©By: John T. Blair (WA4OHZ)
1133 Chatmoss Dr., Va. Beach, Va. 23464; (757) 495-8229
Originally written: circa 1994
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.
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).
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 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.
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?
If the light bulb tests bad, replace it and try the circuit again.
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.
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.
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.
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.
Enjoy your Morgan
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