Halderman J.D., Linder J. Automotive Fuel and Emissions Control Systems

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IGNITION SYSTEM OPERATION AND DIAGNOSIS 361

CAUTION: Ford V-8s use two different firing orders

depending on whether the engine is high output or

standard. Using the incorrect firing order can cause

the engine to backfire and could cause engine dam-

age or personal injury. General Motors V-6 engines use

different firing orders and different locations for cyl-

inder1 between the 60-degree V-6 and the 90-degree

V-6. Using the incorrect firing order or cylinder number

location chart could result in poor engine operation or a

no start. Firing order is also important for waste-spark-

type ignition systems. The spark plug wire can often be

installed on the wrong coil pack, which can create a no-

start condition or poor engine operation.

COMMON IRON

PRIMARY CONTROL

TRANSISTOR

IGNITION

MODULE

B+

IGNITION

FUSE

CENTER

ELECTRODE

SIDE

ELECTRODE

EXHAUST

STROKE

COMPRESSION

STROKE

FIGURE 29–13 A waste-spark system fires one cylinder

while its piston is on the compression stroke and into paired

or companion cylinders while it is on the exhaust stroke. In

a typical engine, it requires only about 2 to 3 kV to fire the

cylinder on the exhaust stroke. The remaining coil energy is

available to fire the spark plug under compression (typically

about 8 to 12 kV).

How Can You Determine the Companion

Cylinder?

Companion cylinders are two cylinders in the same

engine that both reach top dead center (TDC) at the

same time:

• One cylinder is on the compression stroke.

• The other cylinder is on the exhaust stroke.

To determine which two cylinders are compan-

ion cylinders in the engine, follow these steps:

STEP 1 Determine the firing order (such as 165432

for a typical V-6 engine).

STEP 2 Write the firing order and then place the

second half under the first half:

165

432

STEP 3 The cylinder numbers above and below each

other are companion or paired cylinders.

In this case 1 and 4, 6 and 3, and 5 and 2 are

companion cylinders.

FREQUENTLY ASKED QUESTION

Odds Fire Straight

Waste-spark ignition systems fire two spark plugs

at the same time. Most vehicle manufacturers use a

waste-spark system that fires the odd number cyl-

inders (1, 3, and 5) by straight polarity (current flow

from the top of the spark plug through the gap and

to the ground electrode). The even number cylinders

(2, 4, and 6) are fired reverse polarity, meaning that

the spark jumps from the side electrode to the center

electrode. Some vehicle manufacturers equip their

vehicles with platinum plugs that have the expensive

platinum alloy on only one electrode, as follows:

• On odd number cylinders (1, 3, 5), the platinum is

on the center electrode.

• On even number cylinders (2, 4, 6), the platinum is

on the ground electrode.

Replacement spark plugs use platinum on both

electrodes (double platinum) and, therefore, can be

placed in any cylinder location.

TECH TIP

WASTE-SPARK IGNITION

SYSTEMS

PARTS INVOLVED Waste-spark ignition is another name

for distributorless ignition system (DIS) or electronic ignition.

Waste-spark ignition was introduced in the mid-1980s and uses

the ignition control module (ICM) and/or the powertrain control

module (PCM) to fire the ignition coils. A 4-cylinder engine uses

two ignition coils, and a 6-cylinder engine uses three ignition

coils. Each coil is a true transformer because the primary wind-

ing and secondary winding are not electrically connected. Each

end of the secondary winding is connected to a cylinder exactly

opposite the other in the firing order, which is called a compan-

ion (paired) cylinder.

 SEE FIGURE 29–13 .

362 CHAPTER 29

WASTE-SPARK SYSTEM OPERATION Both spark

plugs fire at the same time (within nanoseconds of each other):



When one cylinder (for example, cylinder number 6) is on

the compression stroke, the other cylinder (number 3) is

on the exhaust stroke.



The spark that occurs on the exhaust stroke is called the

waste spark because it does no useful work andis only

used as a ground path for the secondary winding ofthe

ignition coil. The voltage required to jump the spark plug

gap on cylinder 3 (the exhaust stroke) is only 2 to 3 kV.



The cylinder on the compression stroke uses the

remaining coil energy.



One spark plug of each pair always fires straight polarity

(center to side electrode), and the other cylinder always

fires reverse polarity (side to center electrode). Spark plug

life is not greatly affected by the reverse polarity. If there

is only one defective spark plug wire or spark plug, two

cylinders may be affected.

The coil polarity is determined by the direction the coil is

wound (left-hand rule for conventional current flow) and cannot

be changed.

Each spark plug for a particular cylinder always will be fired

either with straight or reversed polarity, depending on its loca-

tion in the engine and how the coils are wired. However, the

compression and waste-spark condition flip-flops. When one

cylinder is on compression, such as cylinder 1, then the paired

cylinder (number 4) is on the exhaust stroke. During the next ro-

tation of the crankshaft, cylinder 4 is on the compression stroke,

and cylinder 1 is on the exhaust stroke:

Cylinder 1 Always fires straight polarity (from the center

electrode to the ground electrode), one time,

requiring 10 to 12 kV, and one time, requiring

3 to 4 kV.

Cylinder 4 Always fires reverse polarity (from the ground

electrode to the center electrode), one time,

requiring 10 to 12 kV, and one time, requiring

3 to 4 kV.

Waste-spark ignitions require a sensor (usually a crank-

shaft sensor) to trigger the coils at the correct time.

 SEE

FIGURE 29–14 .

The crankshaft sensor cannot be moved to adjust ignition

timing because ignition timing is not adjustable. The slight ad-

justment of the crankshaft sensor is designed to position the

sensor exactly in the middle of the rotating metal disc for maxi-

mum clearance.

COMPRESSION-SENSING WASTE-SPARK IGNITION

Some waste-spark ignition systems, such as those used on

Saturns and others, use the voltage required to fire the cylin-

ders to determine cylinder position. It requires a higher voltage

to fire a spark plug under compression than it does when the

spark plug is being fired on the exhaust stroke. The electronics

in the coil and the PCM can detect which of the two compan-

ion (paired) cylinders that are fired at the same time requires

the higher voltage and therefore indicates the cylinder that is

on the compression stroke. For example, a typical 4-cylinder

engine equipped with a waste-spark ignition system will fire

both cylinders 1 and 4. If cylinder 4 requires a higher voltage

to fire, as determined by the electronics connected to the coil,

EST

BYPASS

TACH REF (REF HIGH)

GROUND (REF LOW)

COMPUTER

TACK

LEAD

PINK/BLACK

IGN B

A B C D

DUAL CRANK

SENSOR CONNECTOR

NITION

MODULE

(UNDER COILS)

COIL

ASSEMBLIES

CYLINDER 1

CYLINDER 2

CYLINDER 3

CYLINDER 4

CYLINDER 5

CYLINDER 6

WHITE

TAN/BLACK

PURPLE/WHITE

BLACK/RED

WHITE

LIGHT BLUE/WHITE

GRAY/RED

WHITE/BLACK

FIGURE 29–14 Typical wiring diagram of a GM V-6 waste-spark ignition system.

IGNITION SYSTEM OPERATION AND DIAGNOSIS 363

then the PCM assumes that cylinder 4 is on the compression

stroke. Engines equipped with compression-sensing ignition

systems do not require the use of a camshaft position sensor

to determine specific cylinder numbers.

 SEE FIGURE 29–15 .

TYPES OF COP SYSTEMS There are two basic types of

COP ignition systems:



Two primary wires. This design uses the vehicle compu-

ter to control the firing of the ignition coil. The two wires

include the ignition voltage feed and the pulse ground

wire, which is controlled by the computer. The ignition

control module is located in the PCM, which handles all

ignition timing and coil on-time control.



Three primary wires. This design includes an ignition

module at each coil. The three wires include the following:



Ignition voltage



Ground



Pulse from the computer to the built-in ignition module

Vehicles use a variety of COP-type ignition systems, in-

cluding the following:



Many General Motors V-8 engines use a coil-near-plug

system with individual coils and modules for each in-

dividual cylinder that are placed on the valve covers.

Short secondary ignition spark plug wires are used to

connect the output terminal of the ignition coil to the

GROUND

-10KV

+4KV

A NEGATIVE GOING VOLTAGE

A POSITIVE GOING VOLTAGE

CSI

SIGNAL

CYLINDERS

#2 OR #4

ON WASTE

CYLINDERS

#1 OR #3

ON COMPRESSION

CYLINDERS #4 AND #2 HAVE

BREAKOVER VOLTAGES THAT FALL

FROM ABOVE GROUND TOWARD GROUND

CYLINDERS #2 AND #4 HAVE

FIRING VOLTAGES THAT RISE

A POSITIVE DIRECTION

CYLINDERS #3 AND #1 HAVE

BREAKOVER VOLTAGES THAT RISE

FROM BELOW GROUND TOWARD GROUND

CYLINDERS #1 AND #3 HAVE

FIRING VOLTAGES THAT RISE

A NEGATIVE DIRECTION

FIGURE 29–15 The slight (5 microsecond) difference in the firing of the companion cylinders is enough time to allow the PCM to

determine which cylinder is firing on the compression stroke. The compression sensing ignition (CSI) signal is then processed by

the PCM which then determines which cylinder is on the compression stroke.

TERMINOLOGY Coil-on-plug (COP) ignition uses one

ignition coil for each spark plug. This system is also called

coil-by-plug, coil-near-plug, or coil-over-plug ignition.

 SEE

FIGURES 29–16 AND 29–17 .

ADVANTAGES The COP system eliminates the spark plug

wires that are often the source of electromagnetic interference

(EMI) that can cause problems to some computer signals. The

vehicle computer controls the timing of the spark. Ignition tim-

ing also can be changed (retarded or advanced) on a cylinder-

by-cylinder basis for maximum performance and to respond to

knock sensor signals.

COIL-ON-PLUG IGNITION

364 CHAPTER 29

CKP

SENSOR

B+

IGNITION

SWITCH

INTEGRAL

COIL AND PLUG

PCM

FIGURE 29–16 A typical two wire coil-on-plug (COP) ignition system showing the triggering and the switching being performed

by the PCM from input from the crankshaft position sensor.

INTAKE CAM

PHASER SOLENOID

COIL-ON-PLUG (COP)

COILS

EXHAUST CAM

PHASER

SOLENOID

CAMSHAFT

POSITION (CMP)

SENSOR

FIGURE 29–17 An overhead camshaft engine equipped

withvariable valve timing on both the intake and exhaust

camshafts and coil-on-plug (COP) ignition.

COIL

SPARK PLUG WIRE

TO COMPANION

CYLINDER

FIGURE 29–18 A Chrysler Hemi V-8 that has two spark plugs

per cylinder. The coil on top of one spark fires that plug and,

through a spark plug wire, fires a plug in the companion cylinder.

spark plug, and therefore this system is called a coil-

near-plug system.



In a combination of COP and waste-spark systems, the

systems fire a spark plug attached to the coil and use a

spark plug wire attached to the other secondary terminal

of the coil to fire another spark plug of the companion cyl-

inder. This type of system is used in some Chrysler Hemi

V-8 and Toyota V-6 engines.

 SEE FIGURE 29–18 .

Most new engines use coil-on-plug-type ignition systems.

Each coil is controlled by the PCM, which can vary the ignition

IGNITION SYSTEM OPERATION AND DIAGNOSIS 365

timing separately for each cylinder based on signals the PCM

receives from the knock sensor(s). For example, if the knock

sensor detects that a spark knock has occurred after firing

cylinder 3, then the PCM will continue to monitor cylinder 3

and retard timing on just this one cylinder if necessary to pre-

vent engine-damaging detonation.

ION-SENSING IGNITION In an ion-sensing ignition sys-

tem, the spark plug itself becomes a sensor. An ion-sensing igni-

tion uses a COP design where the ignition control module (ICM)

applies a DC voltage across the spark plug gap after theignition

event to sense the ionized gases (called plasma) inside the cyl-

inder. Ion-sensing ignition is used in the General Motors EcoTec

4-cylinder engines.

 SEE FIGURE 29–19 .

The secondary coil discharge voltage (10 to 15 kV) is elec-

trically isolated from the ion-sensing circuit. The combustion

flame is ionized and will conduct some electricity, which can be

accurately measured at the spark plug gap. The purpose of this

circuit includes the following:



Misfire detection (required by OBD-II regulations)



Knock detection (eliminates the need for a knock sensor)



Ignition timing control (to achieve the best spark timing

for maximum power with lowest exhaust emissions)



Exhaust gas recirculation (EGR) control



Air-fuel ratio control on an individual cylinder basis

Ion-sensing ignition systems still function the same as

conventional COP designs, but the engine does not need to be

equipped with a camshaft position sensor for misfire detection

or a knock sensor because both of these faults are achieved

using the electronics inside the ignition control circuits.

in the engine block. It is this vibration that is detected by the

knock sensor. The signal from the knock sensor is used by the

PCM to retard the ignition timing until the knock is eliminated,

thereby reducing the damaging effects of the abnormal com-

bustion on pistons and other engine parts.

Inside the knock sensor is a piezoelectric element that

is a type of crystal that produces a voltage when pressure

or a vibration is applied to the unit. The knock sensor is

tuned to the engine knock frequency, which is a range from

B+

SPARK

CHARGED TO

80 VOLTS

SPARK EVENT—SPARK CURRENT FLOW

ISIM COMPONENTS ADDED

TO SECONDARY CIRCUIT

ION SIGNAL

B+

DISCHARGING

80 VOLTS

ISIM COMPONENTS ADDED

TO SECONDARY CIRCUIT

MEASUREMENT PERIOD—ON CURRENT FLOW

ION

FLOW

ION SIGNAL

FIGURE 29–19 A DC voltage is applied across the spark

plug gap after the plug fires, and the circuit can determine

if the correct air-fuel ratio was present in the cylinder and

if knock occurred. The applied voltage for ion-sensing

does not jump the spark plug gap but determines the con-

ductivity of the ionized gases left over from the combustion

process.

Never Disconnect a Spark Plug Wire When

the Engine Is Running!

Ignition systems produce a high-voltage pulse nec-

essary to ignite a lean air-fuel mixture. If you discon-

nect a spark plug wire when the engine is running,

this high-voltage spark could cause personal injury

or damage to the ignition coil and/or ignition module.

SAFETY TIP

PURPOSE AND FUNCTION Knock sensors are used to

detect abnormal combustion, often called ping, spark knock,

or detonation. Whenever abnormal combustion occurs, a rapid

pressure increase occurs in the cylinder, creating a vibration

KNOCK SENSORS

366 CHAPTER 29

5 to 10 kHz, depending on the engine design. The voltage

signal from the knock sensor is sent to the PCM. The PCM

retards the ignition timing until the knocking stops.

 SEE

FIGURE 29–20 .

DIAGNOSING THE KNOCK SENSOR If a knock sensor

diagnostic trouble code (DTC) is present, follow the specified

testing procedure in the service information. A scan tool can

be used to check the operation of the knock sensor, using the

following procedure:

STEP 1 Start the engine and connect a scan tool to monitor

ignition timing and/or knock sensor activity.

STEP 2 Create a simulated engine knocking sound by tapping

on the engine block or cylinder head with a soft-faced

mallet or small ball peen hammer.

STEP 3 Observe the scan tool display. The vibration from the

tapping should have been interpreted by the knock

sensor as a knock, resulting in a knock sensor signal

and a reduction in the spark advance.

A knock sensor also can be tested using a digital storage

oscilloscope.

 SEE FIGURE 29–21 .

NOTE: Some engine computers are programmed to ig-

nore knock sensor signals when the engine is at idle

speed to avoid having the noise from a loose acces-

sory drive belt or other accessory interpreted as engine

knock. Always follow the vehicle manufacturer’s recom-

mended testing procedure.

REPLACING A KNOCK SENSOR If replacing a knock

sensor, be sure to purchase the exact replacement needed,

because they often look the same, but the frequency range can

vary according to engine design and location on the engine.

Always tighten the knock sensor using a torque wrench and

tighten to the specified torque to avoid causing damage to the

piezoelectric element inside the sensor.

EXHAUST

MANIFOLD

KNOCK

SENSOR

FIGURE 29–20 A typical knock sensor on the side of the

block. Some are located in the “V” of a V-type engine and are

not noticeable until the intake manifold has been removed.

A 50V AC 1:1 PROBE B 200mV OFF 1:1 PROBE

500uS / DIV SINGLE TRIG:A -2DIV

SINGLE FREE CAPTURE MIN MAX TRIGGER

RECURRENT RUN 10 20 DIV ON A AT 50%

ZOOM

HOLD

FIGURE 29–21 A typical waveform from a knock sensor

during a spark knock event. This signal is sent to the compu-

ter, which in turn retards the ignition timing. This timing retard

is accomplished by an output command from the computer to

either a spark advance control unit or directly to the ignition

module.

The Low-Power Toyota

A technician talked about the driver of a Toyota who

complained about poor performance and low fuel

economy. The technician checked everything and

even replaced all secondary ignition components.

Then the technician connected a scan tool and

noticed that the knock sensor was commanding

the timing to be retarded. Careful visual inspection

revealed a “chunk” missing from the serpentine belt,

which caused a “noise” similar to a spark knock.

Apparently the knock sensor was “hearing” the

accessory drive belt noise and kept retarding the

ignition timing. After replacing the accessory drive

belt, a test drive confirmed that normal engine power

was restored.

Other items that can fool the knock sensor to

retard the ignition timing include the following:

• Loose valve lifter adjustment

• Engine knocks

• Loose accessory brackets such as air-conditioning

compressor, power steering pumps, or alternator

REAL WORLD FIX

IGNITION SYSTEM OPERATION AND DIAGNOSIS 367

The triggering sensor has to work to create a spark from

the ignition coil(s). If there is a no-spark condition, check for

triggering by using a scan tool and check for engine RPM while

cranking the engine:



If the engine speed (RPM) shows zero or almost zero

while cranking, the most likely cause is a defective trig-

gering sensor or sensor circuit fault.



If the engine speed (RPM) is shown on the scan tool while

cranking the engine, then the triggering sensor is working

(in most cases).

Check service information for the exact procedure to

follow for testing triggering sensors.

IGNITION COIL TESTING USING AN OHMMETER If

an ignition coil is suspected of being defective, a simple ohm-

meter check can be performed to test the resistance of the

primary and secondary windings inside the coil. For accu-

rate resistance measurements, the wiring to the coil should be

removed before testing. To test the primary coil winding resis-

tance, take the following steps:

 SEE FIGURE 29–24 .

STEP 1 Set the meter to read low ohms.

STEP 2 Measure the resistance between the positive terminal

and the negative terminal of the ignition coil. Most

coils will give a reading between less than 1 ohm and

3 ohms. Check the manufacturer’s specifications for

the exact resistance values.

FIGURE 29–22 A spark tester looks like a regular spark plug

with an alligator clip attached to the shell. This tester has a

specified gap that requires at least 25,000 volts (25 kV) to fire.

FIGURE 29–23 A close-up showing the recessed center

electrode on a spark tester. It is recessed 3/8 inch into the

shell, and the spark must then jump another 3/8 inch to the

shell for a total gap of 3/4 inch.

IGNITION SYSTEM

DIAGNOSIS

CHECKING FOR SPARK In the event of a no-start

condition, the first step should be to check for secondary

voltage out of the ignition coil or to the spark plugs. If the

engine is equipped with a separate ignition coil, remove the

coilwire from the center of the distributor cap, install a spark

tester and crank the engine. See the Tech Tip “Always Use

aSpark Tester.” A good coil and ignition system should pro-

duce a blue spark at the spark tester.

 SEE FIGURES 29–22

AND 29–23 .

If the ignition system being tested does not have a sepa-

rate ignition coil, disconnect any spark plug wire from a spark

plug and, while cranking the engine, test for spark available at

the spark plug wire, again using a spark tester.

NOTE: An intermittent spark should be considered a no-

spark condition.

Typical causes of a no-spark (intermittent spark) condition

include the following:

1. Weak ignition coil

2. Low or no voltage to the primary (positive) side of the coil

3. High resistances, open coil wire, or spark plug wire

4. Negative side of the coil not being pulsed by the ignition

module

5. Defective pickup coil or crankshaft position sensor

6. Defective ignition control module (ICM)

7. Defective main relay (can be labeled Main, EFI, ASD on

Chrysler products; EEC on Ford vehicle relays)

368 CHAPTER 29

COM

mA A

DIGITAL MULTIMETER

RECORD

MAX MIN

345

MAXMIN

1. INSERT TEST LEADS IN THE INPUT TERMINALS SHOWN.

2. TURN THE ROTARY SWITCH TO Ω.

3. TOUCH THE PROBES AS SHOWN TO MEASURE RESISTANCE

IN PRIMARY WINDINGS.

4. OBSERVE DISPLAY. RESISTANCE SHOULD BE LESS THAN

1 TO 3 OHMS.

5. TOUCH PROBES AS SHOWN TO MEASURE RESISTANCE IN

SECONDARY WINDINGS.

6. OBSERVE DISPLAY. RESISTANCE SHOULD TYPICALLY BE

6,000 TO 30,000 OHMS.

FIGURE 29–24 Checking an ignition coil using a multimeter set to read ohms.

Always Use a Spark Tester

A spark tester looks like a spark plug except it has

a recessed center electrode and no side electrode.

The tester commonly has an alligator clip attached to

the shell so that it can be clamped on a good ground

connection on the engine. A good ignition system

should be able to cause a spark to jump this wide

gap at atmospheric pressure. Without a spark tester,

a technician might assume that the ignition sys-

tem is okay because it can spark across a normal,

grounded spark plug. The voltage required to fire a

standard spark plug when it is out of the engine and

not under pressure is about 3,000 volts or less. An

electronic ignition spark tester requires a minimum

of 25,000 volts to jump the 3/4-inch gap. There-

fore, never assume that the ignition system is okay

because it fires a spark plug—always use a spark

tester. Remember that an intermittent spark across

a spark tester should be interpreted as a no-spark

condition.

TECH TIP

To test the secondary coil winding resistance, follow

these steps:

STEP 1 Set the meter to read kilohms (kΩ).

STEP 2 Measure the resistance either between the primary termi-

nal and the secondary coil tower or between the second-

ary towers. The normal resistance of most coils ranges

between 6,000 and 30,000 ohms. Check the manufac-

turer’s specifications for the exact resis tance values.

MAGNETIC SENSOR TESTING Magnetic Sensor such

as the pickup coil, located under the distributor cap on many

electronic ignition engines, can cause a no-spark condition if

defective. The sensor must generate an AC voltage pulse to the

ignition module so that the module can pulse the ignition coil.

The sensor contains a coil of wire, and the resistance of this

coil should be within the range specified by the manufacturer.

Some common tests for pickup coils and magnetic crank-

shaft position sensors include the following:



Resistance. Usually between 150 and 1,500 ohms, but

check service information for the exact specifications.

 SEE FIGURE 29–25 .



Coil shorted to ground. Check that the coil windings

are insulated from ground by checking for continuity

IGNITION SYSTEM OPERATION AND DIAGNOSIS 369

using an ohmmeter. With one ohmmeter lead attached

to ground, touch the other lead of the ohmmeter to the

pickup coil terminal. The ohmmeter should read OL

(over limit) with the ohmmeter set on the high scale. If

the sensor resistance is not within the specified range

or if it has continuity to ground, replace the pickup coil

assembly.



AC voltage output. The sensor also can be tested for

proper voltage output. During cranking, most sensors

should produce a minimum of 0.25 volt AC.

TESTING HALL-EFFECT SENSORS As with any other

sensor, the output of the Hall-effect sensor should be tested

first. Using a digital voltmeter, check for the following:



Power and ground to the sensor



Changing voltage (pulsed on and off or digital DC volt-

age) when the engine is being cranked



Waveform, using an oscilloscope  SEE FIGURE 29–26 .

TESTING OPTICAL SENSORS Optical sensors will not

operate if they are dirty or covered in oil. Perform a thorough

visual inspection and look for an oil leak that could cause dirty

oil to get on the LED or phototransistor. Also be sure that the

light shield is securely fastened and that the seal is lightproof.

An optical sensor also can be checked using an oscilloscope.

 SEE FIGURE 29–27 .

Because of the speed of the engine and the number

of slits in the optical sensor disk, a scope is one of the only

tools that can capture useful information. For example, a

Nissan has 360 slits, and if it is running at 2,000 RPM, a sig-

nal is generated 720,000 times per minute, or 12,000 times

per second.

FIGURE 29–25 Measuring the resistance of an HEI pickup

coil using a digital multimeter set to the ohms position. The

reading on the face of the meter is 0.796 kΩ, or 796 ohms in

the middle of the 500- to 1,500-ohm specifications.

RECORD

SAVE

RECALL

AUTO

RANGE

AUTOMOTIVE SCOPMETER

-5

VEHICLE

DATA

PRADE

SINGLE

RANGE

HOLD

AUTO

KEYS

RANGE

HALL-EFFECT SENSOR

4.72 V PEAK - PEAK

127 HZ FREQUENCY

42.0 % DUTY CYCLE

3.06 MS PULSE WIDTH

-2V

2MS/DIV

FIGURE 29–26 The connection required to test a Hall-effect

sensor. A typical waveform from a Hall-effect sensor.

SPARK PLUG WIRE

INSPECTION

Spark plug wires should be visually inspected for cuts or de-

fective insulation. Faulty spark plug wire insulation can cause

hard starting or no starting in rainy or damp weather conditions.

When removing a spark plug wire, be sure to rotate the boot of

the wire at the plug before pulling it off the spark plug. This will

help prevent damaging the wire, as many wires are stuck to the

spark plug and are often difficult to remove.

VISUAL INSPECTION A thorough visual inspection should

include a look at the following items:



Check all spark plug wires for proper routing. All plug

wires should be in the factory wiring separators and be

clear of any metallic object that could damage the insu-

lation and cause a short-to-ground fault.



Check that all spark plug wires are securely attached

to the spark plugs and to the distributor cap or ignition

coil(s).

370 CHAPTER 29

ZOOM

HOLD

A 2V DC 1:1 PROBE B 200mV OFF 1:1 PROBE

10ms / TRIG:A -1 DIV

SINGLE FREE CAPTURE MIN MAX TRIGGER

RECURRENT RUN 10 20 DIV ON A AT 50%

ZOOM

HOLD

A 2V DC 1:1 PROBE B 200mV OFF 1:1 PROBE

10ms / TRIG:A -1 DIV

SINGLE FREE CAPTURE MIN MAX TRIGGER

RECURRENT RUN 10 20 DIV ON A AT 50%

(

)(

)

FIGURE 29–27 (a) The low-resolution signal has the same number of pulses as the engine has cylinders. (b) A dual-trace pattern

showing both the low-resolution signal and the high-resolution signals that usually represent 1 degree of rotation.

AIR

GAP

PRIMARY COIL

WINDINGS

TRACKING

TO STEEL

CORE

STEEL

CORE

SECONDARY COIL

WINDINGS

COIL EPOXY

CASE

FIGURE 29–28 A track inside an ignition coil is not a short

but a low-resistance path or hole that has been burned

through from the secondary wiring to the steel core.

Bad Wire? Replace the Coil!

When performing engine testing (such as a compres-

sion test), always ground the coil wire. Never allow

the coil to discharge without a path to ground for the

spark. High-energy ignition systems can produce

40,000 volts or more of electrical pressure. If the

spark cannot spark to ground, the coil energy can

(and usually does) arc inside the coil itself, creating

a low-resistance path to the primary windings or the

steel laminations of the coil.

 SEE FIGURE 29–28 .

This low-resistance path is called a track and

could cause an engine miss under load even though

all of the remaining component parts of the ignition

system are functioning correctly. Often these tracks

do not show up on any coil test, including most

scopes. Because the track is a lower resistance path

to ground than normal, it requires that the ignition

system be put under a load for it to be detected, and

even then, the misfire may be intermittent. If a misfire

was the result of an open circuit in the secondary

circuit, the coil is ruined and must be replaced.

When disabling an ignition system, perform one

of the following procedures to prevent possible igni-

tion coil damage:

1. Remove the power source wire from the ignition

system to prevent any ignition operation.

2. On distributor-equipped engines, remove the sec-

ondary coil wire from the center of the distributor

cap and connect a jumper wire between the dis-

connected coil wire and a good engine ground. This

ensures that the secondary coil energy will be safely

grounded and prevents high-voltage coil damage.

TECH TIP



Check that all spark plug wires are clean and free from

excessive dirt or oil. Check that all protective covers nor-

mally covering the coil and/or distributor cap are in place

and not damaged.



Carefully check the cap and distributor rotor for faults

or coil secondary terminal on waste spark coils.

 SEE

FIGURE 29–29 .

Visually check the wires and boots for damage.  SEE

FIGURE 29–30 .

Check all spark plug wires with an ohmmeter for proper

resistance. Good spark plug wires should measure less than

10,000 ohms per foot of length.

 SEE FIGURE 29–31 .