voltmeter
should read 12V. (If not, and the battery tested good above, the supply
wire is broken. More on this later.) Connect the positive lead from the
voltmeter to point C. With the switch in the open (off) position the
voltmeter will read 0V. When the switch is closed, the voltmeter should
read 12V. If not, the switch is bad. What kind of bad? Does anybody
really care? I do. If the voltmeter always reads 12V then the switch is
shorted out. In other words, the switch is always closed and can't open.
If the voltmeter always reads 0V then the switch is always opened, or the
switch never closes.
If the switch tests bad, replace it, and try it again.
The switch can also be tested using the ohmmeter portion of the
multimeter. This is also called a continuity test. Switch the meter
from the voltage scales to the resistance scale Rx0. (Note: on some
multimeters the red lead will have to be moved from the voltage (V) socket
to the ohm ( W ) socket.) All the wires to the switch must be
disconnected to prevent damaging the ohmmeter. With the switch out of the
circuit, attach one of the leads (it doesn't matter which one) to one of
the connections on the switch and other meter lead to the other connection
on the switch (points B & C). When the switch is in the OFF position, the
meter will read infinite resistance (the needle will not move). When the
switch is placed in the ON position, there will be a closed circuit or a
short, and since we already said that the wire has almost no
resistance, the meter will read 0 ohms (the needle will move to the other
end of the scale). If both of these conditions are met, the switch is
good. Otherwise, the switch is bad. Earlier I said there wouldn't be a
test. Fooled ya! If the ohmmeter always read infinite resistance, what is
wrong with the switch? What if it always reads 0 ohms?
Checking the light bulb
To test the bulb, the ohmmeter function of the multimeter must be
used. Set the multimeter to Rx0 scale.
(Warning: When using the ohmmeter
there cannot be any voltage present, the battery must be disconnected, or
the meter can be damaged. I know I'm repeating myself but this is very
important.) The easiest way to do that is to remove the light bulb from
the socket. If this is not practical, disconnect the battery. The
ohmmeter leads can be connected to the side of the light bulb and to the
connection at the bottom. If the element in the bulb is good it should
read less than 30 ohms. If it is burned out, it will read infinite
resistance.
If the light bulb tests bad, replace it and try the circuit again.
Checking the ground
The easiest way to test for a bad ground is to attach a wire to a
known good ground and to the ground side of the circuit in question. In
our simple example that would mean connecting another wire between the
negative side of the battery (point F) and the ground side of the light
bulb (point E). Now turn on the switch, does the light light? If so,
then one of the ground connections was bad or the ground wire was broken.
The ground can be checked with the voltmeter. Connect the voltmeter's red (positive) lead to point E. If the meter doesn't read 0V, it means that the ground is bad. There appears to be something else (a resistor) in the circuit (as shown in figure 8). A resistor is an electronic component that resists current flow. While there is an actual component for this, any electrical device such as a motor, a light, can electrically be considered a resistor or a load.
To test the ground with the ohmmeter, break the circuit by ensuring the switch is in the off position. The safest and smartest thing is to disconnect the battery. Connect the ground (black) lead of the ohmmeter to a known good ground. Then connect the positive (red) lead to the ground side of the light (point E). If the meter doesn't read 0 ohms, then the ground is bad.
A bad ground is a very common occurrence in a car's electrical system. The only problem is finding out where the ground is. Usually there is a short black wire coming from the load (the light bulb in our circuit) and bolting to either the chassis or some metal part (like a fender well or the firewall) that is mechanically connected (bolted) to the chassis. To fix a ground connection on a car, remove the bolt attaching it to the metal. Clean the connector on the end of the wire and the metal where it attaches with a piece of sandpaper. Both the connector and the metal should be shinny. Reattach the connector to the metal and check the circuit again.
One other note about bad grounds. For lighting circuits, a poor
ground will usually let the light bulb light although dimly. In other
automotive circuits, they can produce all kinds of erratic results, such
as when the right turn signal is turned on, the radio quits playing! Here
the two items, the right turn signal bulb and the radio share the same
ground. Due to the fact that current will take the least resistive path
to ground (back to the battery) the current flows to the light bulb and
not the radio.
Checking the wire
The ohmmeter is also used to check for a broken wire. Suppose the
wire from point C to point D is thought to be broken. To test it, place
one lead at point C and the other lead to point D. The meter should read
0 ohms. If not, the wire is broken and needs to be replaced.
In a Car
In a car the test just described can be more difficult as the wire
might start at one headlight, run across the engine compartment, back
along the fender well, and penetrate the firewall. The lead on the meter
will not reach that far. There are two options. Use a long piece of wire
to attach the remote end of the wire to the meter lead. This wire can be
any kind of wire, such as hookup wire, speaker wire, or an extension cord.
One of the ohmmeter leads would be attached to this long wire, and the
other lead attached to the near end of the wire. The second option is to
touch the remote end of the wire to a metal part that is attached to the
chassis and let the chassis act as the long wire. A short piece of wire
may be needed to reach between the chassis and the wire under question.
If the wire is intact, we have a short between the two ohmmeter leads.
Therefore, the meter should read very close to 0 ohms.
The heart of all this is the battery and the veins and arteries of a car is the wiring harness. The wiring harness can be though of as 3 separate harness', typically a front harness, a rear harness, and a middle harness that connects the rear harness to the main electrical supply or fuse block. Cars that have more auxiliary equipment such as power windows and seats will have additional harness to support the additional equipment.
To protect the wires from the heat, grease, and from getting nicked, the bundles of wires are usually wrapped with something. There are three usual coverings for the wiring harness depending on the type and age of the car.
- The first is like black electrical tape. Older cars used a
cloth tape while the newer cars use a plastic tape.
- The second covering is a cotton weave called loom (use in older cars -
especially British). This is actually spun or woven around the wires in
the harness.
- The third covering, which is used in the more modern cars, is the black, ribbed, split plastic tubing sometimes called conduit. It comes in various diameters. The more wires in a given bundle, the larger the diameter of the loom must be to cover the wires.
To follow a wire from one point in the car to another, the covering may have to be removed. The tape covering can be carefully cut with a knife, a pair of dikes or simply unwound. The plastic conduit or loom pulls off. Find the end of the conduit and look for the slit. Pull on the conduit from the side opposite to the silt and perpendicular to the wires. This conduit can be reused unless it is broken, or very dirty. The conduit is readily available at the auto parts stores.
When working with the electrical system on an older car, be sure
to check the insulation. This is the rubber covering, on a wire itself.
The insulation can be come brittle with age and especially from the heat
of the engine. Look for tiny cracks. In some cases it can be so bad that
the insulation actually falls off of the wire. If the wire has cracks in
the insulation, it should be replaced. Otherwise, the wire can short out
to another wire in the harness or some piece of metal in the car.
Anatomy of wire
The diameter of the wire is an indication of the
amount of current it carries. The larger the wire, the more current it can
carry. As an example of this, look at most of the wires, they are
relatively small. Now look at the main wires coming from the battery.
These wires are quite large as they must supply a lot of current when
trying to start the engine.
The thickness of the insulation is an indication of the voltage in the wire. Again, most of the wires in a car have very thin rubber insulation on them. However, if you look at a spark plug wire, you will see that the insulation is quite thick with respect to the size of the wire. This is because most of the wires, in a car, are for 12 volts, but the spark plug wires carry 20,000 volts or more. To size a wire, I suggest getting an inexpensive wire stripper. Most of these have different size holes in them for the different gauges of wire. Cut off about 1/2" of the insulation (to reveal the bare wire) and find the hole that the wire fits in. That is the gauge wire needed.
Also notice that most of the wire in a car is stranded. This
means that a wire is composed of many smaller wires. (There is also a
solid wire.) Automobiles use stranded wire because
stranded wire bends and withstands flexing much better than the solid wire.
As an example of this, take a lamp cord - be sure the lamp has been
unplugged from its socket. Try to wiggle it, bend it back an forth. It is
easily worked with no adverse results. Now take a coat hanger and cut off
the top hook. Straighten it out. Now try to wiggle or bend it. Once bent,
it holds that bend. If you bend it back and forth several times, the wire
will break. The solid wire can't withstand the constant working. To
replace a piece of wire in a car, be sure to use the correct size
(gauge) of wire and preferably the same color of insulation.
Graduation
There you have it. A short course in basic electricity and
troubleshooting a simple electrical light circuit. The main difference
between the drawing and a car's electrical system is that in the car,
there are more wires. Hopefully, this will helped you get a better
understanding of basic electrical troubleshooting and how to use a
multimeter.
Enjoy your Morgan
John
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