Service Ignition System

COMPETENCY BASED LEARNING MATERIAL

Sector:

AUTOMOTIVE

Distinctive Area of Competence and Qualifications:

Automotive Servicing NC I

Unit of Competency:

Service Ignition System

Module Title:

Servicing Ignition System

Technical Education and Skills Development Authority

East Service Road, South Superhighway, Taguig, Metro Manila

AUTOMOTIVE

INDUSTRY SECTOR

NATIONAL CERTIFICATE LEVEL 1

QUALIFICATION LEVEL

COMPETENCY-BASED LEARNING MATERIALS

No. Basic Competencies Module Title Code
1. Receive and Respond to 1. Receiving and Responding to 500311101

Workplace Communication Workplace Communication 500311102
2. Work with Others 2. Working with Others 500311103
3. Demonstrate Work Values 3. Demonstrating Work Values 500311104

4. Practice Housekeeping 4. Practicing Housekeeping
Procedures Procedures

No. Common Competencies Module Title Code
1. Apply Appropriate 1. Applying Appropriate ALT723201
ALT723202
Sealant/Adhesive Sealant/Adhesive ALT311201
2. Move and Position Vehicle 2. Moving and Positioning ALT311202

3. Perform Housekeeping Vehicle ALT311203
4. Perform Mensuration and 3. Performing Housekeeping ALT723203

Calculation 4. Performing Mensuration and ALT723204
5. Perform Safety Practices Calculation
6 Read, Interpret and Apply ALT723205
5. Performing Safety Practices
Specifications and Manual
6. Reading, Interpreting and
7. Use and Apply Lubricant/Coolant Applying Specifications and
Manual
8. Perform Shop Maintenance
7. Using and Applying Lubricants/
Coolants

8. Perform Shop Maintenance

No. Core Competencies Module Title Code

1. Performing Diesel Engine Tune- 1. Performing Diesel Engine ALT723301

up Tune-up

2. Perform Gas Engine Tune-up 2. Performing Gas Engine Tune- ALT723302

up

3. Service Automotive Battery 3. Servicing Automotive Battery ALT723303

4. Service Ignition System 4. Servicing Ignition ALT723304

System9

5. Test and Repair Wiring/ Lighting 5. Testing and Repairing ALT723305

System Wiring/Lighting System

6. Perform Underchassis Preventive 6. Performing Underchassis ALT723306

Maintenance Preventive Maintenance

HOW TO USE THIS COMPETENCY BASED LEARNING MATERIAL

Welcome to the Module “Servicing and Repairing Ignition System”. This module
contains training materials and activities for you to complete.

The unit of competency “Service Ignition System” contains the knowledge, skills and
attitudes required for an Automotive Servicing course. This is one of the common modules
for National Certificate level I (NC I).

You are required to go through a series of learning activities in order to complete each of
the learning outcomes of the module. In each learning outcome there are Information
Sheets, Operation Sheets and job Sheets (Reference Materials for further reading to
help you better understand the required activities. Follow these activities on your own and
answer the self-check at the end of each learning activity.

If you have questions, don’t hesitate to ask your facilitator for assistance.

Recognition of Prior Learning (RPL)

You may already have some or most of the knowledge and skills covered in this module
because you have:

• been working for some time
• already completed training in this area.

If you can demonstrate to your trainer that you are competent in a particular skill or skills,
talk to him/her about having them formally recognized so you don’t have to do the same
training again. If you have a qualification or Certificate of Competency from previous
trainings show it to your trainer. If the skills you acquired are still current and relevant to this
module, they may become part of the evidence you can present for RPL. If you are not sure
about the currency of your skills, discuss this with your trainer.

After completing this module ask your trainer to assess your competency. Result of your
assessment will be recorded in your competency profile. All the learning activities are
designed for you to complete at your own pace.

Inside this module you will find the activities for you to complete and at the back are the
relevant information sheets for each learning outcome. Each learning outcome may have
more than one learning activities.

At the end of this module is a Learner Diary. Use this diary to record important dates,
jobs undertaken and other workplace events that will assist you in providing further details to
your trainer or an assessor. A Record of Achievement is provided for you by your trainer to
accomplish once you complete the module.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 1

This module is prepared to help you achieve the required competency in servicing
ignition system. This will be the source of information that will enable you to acquire the
knowledge and skills in this particular trade independently at your own pace or with
minimum supervision or help from your trainer.

- Talk to your trainer and agree on how you will both organize the training of this unit.
Read through the learning guide carefully. It is divided into sections which cover all the
skills and knowledge you need to successfully complete this module.

- Work through all the information and complete the activities in each section and
complete the self-check. Suggested references are included to supplement the materials
provided in this module.

- Most probably your trainer will also be your supervisor or manager. He/she is there to
support you and show you the correct way to do things. Ask for help.

- Your trainer will tell you about the important things you need to consider when you are
completing activities and it is important that you listen and take notes.

- Talk to more experienced work mates and ask for their guidance.

- Use the self-check questions at the end of each section to test your own progress.

- When you are ready, ask your trainer to watch you perform the activities outlined in the
learning guide.

- As you work through the activities, ask for written feedback of your progress from your
trainer. After completing each element, ask your trainer to mark on the report that you
are ready for assessment.

- When you have completed this module (or several modules) and feel confident that you
have had sufficient practice your trainer will arrange an appointment with you to asses
you. The result of your assessment will be recorded in your Competency Achievement
Record.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 2

QUALIFICATION : AUTOMOTIVE SERVICING NC I

UNIT OF COMPETENCY : SERVICE IGNITION SYSTEM

MODULE TITLE : SERVICING IGNITION SYSTEM

INTRODUCTION:

This module contains information and practices involved in checking the ignition system
of a vehicle. It includes instructions on how to check up an ignition system prior to trouble
shooting.

You need to complete this module before you can actually start the engine, since this
part is the primary system that lets the engine run.

LEARNING OUTCOMES:

At the end of this module you will be able to:

1. Identify and explain the function of the ignition system components
2. Check ignition coil, ballast resistor, high tension cable resistance
3. Check distributor assembly

ASSESSMENT CRITERIA:

1. Ignition system components are identified
2. Functions of ignition system are understood
3. Standard clearance and resistance of ignition system components are determined
4. Operating principle of the ignition system is explained
5. VOM is properly used
6. Specified resistance value is obtained from service manual
7. Procedure in checking resistance is observed in accordance with the manual of

instructions
8. Distributor terminal and rotor tips are cleaned and lubricated
9. Rotor and distributor cap are checked for cracks / current leakage
10. Condenser is tested by charged and discharged process
11. Dwell angle is adjusted / set in accordance with the manual of specifications and

instructions
12. Cam lube is checked for wear
13. Rotor rotation is determined
14. Advance mechanism operation is checked in accordance with the manual of

instructions

PRE-REQUISITE: NONE

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 3

Program /Course Automotive Servicing NC I
Unit of Competency SERVICE IGNITION SYSTEM
Module SERVICING IGNITION SYSTEM

LEARNING OUTCOME #1

Identify and explain the function of ignition system components.

ASSESSMENT CRITERIA

1. Ignition system components are identified.
2. Functions of the ignition system are understood.
3. Standard clearance and resistance of ignition system components are determined.
4. Operating principle of the ignition system is explained.

RESOURCES

• CDX Industrial Light Vehicle: under Ignition System contact point (CO) VCD
• Automotive Mechanics 10th Edition by Cruise and Anglin (Textbook)
• Internal Combustion Engine (ICE)

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 4

LEARNING EXPERIENCES

Learning Outcome 1 : Identify and explain the functions of ignition system components

LEARNING ACTIVITIES SPECIAL INSTRUCTIONS
What types of ignition systems have you worked?

1. Defining ignition system For more information view
1.1 Read Information sheet LO1-1 “Contact Point • CDX industrial light vehicle
Ignition System”
a) Purpose of the ignition system under ignition
b) Producing the spark • Refer to Information Sheet #1
c) Determining heat range & reach
(Contact Point Ignition
After reading a, b and c answer the Self-Check. System.)
• Automotive Mechanics 10th
What science principles can help us understand edition by Crouse & Anglin
more about ignition inside confined containers & (textbook)
cylinders • Internal combustion engines
• Automotive mechanics 10th
2. Identifying parts & components of the ignition system edition by Crouse & Anglin
2.1 Read a) Components of ignition system (textbook)
• CDX Industrial Light vehicle
3. Describing all functions of the different ignition (CD) ASE
components • From the ignition system
3.1 Identify a) ignition switch mock-up provided, pinpoint the
b) ignition coil components on the left.
c) secondary ignition cables
d) spark plug
e) distributor assembly
f) condenser
g) fusible link
h) ballast resistor

4. Discuss safety procedures in handling high voltage • OHS (Occupational Health &
current • Safety
4.1 Outline procedures paragraph Take actual resistance of the
a) refer to special instructions following components &

5. Measuring clearance circuits
a) contact points • Refer to service manual

5.1 measuring resistance

a ) primary

b ) secondary • See Electricity master
c ) high tension wire experiment manual
d ) ballast resistor

6. Experiment on Electro-magnetism

Read a) primary

b) secondary

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 5

INFORMATION SHEET LO1-1

CONTACT POINT IGNITION SYSTEM

PURPOSE OF IGNITION SYSTEM

The purpose of the ignition system is to ignite the compressed air-fuel mixture in the engine
combustion chambers. This should occur at the proper time for combustion to begin. To start
combustion, the ignition system delivers an electric spark that jumps a gap at the combustion-
chamber ends of the spark plugs. The heat from this arc ignites the compressed air-fuel
mixture. The mixture burns, creating pressure that pushes the pistons down the cylinders so the
engine runs. The ignition system may be either a contact-point ignition system or an electronic
ignition system.

PRODUCING THE SPARK Fig 1. Schematic diagram of the primary circuit in the
contact-point ignition system (ATW).
The ignition system consists of two separate
but related circuits: the low-voltage primary
circuit and the high-voltage secondary circuit.
The ignition coil (Fig. 1) has two windings.
The primary winding of a few hundred turns of
heavy wire is part of the primary circuit.

The secondary winding at many thousand
turns of fine wire is part of the secondary
circuit, When the ignition key is ON and the
contact points closed, current flows through
the primary winding (Fig. 1). This produces a
magnetic field around the primary windings in
the coil.

When the contact points open, current flow steps and the magnetic field collapses. As it
collapses, it cuts across the thousands of turns of wire in the coil secondary winding.
This produces a voltage in each turn. These add together to produce the high voltage delivered
through the secondary circuit to the spark plug (Fig. 5).

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 6

SPARK PLUG HEAT RANGE AND REACH

Spark plugs are made in different heat
ranges (Fig. 2). The heat range indicates
how fast the plug transfers heat from the
combustion chamber to the cylinder head.
This is primarily determined by the length of
the lower insulator. The longer the head path,
the hotter the plug will run. A short path
transfers the heat faster so the plug runs
cooler.

Sooty deposits accumulate on the firing end
of the spark plug if it runs too cold. The end
does not get hot enough to burn away the
deposits. Then the high-voltage surges short
across the deposits instead of jumping the
spark-plug gap. A plug that runs too hot
burns away the electrodes more rapidly. This
can widen the gap so much that the spark
cannot jump it and miss occurs.

Spark plug reach is the distance from the Figure 2. Heat range and reach of spark plugs. The
shell gasket seat (or top of the tapered seat longer the heat path, as indicated by arrows, the hotter
to the end of the threads. If the reach is too the plug runs. (AC Spark plug Division of General Motors
long, the plug electrodes too far into the
combustion chamber The plug could interfere Corporation)
with moisture turbulence or be struck by
valve or piston. A plug that does not reach far
enough fail to will fail to ignite the mixture
properly. The recommended spark plugs for
an engine have the correct reach.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 7

INFORMATION SHEET LO1-2

COMPONENTS IN CONTACT-POINT IGNITION SYSTEM

The ignition system (Fig. 3) includes the battery, ignition switch, ignition coil, ignition
distributor (with contact points and condenser), secondary wiring, and spark plugs.

BATTERY STARTER RELAY IGNITION
TO STARTER SWITCH S
A
SECONDARY CONNECTOR
CABLE L
OFF
DISTRIBUTOR CAP R

COIL WIRE RESISTANCE
- WIRE

SPARK COIL
PLUG TOWER
TOWER
+

SPARK CONTACT SECONDARY
PLUG ROTOR WINDING

VACUUM COIL PRIMARY
ADVANCE WINDING
MECHANISM

GAP CONDENSER
CAM
MOVABLE ARM PIVOT ARM
CENTRIFUGAL POINT
GROUND ADVANCE RUBBING
ELECTRODE MECHANISM STATIONARY BLOCK
POINT
WEIGHT DISTRIBUTOR CAM
SPRING SHAFT CAM LOBE

DISTRIBUTOR
HOUSING

Figure 3. Components in the contact-point ignition system (Ford Motors Company)

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 8

1. IGNITION SWITCH The ignition switch connects
the ignition coil to the battery when the ignition
key is ON. When the key is turned to START,
the starting motor cranks the engine for starting.

2. IGNITION COIL The ignition coil (Fig. 4) is a
step-up transformer that raises the battery
voltage to a high voltage that may reach 25,000
volts. In some electronic ignition systems, the
voltage may go up to 47,000 volts or higher. The
high voltage causes sparks to jump the gap at
the spark plugs.

Fig.4 Ignition coil, cutaway to show the windings.
(Delco-Remy Division of General Motors
Corporation)

3. IGNITION DISTRIBUTOR The ignition
distributor does two jobs. First, it has a set of
contact points or breaker points, (lower right)
that work as a fast-acting switch. When the
points close, current flows through the coil,
When the points open, current flow stops and
the coil produces a high-voltage surge. A
condenser connects across the points. It aids in
the collapse of the magnetic field and helps
reduce arcing that burns away the points.

Fig. 5 Ignition distributor with vacuum-advance
unit and cap (Delco-Remy Division of General

Motors Corporation)

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 9

Second, the distributor distributes the high-
voltage surges to the spark plugs in the correct
firing order. A coil wire delivers the high-voltage
from the coil to the center terminal of the
distributor cap. Inside the cap, a rotor (Fig. 6) is
on top of the distributor shaft.

Fig. 6 Cutaway ignition distributor and vacuum-
advance unit, with rotor and cap in place.

In most contact-point distributors, the distributor
shaft is driven from the engine camshaft by a
pair of spiral gears (Fig. 7). The rotor has a
metal blade. One end of the blade contacts the
center terminal of the distributor cap (Fig. 5).
When the rotor turns, the other end passes close
to the outer terminals in the distributor cap.
These are connected by spark-plug wires to the
spark plugs. The high-voltage surge jumps the
small gap from the rotor blade to the terminal.
The spark-plug wires carry the high-voltage
surge to the spark plug in the cylinder that is
ready to fire.

Fig. 7 Simplified secondary circuit, The coil
secondary winding is connected through the
distributor cap, rotor, and wiring to the spark

plugs.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 10

4. SECONDARY IGNITION CABLES The second
ignition cables or wiring include the coil wire and
the spark-plug wires. These cables connect
between the center of the ignition coil and the
distributor cap, and between the distributor cap
and the spark plugs. Figure 8 compares the
construction of an older, spark-plug wire with the
resistance cable now used on all cars.
Secondary cables for contact-point ignition
systems have a 7mm (0.276 inch) diameter.
Many electronic ignition systems require 8mm
(0.315 inch) cables. The use of a silicone
insulating jacket makes these cables larger.

Fig. 8 Silicone resistance cable or spark-plug wire
(left) now used on all vehicles compared with

older solid-conductor (right) spark-plug wire, (AC-
Delco Division of General Motors Corporation)

5. SPARK PLUGS The spark plug has two solid-
metal conductors called electrodes positioned to
form a gap. The gap is between the insulated
center electrode and the ground electrode. The
spark jumps the gap to ignite the compressed
air-fuel mixture in the engine cylinders.

Fig. 9 Spark plug electrodes.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 11

6. DISTRIBUTOR ASSEMBLY

a. Contact-Point Ignition Distributor

The contact-point set mounts on the breaker
plate in the distributor (Fig. 10). The points are
operated by a breaker cam on top of the
distributor shaft. The cam has the same number
of lobes as there are cylinders in the engine. As
the cam revolves and the points close and
open, they act as mechanical switch to make
and break the primary circuit.

One contact point mounts on the grounded
breaker plate and is stationary. The other point
mounts on the end of an insulated movable
arm. The arm swings back and forth on a pivot
as the cam lobes push on the rubbing block to
open the points. A spring attached to the
movable-point arm closes the points.

When the points close, this connects the coil Fig. 10. Top view of a contact-point ignition distributor
primary winding to the battery. A magnetic field with the cap and rotor removed. Note that the cam has
builds up in the coil. As the breaker cam
rotates, the next lobe pushes the movable arm six lobes (for a six cylinder engine). The drawings at
away from the stationary contact point. This the top show the cam action. When a lobe moves
opens the points and stops the current flow. under the rubbing block, the contact arm is moved,
The magnetic field collapses and a high-voltage separating the points. (Delco-Remy Division of
surge results. The length of time in degrees of General Motors Corp).
distributor-shaft rotation that the contact points
remained closed in the dwell. The distance that
separates the points when they are fully open is
the gap. Points are normally adjusted by dwell
or gap measurements.

The distributor shaft and cam are driven by
the engine camshaft which turns at one-half
crankshaft speed. It takes two complete
revolutions of the crankshaft to rotate the
distributor shaft one complete revolution.

The relationship between piston position and spark plug firing is ignition timing.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 12

b. Electronic Ignition System

Contact-point and electronic ignition systems are similar is operation and also often in
construction. Both distributors may have centrifugal and vacuum-advance mechanisms. The
major difference is the use of an electronic switch instead of a mechanical switch (contact
points) to control the primary current. The electronic distributor has an armature or reluctor and
a magnetic sensor or pick-up coil instead of a breaker cam and contact points. Like the breaker
cam, the reluctor has the same number of teeth or tips as there are cylinders in the engine.
When the reluctor rotates, each tooth creates a voltage pulse in the pickup coil. This signals the
ignition module to open the primary circuit.

Fig. 11. Components in an electronic-ignition using a pick-up coil distributor, with a simplified electronic control
module (ECM). The pick-up coil voltage signal is shown at the lower right. (General Motors Corporation).

The ignition module may be a separate ignition unit or mounted on or in the distributor.
Engines with an electronic-engine-control system may not have a separate ignition module. The
engine controller or electronic control module (ECM) completely controls the ignition system.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 13

c. High-Energy Ignition (HEI) System

Many electronic ignition systems produce
higher secondary voltage than contact-point
ignition. The voltage in these high-energy
ignition (HEI) systems can reach 47,000 volts
or higher. This allows the use of spark plugs
with wider gaps. The longer spark can ignite
leaner air-fuel ratios. These provide better fuel
economy and reduced exhaust emissions.

The secondary circuits are basically the same Fig. 12. Distributor, rotor and cap for a high-energy
in both ignition systems, however, the ignition (HEI) system. Note wider distributor and higher
distributor, ignition coil, and secondary wiring more widely spaced terminals on the cap that reduces
are redesigned to handle the higher voltage.
Distributor caps are larger with the spark plug arcing between terminals. (Delco-Remy Division of
terminals taller and farther apart. This reduces General Motors Corporation)
the possibility of arcing between terminals.
Arcing can cause engine miss and can
damage the cap. The capacitor reduces radio
interference (static). HEI systems also use the
larger 8-mm silicone spark-plug wires and coil
wire. Some General Motors HEI systems for
V-type engines mount the ignition coil in the
top of the distributor cap. This simplifies
ignition wiring. The HEI coil has a different
shape and produces a higher voltage than the
coil of contact-point systems. However, its
basic operation is the same.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 14

d. Distributor with Electronic Spark Advance

The electronic ignition systems describe above are usually on cars without an electronic-
engine control system. Cars with an electronic-engine-control system and a distributor usually
have electronic spark advance. Centrifugal and vacuum-advance mechanisms are not used.
The computer or ECM controls spark advance based on inputs from various sensors.

Fig. 13. Electronic ignition with electronic spark advance (Ford Motor Company).

Electronic spark advance affects only the primary circuit of the ignition system. There is no
change in the secondary circuit.

Figure 13 shows an electronic ignition system using electronic spark advance. The ignition
module and the ECM work together to control spark timing. The ignition module mounts on the
distributor housing. Inside the distributor, a profile ignition picks or PIP sensor signals the
ignition module as each piston nears top dead center (TDC). The ignition module shares this
information with the ECM which then computes spark advance. The ECM produces a new
signal that Ford calls the spark output (SPOUT) signal. It is sent back to the ignition module,
which opens the primary circuit at the proper time to fire the spark plug.

Sometimes the ECM or other part fails in the electronic engine control system. This may
cause the engine to enter in limp-in or limited-operating strategy (LOS) mode. Then the ignition
module uses only the signal from the sensor in the distributor to open and close the coil primary
circuit.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 15

PRIMARY RESISTANCE

Excessive current flow in the primary circuit causes arcing and burning of the contact points. To
prevent this, a resistance is placed between the ignition switch and the coil primary winding.
The resistance may be a separate resistor or a special resistance wire. For easier starting, the
resistance is bypassed and full battery voltage reaches the coil during cranking. After the
engine starts, the resistance reduces coil voltage from 5 to 8 volts.

SECONDARY VOLTAGE

The high voltages and high rate of charge
prevent many voltmeters from measuring
secondary voltage. Figure 9 shows a
computerized engine analyzer that
includes two cathode-ray tubes (CRTs).
The CRT is an oscilloscope or scope. It
can display the primary and secondary
voltage patterns and the length of time
this occur.

Figure 15 shows the basic pattern for on
spark plug firing cycle in the secondary
circuit. At the left, the points open. This
stops the current flow through the coil
primary winding. The magnetic field then
collapses, causing a sudden high voltage
(A to B) in the secondary winding. This is
the firing voltage that starts the spark
jumping the gap at the spark plug. The
volt quickly drops from B to C because it
takes less voltage to sustain the spark
than it does to start it. The spark
continues from C to D, where it stops.

Figure 14. Computerized Engine Analyzer.

This is the spark line burn time. It lasts for Figure 15. Oscilloscope waveform or trace, showing one
about 20 degrees of crankshaft rotation, complete spark-plug firing cycle in the secondary circuit. The
or 1 to 2 milliseconds (thousands of a
second). The remaining energy causes dwell section is the time period during which current flows
voltage ripples or oscillations from D to E through the primary circuit.
in Fig. 15. However, the voltage is not
great enough to restart the spark. At E,
the points close and current starts flowing
through the coil primary winding. The
expanding magnetic lines of force pass
through the primary winding, creating a
slight alternating voltage in it. This is
shown by the dip and small ripples to the
right of E in Fig. 15.

From E to F is the dwell section.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 16

Dwell is the length of time the points are closed and current flows through the primary winding
of the coil. Then at F, the points open and the cycle begins again at A as the spark occurs at the
spark plug. The whole procedure repeats continuously as long as the engine runs.

ADVANCING THE SPARK

When the engine is idling, the spark is timed to reach the spark plug just before the piston
reaches TDC on the compression stroke. At higher speeds, the spark must occur earlier. If it
does not, the piston will be past TDC and moving down on the power stroke before combustion
pressure reaches its maximum. The piston is ahead of the pressure rise which results in a weak
power stroke. This wastes much o the energy in the fuel.

To better use the energy in the fuel, the spark takes place earlier as engine speed increases.
This spark advance causes the mixture to burn producing maximum pressure just as the piston
moves through TDC. Most contact-point distributors have two mechanisms to control spark
advance. A centrifugal-advance mechanism adjusts the spark based on the engine speed. A
vacuum advance mechanism adjusts the spark based on engine load. On the engine, both work
together to provide the proper spark advance for the engine operating conditions.

CENTRIFUGAL ADVANCE

The centrifugal-advance mechanism
advances the spark by pushing the
breaker cam ahead as the engine
speed increases. Two advance weights,
two weight springs, and a cam
assembly provide this action. The cam
assembly includes the breaker cam and
an oval-shaped advance cam (Fig. 16).
At low speed, the springs hold the
weights in. As engine speed increases,
centrifugal force causes the weights to
overcome the spring force and pivot
outward (Fig. 16). This pushes the cam
assembly ahead. The contact points
open and close earlier, advancing the
spark.

Figure 16. Parts of a centrifugal-advance mechanism for a
contact-point distributor.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 17

VACUUM ADVANCE

When the throttle valve is only partly
open, a partial vacuum develops in the
intake manifold. Less air-fuel mixture
gets into the engine cylinders. Then the
fuel burns slower after it is ignited. The
spark must be advanced at part throttle
to give the mixture more time to burn.

Figure 17. Centrifugal-advance mechanism in
no-advance and full-advance positions. In the
example shown, the ignition is timed at 8
degrees before TDC on idle. There is no
centrifugal advance at 1,000 engine rpm. At
4,000 engine rpm, there is a total of 28 degrees
advance (8 degrees original timing plus 20
degrees centrifugal advance).

The vacuum-advance mechanism Figure 18. When the throttle valve is closed, there is no vacuum
Fig. 16 and 18) advances spark advance. The ported vacuum passage is above the closed throttle
timing by shifting the position of the
breaker plate. The vacuum-advance valve.
passage connects the diaphragm to
a port just above the closed throttle
valve. When the throttle valve moves
past the vacuum port, the intake
manifold pulls on the diaphragm.
This rotates the breaker plate so the
contact points open and close earlier
(Fig. 18). Any vacuum port above the
throttle valve provides ported
vacuum.

Figure 19. Operation of the vacuum-advance unit. When the
throttle valve moves past the port, intake-manifold vacuum is
admitted to the vacuum-advance unit on the distributor. The

breaker plate then rotates to advance the spark.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 18

COMBINED CENTRIFUGAL AND VACUUM ADVANCE

At any speed above idle, there is some
centrifugal advance. Depending on intake-
manifold vacuum, there may also be some
vacuum advance. The total advance curve in
Fig. 20 shows how the centrifugal and
vacuum advance combined. At 40 miles per
hour (64 km/h), there are 15 degrees of
centrifugal advance. The vacuum advance
can produce up to 15 degrees of additional
advance at part throttle. The advances
shown in Fig. 20 combine to produce a
maximum advance of 30 (15 + 15) degrees.

When the engine runs at wide-open throttle, Figure 20. Centrifugal and vacuum advance curves for
intake manifold vacuum drops to zero. There one engine.
is no vacuum advance. Normally, the total
advance varies between the straight line
(centrifugal advance) and the curved line
(centrifugal plus vacuum advance) in Fig.20.

SPARK PLUGS

The spark plug has a metal outer shell
enclosing a ceramic insulator. Centered in
the insulator is the center electrode which
carries the high-voltage current from the
ignition coil. A ground electrode attaches to
the metal shell and is bent inward to produce
the proper spark gap. The gap varies from
0.035 inch (0.9 mm) for contact-point ignition
systems to 0.080 inch (2.03 mm) for some
electronic ignition systems. The spark jumps
from the center electrode to the ground
electrode. The wider the gap, the higher the
voltage required to jump it.

Spark plugs may have a suppressor or Figure 21. Cutaway resistor-type spark plug. The center
resistance built into the center electrode (Fig. electrode is insulated. The side electrode is grounded
21). It reduces television and radio through the engine. (AC Spark Plug Division of General
interference (static) caused by the ignition
system. Spark plugs may require gaskets Motors Corporation)
when installed to assure a leak proof seal.
Many engines use spark plugs with tapered
seats which seal without a gasket. Some
spark-plug threads are coated with an anti-
seize compound. This makes plug removal
easier, especially from aluminum cylinder
heads.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 19

Some engines have two spark plugs in each
combustion chamber. Both plugs may fire
together or one slightly ahead of the other.
The additional plugs help reduce exhaust
emissions and increase engine power.

Most spark plugs have electrodes made of Figure 22. Spark plugs with center electrodes that have a
nickel and chrome alloys that resist (A) copper core and a (B) platinum tip. (Robert Bosch
corrosion. Some ground and lower center Corporation)
electrodes have a copper core. Other also
have a thin-wire platinum tip. These foul and
misfire less often, last longer, and have a
greater heat range than other plugs.

IGNITION SWITCH

The ignition switch does several jobs. Figure 23. Combination ignition-switch and steering-wheel lock,
It turns the ignition system on and off. showing the opening mechanism inside the steering column.
It has a START position for operating (General Motors Corporation)
the starting motor. It operates the
steering-wheel lock, an audible or light
signal if the ignition key is in the
ignition lock when a door opens or if
seat belts are not buckled. On many
vehicles, the electric fuel pump
connects to the battery through the
ignition switch. Other accessories such
as radio and heater blower motor also
receive current through the ignition
switch.

In most cars, the ignition key is placed in the ignition lock or lock cylinder in the steering column
(Fig. 19). The ignition switch may attach to the lock, or go further down the steering column.
Turning the ignition key moves an actuator rod that operates the ignition switch.

The five positions of the ignition lock are ACCESSORY, LOCK, OFF, RUN, and START. Figure 19
shows how the ignition lock operates the steering-wheel lock. The notched disk is splined to the
top end of the steering shaft. With the automatic transmission in PARK, the driver turns the key
to LOCK. The spring-loaded plunger moves up into a notch in the disk, locking the steering
wheel. This also locks the transmission in park.

If the key will not move to LOCK, turn the Figure 24. Ignition-system oscilloscope pattern.
steering wheel until the plunger and a notch
align. Then the spring will force the plunger
up and the key will turn to LOCK. Turning the
key from LOCK to OFF pulls the plunger out of
the notch in the disk. This unlocks the
steering wheel and the transmission.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 20

SELF-CHECK LO1-1

I. Multiple Choice Test

Instruction: Select the best answer to each question

1. The ignition system performs all the following jobs except
a. controls the compression pressure
b. produces the spark that jumps the spark-plug gap
c. ignites the compressed air-fuel mixture
d. delivers the spark at the proper time

2. The contact-point distributor has two major jobs
a. to advance and retard spark
b. to distribute the high voltage surges and switch the current to the coil on
and off
c. to distribute the battery voltage and switch the current to the spark plugs on
and off
d. to provide centrifugal advance and vacuum advance

3. Technician A says the spark occurs when the contact points open. Technician
B says the spark occurs when the coil magnetic field collapses. Who is right?
a. A only
b. B only
c. both A and B
d. neither A nor B

4. Cross-firing may be caused by all the following except
a. wet distributor cap or rotor
b. defective insulation on secondary cables
c. improper routing of spark-plug cables
d. a fouled spark-plug

II. Identification

1. List the five (5) major components in the contact point ignition system

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 21

ANSWER KEY LO1-1

I.
1. a

2. b

3. c

4. d

II.
1. ignition switch
2. ignition coil
3. distributor assembly
4. secondary cable/high tension cables
5. spark plugs

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 22

QUALIFICATION : AUTOMOTIVE SERVICING NC I
SERVICE IGNITION SYSTEM
UNIT OF COMPETENCY : SERVICING IGNITION SYSTEM

MODULE TITLE :

LEARNING OUTCOME #2: Check Ignition coil, ballast resistor, and high tension cable
resistance

ASSESSMENT CRITERIA :

1. VOM is properly used

2. Specified resistance value is obtained from service manual

3. Procedure in checking resistance is observed in accordance with
manual instruction

RESOURCES :

• Automotive Mechanics 10th Edition by Cruise and Anglin (Textbook)

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 23

LEARNING EXPERIENCES

Learning Outcome #2: Check Ignition coil, ballast resistor and high tension cable
resistance

LEARNING ACTIVITIES SPECIAL INSTRUCTIONS

- Differentiate analog from digital testers
- Have you tried using VOM testers

The next activity will require you to take actual resistance Read “Ignition-System Trouble
of the following ignition system components. Diagnosis” Automotive Mechanics
10th ed., pp. 411-422
1. Conducting ignition system component system testing
on: - refer to self-check on LO2-1
a. ignition coil
b. ballast resistor
c. high tension cable

Testing Cable Resistance

To determine that the internal structure of a cable is
sound, check its resistance with an ohmmeter. The
normal range of resistance in a new cable is about 5,000
to 10,000 ohms per foot. A defective cable can have
50,000 (50K) or 100,000 (100K) ohms of resistance or it
can be totally open (infinite resistance).

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 24

SELF-CHECK LO2-1

1. List the three categories of ignition-system failure and give examples of it.
2. What is the difference in making a spark test as the plug end of the spark-plug cables

and at the coil?
3. Describe how to perform a triggering test and explain what it tells you.
4. Describe how to time the distributor to the engine after the crankshaft has turned while

the distributor was out.
5. Explain how to set the ignition timing using a timing light and magnetic timing tester.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 25

QUALIFICATION : AUTOMOTIVE SERVICING NC I
SERVICE IGNITION SYSTEM
UNIT OF COMPETENCY : SERVICING IGNITION SYSTEM

MODULE TITLE :

LEARNING OUTCOME #3 : Check Distributor Assembly

ASSESSMENT CRITERIA :

1. Distribution terminals and rotor tip cleaned & lubricated

2. Rotor and distributor cap check for crack / current leakage

3. Condenser is tested by charged and discharged process

4. Dwell angle is adjusted / set in accordance to manual
specifications & instructions

5. Cam lube is checked for wear

6. Rotor rotation is determined

7. Advance mechanism operation is checked in accordance to
manual instructions

RESOURCES :

• Automotive Mechanics 10th Edition by Cruise and Anglin (Textbook)

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 26

LEARNING EXPERIENCES

Learning Outcome #3 : Check distributor assembly

LEARNING ACTIVITIES SPECIAL INSTRUCTIONS

• Have you tried removing the distributor from the
engine

1. Servicing the distributor cap & rotor • Refer to manufacturer service
1.1 removing the distributor cap & rotor manual on distributor servicing

1.2 cleaning of terminals & tips

2. Testing the condenser for charging & discharging • Self-check see LO3-1 (see

& leak conditions back for table)

2.1 removing the condenser without damaging
rebated parts.

2.2 replacing the condenser with the approved • Refer to manufacturer service
replacement manual

2.3 testing the newly replaced condenser
accurately

2.4 adjust dwell angle according to specification • Refer to manufacturer service
manual
• Have you seen the internal parts of the distributor
• Have you tried disassembling its pats • Read p. 392 fig. 3-11 of
Automotive Mechanic Crouse
3. Inspecting Internal parts of the distributor & Anglin 10th ed.
3.1 Inspect a) cam lobe for wear
b) rotor for rotation • Read Auto Repair Manual for
c) advance mechanism for normal Toyota Corolla Series by
operation Haynes for general procedures
on disassembling &
assembling

After disassembling all of the parts under 3.1 you are
now going to apply the procedure suggested in the
service manual

Note: Inform your trainer after you have completed • See self-check LO3-1
the assembling procedure.

3.2 Answer self-check after assembling

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 27

SELF-CHECK

Answer the following test items

1. Enumerate briefly the disassembly-assembly procedures that you have performed

1.1____________________________________________
1.2____________________________________________
1.3____________________________________________
1.4____________________________________________
1.5____________________________________________
1.6____________________________________________
1.7____________________________________________
1.8____________________________________________
1.9____________________________________________
2.0____________________________________________

2. State your inspection results in the table provided below

PARTS RESULT/DESCRIPTION

Cam Lobe

Rotor

Advance
Mechanism

Please record your actual data on the space provided under reading

TEST CAPACITOR TEST REMARKS
READING

Charging

Discharging

Leak

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 28

SELF-CHECK

Answer the following test items

I. Enumerate briefly the disassembly-assembly procedures that you have just performed

1.1___________________________________________
1.2___________________________________________
1.3___________________________________________
1.4___________________________________________
1.5___________________________________________
1.6___________________________________________
1.7___________________________________________
1.8___________________________________________
1.9___________________________________________
2.0___________________________________________

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 29

PERFORMANCE TEST

Learner's Name Date
Competency: Service Ignition System
Test Attempt
1st 2nd 3rd

Directions: OVERALL EVALUATION

CALL INSTRUCTOR and ask Level PERFORMANCE LEVELS
him/her to assess your Achieved
performance in the following critical 4 - Can perform this skill without supervision and with
task and performance criteria initiative and adaptability to problem situations.
indicated below 3 - Can perform this skill satisfactorily without
assistance or supervision.
You will be rated based on the 2 - Can perform this skill satisfactorily but requires
overall evaluation outlined on the some assistance and/or supervision.
right side. 1 - Can perform parts of this skill satisfactorily, but
requires considerable assistance and/or supervision.

Instructor will initial level achieved.

PERFORMANCE STANDARDS Yes No N/A
For acceptable achievement, all items should receive a "Yes" or
"N/A" response.
• Ignition system components are identified
• Functions of ignition system are understood
• Standard clearance and resistance of ignition system components are

determined
• Operating principle of the ignition system is explained
• VOM is properly used
• Specified resistance value is obtained from service manual
• Procedure in checking resistance is observed in accordance with the

manual of instructions
• Distributor terminal and rotor tips are cleaned and lubricated

• Rotor and distributor cap are checked for cracks / current leakage

• Condenser is tested by charged and discharged process

• Dwell angle is adjusted / set in accordance with the manual of
specifications and instructions

• Cam lube is checked for wear

• Rotor rotation is determined

• Advance mechanism operation is checked in accordance with the
manual of instructions

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 30

Learner’s diary

DIARY NOTES

Record important dates, jobs undertaken and other workplace events that will assist you in providing further
details to an Assessor.
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................
....................................................................................................................................................................

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 31

Record of Achievement

Module: Servicing Ignition System

LO#1: Identify and explain the function of the ignition system components.

Performance Criteria:

1. Ignition system components are identified.
2. Functions of ignition system are understood.
3. Standard clearance and resistance of ignition system components are determined.
4. Operating principle of the ignition system is explained.

Comments:
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................
.....................................................................................................................................................................

Learner has satisfied the above performance criteria.
Learner’s signature: .....................................................................
Trainer’s signature: ......................................................................
Date: ............................................................................................

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 32

Record of Achievement

Module: Servicing Ignition System

LO#2: Check ignition coil, ballast resistor, high tension cable resistance.

Performance Criteria:

1. VOM is properly used.
2. Specified resistance value is obtained from service manual
3. Procedure in checking resistance is observed in accordance with the manual of

instructions

Comments:
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................

Learner has satisfied the above performance criteria.
Learner’s signature: .....................................................................
Trainer’s signature: ......................................................................
Date: ............................................................................................

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 33

Record of Achievement

Module: Servicing Ignition System

LO#3: Check distributor assembly.

Performance Criteria:

1. Distributor terminal and rotor tips are cleaned and lubricated
2. Rotor and distributor cap are checked for cracks / current leakage
3. Condenser is tested by charged and discharged process
4. Dwell angle is adjusted / set in accordance with the manual of specifications and

instructions
5. Cam lube is checked for wear
6. Rotor rotation is determined
7. Advance mechanism operation is checked in accordance with the manual of

instructions

Comments:
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................
..................................................................................................................................................................

Learner has satisfied the above performance criteria.
Learner’s signature: .....................................................................
Trainer’s signature: ......................................................................
Date: ............................................................................................

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 34

Terms and Definitions

Ballast resistor: (BAL RES) A resistor constructed of a special type wire, the properties of
which tend to increase or decrease the voltage in direct proportion to the heat of the
wire.

Centrifugal advance: A device found on the distributor which, through the action of
centrifugal force on two weights, advances or retards the ignition spark to correspond
with changes in engine speed and load.

Condenser: [1] A small metal cylinder which is usually located in the distributor. It is installed
between the breaker points and coil to prevent arcing at the breaker points by
absorbing or storing the excess current. A condenser (also called a "capacitor") has
the ability to absorb and retain surges of electricity. It is constructed of two metal plates
separated by an insulator.

Contact points: Two movable points or areas that when pressed together, complete a circuit.
These points are usually made of tungsten, platinum, or silver.

Distributor: A unit in the ignition system designed to make and break the ignition primary
circuit and to distribute the resultant high voltage to the proper cylinder at the correct
time. The high voltage comes from the coil to the center terminal of the distributor cap
and passes down the rotor. As the rotor turns, contact is made with each successive
terminal on the circumference of the distributor cap. From there, the voltage goes into
the spark plug wires and to the spark plug. Generally when your vehicle has its timing
adjusted, it is the distributor that is adjusted. Also called "ignition distributor."

Dwell: [1] In a contact breaker ignition system, the number of degrees the breaker cam
rotates from the time the breaker points close until they open again. Also called the
"dwell angle" or "dwell period."
[2] In a breakerless ignition system, the time during which the electronic control unit
allows current to flow through the primary winding of the coil, which in ignition systems
with a Hall generator is determined by the width of the vanes.

Electromotive force: (EMF) A source of electrical energy required to produce an electric
current, produced by devices such as batteries or generators and measured in volts.

Electronic spark advance: (ESA) the part of an ECU that controls ignition timing and dwell
angle

Fusible link: Special length of smaller gauge wire designed to "blow" if heavy current flows in
circuit

Ignition: A process which initiates the combustion of the compressed air/fuel mixture in the
combustion chamber. In a spark-ignition engine, the mixture is ignited by an electric
spark; in a diesel engine, the self-igniting mixture must be preheated by glow plugs
when a cold start is performed.

Ignition advance: The extent to which the ignition spark is made to occur earlier. The
opposite is ignition retard.

Ignition cable: A general term to designate the high-voltage cables of the ignition system,
from the ignition coil to the distributor and from the distributor to the spark plugs. Also
called "ignition leads"

Ignition coil: A pulse transformer which is a part of the ignition system. It receives a small
amount of electrical voltage from the battery and steps up the low "primary" voltage
and amplifies it into a big jolt of voltage of about 20,000 volts, and sends it to the spark
plugs via the distributor. It is made of two windings and a core of iron. The primary coil
has about 200 turns of relatively heavy wire. The secondary windings may have as
much as 22,000 windings of fine wire. As electricity travels through the primary
winding, it produces a magnetic field in the coil. When the points open, the magnetic
field collapses and the movement of the magnetic field induces current in the
secondary windings of the coil. The voltage is stepped up in proportion to the ratio of

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 35

secondary to primary turns and the distributor directs this high voltage to the spark
plug. Also called just "coil."
Ignition switch: A control device that is generally operated with a key that closes and opens
an electrical current to connect and disconnects the ignition system from the battery so
that the engine can be started and stopped as desired. The key should not be
removable when the engine is running. It usually has various positions which, besides
starting the engine, allow the user to operate the accessories without engaging the
engine or to check the bulbs in the warning lights on the dashboard.
Ignition system: The system that provides the electrical current or spark to ignite the air-fuel
mixture in the combustion chambers. It includes the battery or magneto which sends
electricity to the ignition coil if the ignition switch is on. The coil amplifies the electrical
charge and sends it to the distributor where it goes through the points and is modified
by the condenser. The distributor sends it along the high tension lines (spark plug
wires) to the spark plugs where it ignites the air-fuel mixture.
Reluctor or Trigger wheel: A rotor of a magnetic pick-up integrated into the distributor, with
as many teeth as the engine has cylinders
Rotor: A small rotating cap-like unit at the end of the distributor shaft. It is located on the
breaker cam inside the cap. It connects between the center electrode and the various
outer spark plug terminals as it turns, thus distributing the high voltage from the ignition
coil secondary winding to the proper spark plug.
Spark plug: A device containing two electrodes across which electricity jumps to produce a
spark to fire the fuel charge. The center electrode is insulated from the spark plug shell
by means of a molded insulator resembling porcelain. The side electrode protrudes
from the bottom edge of the spark plug shell and is so positioned that there is a gap
between it and the center electrode.
Vacuum advance: A mechanism on the side of the distributor that automatically varies the
instant at which the spark occurs as a function of intake manifold vacuum. Vacuum
advance provides the additional advance that is needed when the engine is operating
at part throttle. At part throttle less air-fuel mixture gets into the cylinders and the
mixture takes longer to burn after it is ignited. Because the mixture burns more slowly,
the piston will be past top dead center and moving down before the mixture has a
chance to burn and produce high power. As a result much of the power in the fuel will
be lost. The vacuum advance mechanism consists of a flexible spring-loaded
diaphragm connected by a linkage to the breaker plate on which the points are
mounted. The sealed side of the diaphragm is connected by a tube to the carburetor.
The throttle valve is below the vacuum passage in the carburetor air horn so there is
no vacuum advance when the engine is idling because the throttle is closed. However,
when the throttle is partly open, intake manifold vacuum pulls the diaphragm in and this
causes the breaker plate to rotate a few degrees and advance the timing. With wide-
open throttle there is very little vacuum in the intake manifold so there will be no
vacuum advance. In most instances the vacuum advance is disconnected before
checking the timing and point gap.
Voltage: A difference in electrical potential between one end of a circuit and the other. Also
called "electromotive force (EMF)." Voltage causes current to flow. Measured in volts.

Volt meter: an instrument used to measure the electromotive force or voltage of a battery.

Code No. Servicing Ignition System Date: Developed Date: Revised Page #
ALT723304
Aug. 15, 2003 36