© 2012 Delmar, Cengage Learning

Ignition System Service

Chapter 38

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Objectives• Diagnose common ignition system problems• Service ignition systems and distributors

correctly• Install a distributor and adjust ignition timing• Operate an oscilloscope and interpret scope

patterns

© 2012 Delmar, Cengage Learning

Ignition System Service and Repairs

• Tune-up– Originated when cars had ignition points– Today: referred to as a 30,000- or 60,000-mile

service• Can include replacement of all filters, belts, hoses,

and fluids• A few engines require mechanical valve

measurement and adjustment• Timing belt can also be part of a 60,000- or

90,000-mile service

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Spark Plug Service• Important points

– Use correct replacement plugs– Use the correct spark plug heat range– Check the size of the threads– Check to see if the old spark plug used a gasket– Check the reach or thread length– Tapered seat plugs can be difficult to remove

© 2012 Delmar, Cengage Learning

© 2012 Delmar, Cengage Learning

Spark Plug Diagnosis• Temperature of spark plug

– Relates directly to combustion chamber temperature

• Condition of used spark plugs – Indicates condition of engine and fuel system– Spark plug wear

• Some use double plat plugs• Worn rounded electrode more surface area to be

charged– Fouled plugs

• Oil and ash deposits

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Spark Plug Diagnosis (cont’d.)• Checking and adjusting the spark plug gap

– Gap is set at the factory• Always double-check before installation

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Installing Spark Plugs• Seal of the plug

– Can be ruined by overtightening• Today's spark plugs

– Longer lifetime• Indexing a spark plug

– High-performance procedure • Usually used in high-compression racing engines

– Platinum plating on spark plug electrodes reduces wear

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Repairing Damaged Spark Plug Threads

• Spark plug threads are sometimes stripped– Can be avoided if a ratchet is never used– Stripped threads can be replaced with a thread

insert• Tapered seat spark plugs

– Cannot be repaired with thread inserts

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Spark Plug Cable Service• Removing spark plug cables

– Must be a careful operation• Determine internal structure of a cable is sound

– Check resistance with an ohmmeter• Cable ends

– Terminal is crimped tightly to the cable • Spark plug cables

– Installed in the holes around the distributor cap

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Distributor Ignition Service• Distributor ignition systems

– Unique maintenance requirements• Distributor cap and rotor service

– Distributor is cracked: carbon trail often forms– Excessive resistance in secondary system:

creates a hole at the tip of the rotor– Distributor cap has an aligning feature

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Ignition Timing• Ignition timing on electronic ignition (EI) systems

– Controlled by the powertrain control module– A timing light is a strobe light– Timing is changed by loosening the distributor

body and rotating in one direction or the other– Older cars with points: point gap adjusted with a

feeler gauge

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Electronic Ignition Distributor Service

• Normal electronic ignition service on DI systems– Limited to replacement of the rotor

• No spark – Trigger may not be opening primary circuit– Bad module– Bad ignition coil

• Best test for a bad module – Test the other parts of the system first

© 2012 Delmar, Cengage Learning

© 2012 Delmar, Cengage Learning

Other Distributor Repair Service• Distributors have gears and bushings

– Wear out or break• Distributor drive gear

– Can become stripped• Shaft bushings

– Can wear out

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Distributor Installation• Align the timing mark on the damper with the

pointer on the timing cover– If distributor is installed 180 degrees off,

backfiring will occur and engine will not run• On pushrod engines the distributor will not drop

all the way into the block– Bottom is not aligning with the drive lug on the oil

pump

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Static Timing• Ignition system can be static timed

– With the number one cylinder on its compression, align the timing indicator at the crank

– Install distributor with rotor pointing to the number one plug cable

– Rotate distributor body until the armature pole piece aligns with permanent magnet

– Tighten the distributor holddown

© 2012 Delmar, Cengage Learning

© 2012 Delmar, Cengage Learning

Electronic Ignition System Service

• Determine problem cause– Before replacing the part

• Important concepts– Ignition problem diagnosis: visual inspection– Ignition-related no start: use scan tool– Signal generator testing: different sensor types– Pickup coil testing and replacement: use

ohmmeter to test continuity– Air gap measurement: use feeler gauge– Hall switch testing: use scan tool

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Electronic Ignition System Service (cont’d.)

– Scopes: oscilloscope is helpful for pinpointing ignition and engine problems

– Ignition patterns: different scope pattern types can be selected

– Spark line: horizontal line that begins at the voltage level where electrons start to flow across the spark plug gap

– Dwell section: represents the time that the primary current is switched on

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Types of Scope Patterns• Raster pattern

– Displays all cylinders• Superimposed pattern

– Compares all cylinders• Patterns are displayed one on top of the other

• Display pattern – Displays all cylinders side by side

• Firing line heights can be compared

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Oscilloscope Tests• Voltage test

– Popular with older contact point systems• Reading a parade pattern

– Run the engine at 1,200-1,500 rpm for oscilloscope tests

– Perform an acceleration load test by rapidly snapping the throttle open and closed

• Reading a raster pattern– Spark section– Dwell section

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Oscilloscope Tests (cont’d.)• Scope diagnosis of engine problems

– Engine problems can show up in a scope pattern• Scope diagnosis of ignition problems

– When the engine is off, use a ground probe to ground the spark plug end of a plug cable

• Stress test– Parts are cooled or heated

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Oscilloscope Tests (cont’d.)• Primary voltage ignition pattern

– Secondary pattern is the best pattern for displaying an ignition system

• Shows primary as well as secondary current flow• Primary current ramp test

– Allows you to watch the current• As it ramps up and at its peak

• Vacuum waveforms– Problem cylinders will have less vacuum than

others

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Other Scope Tests• Tests of individual components

– Can be done with a scope• Hall switch, contact points, and LED

– Give a square wave• Pickup coil

– Gives an AC sine wave– Increases with speed

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Other Diagnostic Instruments• Other test instruments have become common

– Computer systems• Store trouble codes

– Handheld tools• Read codes and interpret data

– DSO• Tool used to diagnose electronic problems