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The purpose of the ignition system is to light the fuel/air mixture on fire at the right
time.
Three types of systems have
been used in modern times:
 | The Breaker Point System
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| The Electronic System
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| The Computerized System
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| The Distributorless System
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We
will discuss them all, but the one we will deal with in the greatest detail, is the
breaker point system. The way they create the high voltage spark is the same in all types
of systems, the only thing that differs is the way they are controlled.
All
ignition systems have two circuits;
The Primary Circuit
The primary circuit is the low voltage
circuit that controls the ignition system.
The primary circuit consists of:
| Battery - provides the power to run the system.
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| Ignition Switch - allows the driver to turn the system on and off.
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| Ballast Resistor - reduces battery voltage from 12 volts to 9 volts.
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| Points - a mechanical switch that acts as the triggering mechanism.
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| Condenser - protects the points from burning out.
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| Primary Coil - produces the magnetic field which creates the high voltage in the
secondary coil.
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| Wires - join all the components together.
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The Secondary Circuit
The secondary
circuit is the circuit which converts magnetic induction into high voltage electricity to
jump across the spark plug gap, firing the mixture at the right time.
The Secondary
Circuit consists of:
| Secondary Coil - the part of the coil that creates the high voltage electricity.
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| Coil Wire - a highly insulated wire, that takes the high voltage from the coil, to
the distributor cap.
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| Distributor Cap - a plastic cap which goes on top of the distributor, to hold the
high tension wires in the right order.
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| Rotor - spins around on the top of the distributor shaft, and distributes the spark
to the right spark plug.
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| Spark Plug Wires - another highly insulated wire that takes the high voltage from
the cap to the plugs.
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| Spark Plugs - take the electricity from the wires, and give it an air gap in the
combustion chamber to jump across, to light the mixture.
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Electrical Terms and Principles
Terms;
1. A Circuit is the continuous path that the electricity goes through. It must be
complete from the source, to the switch, to the load, and back to the source again.
2. Ground is the part of the circuit which is not wires, but a part of the car's metal
body. This is almost always the negative side of the battery.
3. Voltage is the electrical pressure that makes electrons move through a wire. High
voltage requires lots of insulation to prevent electrons leaking to ground. An example of
a high voltage circuit, is the secondary circuit. Voltage is measured in volts. An
ignition system can produce as much as 45,000 volts, a battery, 12 volts.
4. Current is the actual amount of electrons flowing. Large amounts of current require
lots of copper to travel through. An example of a circuit with a large amount of current,
is the cables from the battery to the starter. Current is measured in amperes, or amps for
short. A starter can draw 200 amps, an ignition system, less than 5 amps.
5. Resistance is the opposition to current flow, and is measured in ohms.
6. Magnetic Field can best be described by imaginary lines of force between one pole
of the magnet and the other.
Principles
1. When electricity flows through a wire, a magnetic field is built up around the
wire.
2. When a wire passes through magnetic lines of force, cutting them, a voltage is
induced in the wire.
3. Three things are needed to produce electricity:
1. Magnetic Field
2. Circuit - a path for
the electricity to go through.
3. Motion - either the
wire, or the magnetic field, has to move.
...So, How Does The Ignition System Work
Anyway?
Electrons, supplied by the battery when the engine is starting,
or by the alternator when the engine is running, are supplied to the primary circuit at about
12 volts electrical pressure. When the circuit is completed by turning on the ignition
switch, and the breaker points are closed, those electrons flow through the primary coil,
across the points to ground, and back to the battery again.
When electrons flow through a wire, a magnetic field is built up
around the wire. Make the wire into a coil, and the magnetic field increases by the number
of loops in the coil. This magnetic field takes a relatively long period of time to build
up. It isn't instantaneous. The time the coil is charging up is called coil
saturation, and is controlled by the amount of time the breaker points are
closed, or "dwell". the longer the points are closed for, the longer
the dwell, and the stronger the magnetic field becomes.
The coil is actually
named wrong. It shouldn't be called the coil. It should be called the "coilS".
The primary coil is the one that builds up the magnetic field. It has a few
hundred turns of relatively large wire in it.. The secondary coil has a few
thousand turns of small diameter wire in it because it is the one that will make
the high voltage, but low current, and fire the spark plugs.
So when the points
are closed and the ignition switch is turned on, a magnetic field is built up
around the coil. When the points are opened by the distributor cam, electrons
can no longer flow, so the magnetic field collapses toward the center of the
coil at the speed of light. When it collapses, it moves through the secondary
coil. Since the secondary coil has so many turns of wire, and the speed of
the magnetic field is so high, a great deal of voltage is induced into it.
Not all of the
electrical energy is actually used though. Voltage only builds up until there is
enough to ionize the air in the gap between the positive and ground electrodes
of the spark plug. When there is enough voltage the spark plug fires and
releases the energy to ground. It will always take between 5,000 (5KV) and
15,000 (15KV) volts to jump across the spark plug gap. If it takes more, there
is too much resistance in the plug circuit, or there is too wide a spark plug
gap. If it takes less than 5KV to fire the plug, there is a short, caused by a
shorted plug wire, too small a spark plug gap, or a fouled plug.
The high voltage
electricity produced in the secondary coil goes from the coil tower, through the
coil high tension wire to the distributor cap, from the center of the cap across
the rotor to the outer terminal of the cap, through the spark plug high tension
wire to the spark plug, across the plug gap to ground firing the mixture in the
combustion chamber. This all takes place at the speed of light.
The coil is actually
a transformer. It transforms a twelve volts or so, into as much as 45,000 volts.
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A breaker point ignition system is capable of producing between 20,000 and
30,000 volts of electrical pressure. There is very little actual current flow.
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| Electronic ignition systems were first used as standard equipment in
1975 because of the 50,000 mile emission durability test required by the
Environmental Protection Agency.
The problem with the old system which had been used for seventy five years,
was the points, which started to deteriorate after 1,000 miles, and were
totally worn out by 20,000 miles. An electronic ignition system uses a
transistor to turn on and off primary power. Transistors are electronic
switches that either work or don't, they don't just deteriorate in use.
Electronic systems are capable of producing up to 45,000 volts and much
higher amounts of current than the breaker point system.
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The spark will take place just before Top Dead Center on the compression
stroke.
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Parts of the Primary Circuit
Points
The points are not anything
mysterious. they are simply a mechanical switch that turns on and off the
ignition coil. The are opened by the distributor cam, and closed by the point
spring. When they are closed, the electricity flows from the battery to the
ignition switch on the steering column, to the positive side of the primary
coil, and across the points to ground. The only way the electricity can get to
ground is across the points, so when the points open, electrons can no longer
flow and the magnetic field around the coil collapses.
The points are the weak link in
an ignition system that includes them. After as little as 1000 miles they have
deteriorated significantly, and gone out of adjustment. By 20,000 miles the
engine is not likely to run at all.
The points must be replaced, and
adjusted at the time of a tune-up. They are set by adjusting "dwell, which
is the number of degrees of distributor cam rotation the points are closed for.
Dwell, coil saturation time, and cam angle are all the same thing.
 
As you can see in the diagram,
the closer the points are to being closed, the longer they stay closed for and
therefore the longer Dwell is. To adjust the points, simply hook up a dwell
meter to the coil. ( red lead to negative, black to engine ground) Crank the
engine with the distributor cap and rotor off, and adjust the fixed contact of
the points until the correct dwell reading is obtained.
If there is a range in
the dwell specification, adjust the points to the low end of the range because
dwell will always increase as the rubbing block wears down.
Condenser
The sole purpose of the condenser
is to protect the points, and keep them from burning out prematurely.
The collapsing magnetic field not
only collapses through the secondary coil, but also through the primary coil.
The collapsing field induces a few hundred volts in the primary coil. These
electrons have to go somewhere, they are just trying to get to ground by
the easiest means possible. If they were allowed to jump across the points, they
would burn them out in as little as 100miles. They see the condenser as an easy
way to ground but what it really does is store them for a fraction of a second.
Meanwhile the points have opened far enough that the 300 volts or so, can't jump
across them.
The condenser is just a little
can with a strip of tin foil and a strip of waxed paper, rolled together inside
a little can. A wire is attached to the roll of tin foil and the waxed paper is
there to separate one roll of the foil from the next one. The condenser is
merely a storage room for electrons.
Ballast Resistor (or resistor
wire)
The coil is designed to operate
on 9 volts. Battery voltage (12 volts) is reduced to
9 volts by the Ballast Resistor. When the ignition switch is in the
"run" position, the coil is powered through the Ballast Resistor
feeding it 9 volts; but when the ignition switch is turned to "start",
the Ballast Resistor gets by-passed. This feeds full battery voltage to the coil
for better starting. The starter motor is drawing battery voltage down to about
10 volts at this time.
Battery
Don't forget, without a good
battery, you don't have a good ignition system.
Primary Coil
The primary coil has a few
hundred turns of relatively large wire. Its positive side is connected to the
ballast resistor, and its negative side is connected to the distributor (or the
module in an electronic system). The primary coil is the one that builds the
magnetic field around it.
Parts of the Secondary Circuit
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All parts of the Secondary circuit are highly insulated to
prevent the high voltage electricity from escaping to ground.
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Secondary coil
The secondary coil has a few
thousand turns of hair fine wire. It is the coil that the magnetic field moves
through to produce the high voltage electricity. Because of the high number of
turns of wire, and because of the extremely high speed the magnetic field is
moving at, ( the speed of light ) extremely high voltage is produced, but
because the current flow is very small, the wire only needs to be small too. The
positive side of the secondary coil attaches to the positive primary wire, and
the negative side goes to the coil tower where the coil high tension wire plugs
in.
Rotor
The rotor spins around on the top
of the distributor shaft and distributes the spark from the center terminal of
the cap to each insert around the outside in the firing order. Is snaps onto, or
is held by screws, to the top of the distributor shaft, and only goes on one
way.
The rotor is made of
"Bakelite", a type of plastic. Bakelite differs from most plastics in
that is is capable of withstanding a fair amount of heat. It is, however, quite
brittle, but it does have high dialectic strength or resistance to current flow.
| During a tune-up, the rotor should be checked for worn electrodes,
cracks, and evidence of punctures. These are places where the electricity
has burned through the rotor to the distributor shaft.
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Distributor Cap
The distributor cap is
also made of bakelite. It has brass, copper, or aluminum inserts in it to
conduct the electricity to and from the rotor and the high tension wires. The
cap usually has ribs on the inside to prevent flashover between the terminals.
There is one insert in the center for the coil HT wire, and inserts around the
outside for the spark plug HT wires. The plug wires are pushed into these
terminals in the firing order.
Good quality caps have copper or
brass inserts and not aluminum. There is arcing between the cap and rotor and
that arcing causes oxidation of the inserts. Aluminum oxide is a very effective
abrasive and causes wear on the distributor shaft bearings.
| During the tune-up, the cap should be checked for any
wear on the inserts, and evidence of "carbon tracking". These are
places where the electricity has made another way to ground, or one of the
other terminals in the cap.
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| Both cap and rotor should be checked during a tune-up,
but they don't necessarily have to be replaced unless they show wear.
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High Tension Wires
The HT leads are
highly insulated to prevent the electricity taking a short circuit to ground.
There are usually one plug wire going to each spark plug, and one coil wire
going from the coil to the center of the cap, although on GM's High Energy
ignition system the coil wire has been eliminated by placing the coil directly
on the top of the cap.
Ht leads are usually carbon core;
very much like a little piece of string impregnated with graphite. There is very
rarely an actual conductor made of copper. The insulation makes up a large
percentage of the diameter of the wire, and is usually made of silicon in modern
wire sets.
| During the tune-up the wires should be checked for
evidence of burn through, deterioration of the wire, or boots, or any
abnormality.
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| The plug wires should be separated from each other, and
never bundled together. Bundling the wires causes cross-fire between the
plug leads and therefore the spark plugs.
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| The plug wires can be checked on an engine analyzer, or
oscilloscope for high or low firing lines. High firing lines indicate open
circuits caused by a broken wire or spark plug, or a wide spark plug gap.
Low firing lines indicate a short , caused by leakage to ground. This could
be a wire laying across an exhaust manifold or the cylinder head. If you
don't have an oscilloscope, HT leads can be checked with an ohm-meter. There
should be about a thousand ohms of resistance per foot of wire.
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| When pulling plug wires off the spark plugs, twist and
pull the boot, don't yank on the wire itself. This will cause the wire to
break inside and although it will still work right then, it will give
problems down the road as the wire burns back in both directions from the
break.
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| Wire sets don't necessarily have to be replaced during a
tune-up, but they should always be checked.
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Spark Plugs
The spark plugs are
the last remaining part of a modern ignition system that need servicing on a
regular basis.
The plugs must
have the correct "reach", or length of the threads, diameter, sealing
method, and heat range.
The plug on the left requires the use of a seal ring, or gasket to prevent
compression leakage past the threads. The plug on the right does not require a
gasket.
The spark plug must
run at the correct temperature. If the plug runs too hot, above 900 degrees Celsius,
it will glow red hot, and the fuel mixture will start on fire all by itself, not
when the plug fires. This is called pre-ignition, and must be avoided at all
costs. If the plug runs too cool, below 450 degrees Celsius, it will foul up
with crud because it never cleans itself. High performance and high compression
engines have a great deal of heat in the combustion chamber (remember, its the
heat that pushes the pistons down) and so don't require any
"artificial" heat created by the plug to keep it hot. High performance
engines use cold plugs. Low performance, low compression engines don't have a
great deal of heat in their combustion chambers and therefore need to keep the
plugs hot in another way. They use hot spark plugs.
Note the short heat path in the plug on the left. Remember, the hottest part of
the plug is the center electrode. The shorter the distance the heat has to
travel to the coolant, the cooler the plug runs. A hot spark plug has a longer insulator
nose.
Projected nose or
extended tip plugs
take the whole insulator and move it further out into the combustion chamber.
This moves the tip into the swirling gasses in the combustion chamber and the
tip keeps cleaner than a normal plug and prevents fouling.
A technician can tell a great deal about the engine he is
working on simply by "reading" the spark plugs
Worn plugs, like the one on the right in the drawing above,
should be replaced.
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The spark plug air gap must be set when the plugs are
installed.
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Spark plugs are not normally cleaned and re-gapped anymore.
Sand blasting the insulator gives it a rough finish and it fouls up easier
than when new.
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Timing
To give the maximum
cylinder pressure and therefore the maximum horsepower, burning of the gasses
must be finished by shortly after Top Dead Center. If the piston is allowed to
go too far down the cylinder, the combustion chamber volume will have become too
big, the pressure will drop and so will the power and economy.
In a breaker point ignition system,
point opening triggers when the spark happens, so by changing where the points
open in relation to the distributor cam, we change when the spark happens. The
same thing happens in an electronic system when the transistor turns the coil
off. Timing very rarely needs adjusting in an electronic system because there is
nothing that goes out of adjustment. If timing is off, then someone probably
adjusted it wrong last time.
Basic or Initial Timing
Basic timing is the
starting point for the automatic timing advance systems so it is imperative that
it be set correctly.
To set the timing:
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The engine must be at idle speed to make sure mechanical
advance isn't operating.
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The vacuum line to the distributor vacuum chamber must be disconnected
to make sure vacuum advance isn't operating.
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Basic timing is set by moving the distributor body which of
course has the points attached to it. Loosen the clamp that holds the
distributor down so the distributor can be moved but leave it tight enough that
it can't move itself. Attach a timing light to the engine and shine it down at
the timing marks. The timing light will "freeze" the timing mark on
the balancer whenever #1 spark plug fires. Adjust the distributor body so the
timing mark on the balancer lines up with the correct mark on the scale, and
tighten down the distributor clamp. Timing marks can also be located on the
engine's flywheel, especially in front wheel drive cars.
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If basic , or initial timing is wrong; both vacuum and
centrifugal advance will be off too.
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Centrifugal Advance
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Centrifugal advance affects power.
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The fuel in the combustion chamber takes
approximately the same amount of time to burn no matter what speed the engine is
running at for equal throttle opening. Remember that the spark only starts
the fuel burning. Once we have lit it on fire with the spark, it keeps burning
all by itself, and that takes time. It takes time for the flame to travel across
the cylinder. At low RPM, the piston doesn't travel very far, and therefore the
spark can happen quite late; but as the engine speeds up, the piston is traveling
faster, and if the spark does not happen earlier, the piston will have traveled
too far down the cylinder, the volume of the cylinder will be too large,
pressure will be low, and the engine will be low on power. The spark must be
timed earlier to allow for the increased piston speed.
   
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Notice that at idle, the spark fires at Top Dead Center. The
piston only travels 10degrees before the fuel has finished burning.
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At 1000 RPM the spark fires the mixture at 8 degrees before top
dead center to have the combustion complete by 10 degrees after TDC.
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At 2000 RPM the spark must fire at 26 degrees before TDC, to
have the fuel burned by 10 degrees after TDC.
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To do this, small springs hold back advance weights in the distributor,
either above or below the breaker plate. As distributor speed increases, these
weights are able to overcome the tension of the springs, the weights fly out,
and the distributor cam, or timer core advances in relation to the distributor
shaft.
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Centrifugal advance can be checked very easily, and should
always be checked during a tune-up. When you've got a timing light on the
engine, rev it up a bit, the timing should advance as RPM increases.
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Vacuum Advance
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Vacuum advance affects fuel economy.
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To control
the speed of a gasoline engine, a throttle, or butterfly valve is placed
somewhere in the air intake system of the engine. When the throttle valve is
open, the engine can take in as much air as it wants. When the throttle valve is
closed, air supply to the engine is restricted, and this restriction creates a
vacuum under the throttle plate called "manifold vacuum". Wide open
throttle, no vacuum. Closed throttle, high vacuum. When the throttle is closed,
the amount of fuel and air in the combustion chamber is much less than at wide
open throttle, so the molecules of fuel are further apart. Remember, the spark
only starts the fuel burning; once it starts burning, it keeps burning all by
itself. If the molecules of fuel are further apart, then one has to burn for a
slightly longer time before the next one starts on fire, and so on down the
line. This makes a difference in the time it takes to burn the fuel. The more
wide open (low vacuum) the throttle is, the less time it takes to burn. The more
closed the throttle is (high vacuum), the longer the time it takes to burn.
Ported vacuum, taken from above the throttle plate at idle, (you don't want
vacuum advance at idle, it makes the engine hard to start) acts on a vacuum diaphragm,
to move the breaker plate that the points are attached to. This advances the
timing at part throttle for better fuel economy.
It is very common for vacuum diaphragms to rupture because of years of being
exposed to gas fumes. This causes a sudden decrease in fuel economy.
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Vacuum advance can be checked very easily, and should be
checked as a part of every tune-up. You don't need a timing light or
anything. With the engine idling, apply a vacuum with w vacuum pump, or by
sucking on the vacuum advance line. The engine RPM should change. If it
doesn't change, the vacuum advance diaphragm is ruptured and should be
replaced.
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Centrifugal, and vacuum advance work together to modify the advance curve of the
ignition system to maximize fuel economy, emissions, and power.
See If You Understand How the Ignition System Works.
In multiple choice questions there may be more than one correct
answer. Only the best one will be considered correct
Name the parts of the primary circuit:
Click on the correct number by the part name.
Ballast Resistor 1
2 3
4 5
6 7
Points
1 2
3 4
5 6
7
Ignition Switch 1
2 3
4 5
6 7
By-pass wire 1
2 3
4 5
6 7
Primary coil 1
2 3
4 5
6 7
Battery
1 2
3 4
5 6
7
Condenser
1 2
3 4
5 6
7
The ballast resistor cuts this voltage down
to how many volts when the engine is running?
a) 5KV to 15KV
b) 25,000volts
c) 9 volts
d) 12 volts
When the ignition switch is turned to " on ", and the
points are closed, what is flowing through the primary circuit?
a) magnetic field
b) electrons
c) ohms
d) radio waves
What happens around a wire whenever electricity passes through
the wire?
a) magnetic field
b) electrons
c) ohms
d) radio waves
What is being built up around the coil when the
points are closed?
a) magnetic field
b) electrons
c) ohms
d) radio waves
How many windings in the primary coil? ( approximately )
a) 50
b) 500
c) 5000
d) 50,000
How many windings in the secondary coil?
a) 50
b) 500
c) 5000
d) 50,000
How fast does the distributor shaft turn compared to the crankshaft?
a) half as fast
b) same speed
c) twice as fast
d) faster
The primary circuit always grounds through the points.
Therefore the wire to the
distributor is always connected to which terminal
of the coil?
a) positive
b) negative
c) high voltage
d) none of the above
Dwell is the time the coil is turned On
or Off ?
a) on
b) off
Dwell, Coil saturation, and Cam
angle, all mean the same thing.
a) True
b) False
Dwell should always be set before timing,
because dwell affects timing, but
timing doesn't affect dwell?
a) True
b) False
Rather than using a point set to turn on and off the coil, an
electronic ignition system uses what?
a) transistor
b) reluctor
c) pickup coil
d) ignition switch
What tool do you use to " Freeze
" the timing marks so you can read them?
a) test lead
b) test light
c) dwell meter
d) timing light
In centrifugal advance, weights held in by spring tension at
low RPM, fly out at high RPM and advance what to advance RPM?
a) distributor cam
b) breaker plate
c) distributor shaft
d) distributor
In vacuum advance, the vacuum diaphragm, normally held in place
by a spring, is pulled at part throttle by ported vacuum and advance what to
advance the timing?
a) distributor cam
b) breaker plate
c) distributor shaft
d) distributor
Which plug is the cold plug? ( circle one )
Name the parts of the secondary circuit:
Click on the correct number by the part's name:
Distributor
cap
1 2
3 4
5 6
Coil
1 2
3 4
5 6
Rotor
1 2
3 4
5 6
Spark
Plug
1 2
3 4
5 6
Coil High Tension Wire 1
2 3
4 5
6
Spark Plug
Wire
1 2
3 4
5 6
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