CH28 ignition system diagnosis and service STUDENT VERSION r

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Chapter 28

Ignition System

Diagnosis and Service

Service Precautions

• Always turn the ignition switch off before disconnecting any system wiring

• Do not touch any exposed connections

while the engine is running or cranking

• Ignition misfires can be caused by weak

spark, fouled plug, faulty coil, primary or

secondary faults, or incorrect plug gap

Causes of Detonation

• Excessively advanced ignition timing

• Engine overheating

• Excessively lean mixtures

• Gasoline with too low octane rating

Preignition

Types of Ignition System Problems

• Common

– Problems affect all cylinders

– Components include parts of the primary circuit and secondary circuit up to the distributor rotor

• Uncommon

– Problems affect one or more but not all cylinders

– Components include plug wires and plugs

Visual Inspection

• No-start conditions mostly caused by

primary circuit faults

• Common symptoms of ignition problems include:

– Hard starting

– Rough idle

– Engine stalling

Primary Circuit

• Inspect wiring and connections

• Perform wiggle test

• Examine ignition switch

• Check charging system and battery

• Inspect for moisture affecting wiring

• Inspect magnetic pulse generators and effect sensors and wiring

Hall-• The non-magnetic reluctor can become

magnetized and affect pulse generator

signal

Control Modules

• Use transistors as switches

• Modules can be mounted in distributor,

remotely, or as part of PCM

• Check for clean and tight mounting

• May require heat-conductive silicone grease

on mounting surface

Secondary Circuit

• Inspect plug wires

and boots for damage

• Ensure correct wire

No Spark Diagnosis

• Visual inspection:

– Check for loose or corroded connections – Look for signs of moisture

• Check for trouble codes

• Check fuel system operation

• Check engine mechanical condition

Basic No-Start Diagnosis

• Test spark at spark plug

– If no spark, test at coil output terminal

• Check for voltage at coil negative terminal

• Crank engine and check for coil pulse

– If no pulse, problem is with control module or pickup

No-Start Diagnosis of EI System

• Determine whether problem is ignition

related by using a spark tester

• Check for voltage at each coil’s primary

circuit

• Check crankshaft sensor output with a

voltmeter or oscilloscope

Cylinder Performance Test

• Also called a power balance or contribution test

• Each cylinder is tested

• Spark is stopped and the RPM drop is

recorded

• A weak cylinder will have little RPM drop

Ignition Performance Tests

• Most analyzers include primary and

secondary tests:

– Kilovolt (KV) tests

– Acceleration (snap) tests

– Cylinder burn time

– Display patterns

Understanding Single

Cylinder Patterns

Ignition Pattern Components

• Firing Line

– Represents the voltage needed to overcome the

resistance in the secondary circuit

• Spark Line

– Represents the time the spark lasts

• Intermediate or Coil Section

– Shows the remaining coil oscillations

• Dwell Section

– Shows the length of time current is flowing through the primary circuit

Pattern Display Modes

Spark Plug Firing Voltage

• Firing line is affected by anything that adds resistance to the secondary circuit

– Plugs, plug wires, engine temp, fuel mixture, and compression pressures

• Normal height between 7 – 13 kV with no more than 3 kV variation

• Snap throttle test should increase 3 – 4 kV

Spark Duration

• The amount of time the plug is firing

• Most engines have spark duration of about 1.5 ms

• Short duration cannot provide complete

combustion

• A long spark line and short firing line may indicate a fouled plug, low compression, or narrow plug gap

Coil Condition

• The coil section shows voltage reserve

• Look at height of oscillations

• Should uniformly decrease to zero value

• Abnormal oscillations may indicate a short

in the coil

Primary Circuit Checks

• Firing section should

Stress Testing Components

– Affect vacuum readings

– Advanced timing can cause pinging or knock

Checking Ignition Timing

• Follow the manufacturer’s procedures

• Make sure the spark plug gap and idle speed are correct before checking timing

• The engine must be at operating

temperature

Coil Resistance

Pickup Coil Waveform

Hall-Effect Sensors

• May be tested by connecting 12 volts across positive and negative connection and a

DMM across negative and signal wires

• Insert a steel feeler gauge between sensor

and magnet and watch DMM voltage

• A logic probe can be used to check output

Control Module

• Can be tested with a module tester

• Simulates operating conditions

• Usually very reliable

Distributor Service

• May need to be removed before service

• Inspect all wiring and connections

• Test advance mechanisms

• Check gear for wear or chipped teeth

• Inspect reluctor for damage

Installing and Timing the Distributor

• Lubricate O-ring

• Position rotor to align with marking

• Lower and fully seat distributor

• Install hold-down, cap, and wires

Distributor Cap and Rotor Service

• Arcing between cap and rotor terminals deteriorates terminals

• Replace both cap and rotor together

• Make sure both are fully seated

• Arrange plug wires properly on cap

DIS Systems

• A problem on one cylinder may affect its companion cylinder

• Separate primary circuit for each coil

• Test each coil’s primary and secondary wiring

• Proper plug torque can affect ground

circuit

Coil-On-Plug Systems

• Individual coil problems affect only one cylinder

• Coils can be tested with ohmmeter

• Cylinder misfire codes can result from injector, plug, coil, or engine mechanical problems

• Remove the plug and inspect threads and

cylinder head for damage

• Never remove plugs from a hot engine

Inspecting Spark Plugs

• Note deposits and degree of electrode wear

• Normal color is grey or tan

• Gap should increase no more than 001” for every 10,000 miles of service

• Reading spark plugs can reveal a variety of engine conditions

Worn Spark Plug

Regapping Spark Plugs

• The gap of both new and used plugs should

be checked

• Do not reduce gap by tapping plug, use

needle-nose pliers or gapping tool

• Keep ground electrode in line with positive

Spark Plug Installation

• Wipe dirt from plug seats

• Verify plugs are correct replacements

• Adjust the gap as needed

• Check whether anti-seize should be used

• Install and hand tighten

• Torque to specifications

Secondary Ignition Wires

• Inspect for cracks and worn insulation

• Check boots for cracks and hard, brittle conditions

• Check resistance with an ohmmeter

• Replace as a set

• Properly route to avoid cross-firing