After studying Chapter 11, the reader should be able to:
Prepare for ASE Engine Performance (A8) certification test content area “B” (Ignition System Diagnosis and Repair).
Describe the procedure used to check for spark.
Discuss what to inspect and look for during a visual inspection of the ignition system.
List the steps necessary to check and/or adjust ignition timing on engines equipped with a distributor.
Describe how to test the ignition system using an oscilloscope.
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.
A good coil and ignition system should produce a blue spark at the spark tester.
Typical causes of a no-spark (intermittent spark) condition include the following:
Weak ignition coil
Low or no voltage to the primary (positive) side of the coil
High resistance or open coil wire, or spark plug wire
Negative side of the coil not being pulsed by the ignition module
Defective pickup coil
The voltage required to fire a standard spark plug when it is out of the engine and not under pressure is about 3000 volts or less. An electronic ignition spark tester requires a minimum of 25,000 volts to jump the 3/4-in. gap.
Figure 11-1 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 11-2 A close-up showing the recessed center electrode on a spark tester. It is recessed 3/8 in. into the shell and the spark must then jump another 3/8 in. to the shell for a total gap of 3/4 in.
IGNITION COIL TESTING USING AN OHMMETER
To test the primary coil winding resistance, take the following steps:
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 1 and 3 ohms; however, some coils should indicate less than 1 ohm. Check the manufacturer's specifications for the exact resistance values and be sure to zero the ohmmeter before measuring the coil.
Figure 11-3 Checking an ignition coil using a multimeter set to read ohms. (Courtesy of Fluke Corporation)
To test the secondary coil winding resistance, follow these steps:
Step 1: Set the meter to read kilohms (kW).
Step 2: Measure the resistance between either primary terminal and the secondary coil tower. The normal resistance of most coils ranges between 6000 and 30,000 ohms. Check the manufacturer’s specifications for the exact resistance values.
PICKUP COIL TESTING
The pickup coil must generate an AC voltage pulse to the ignition module so that the module can pulse the ignition coil.
Some common specifications include the following:
(Continued) 150 - 900 (orange and black leads) Chrysler Brand 400 - 100 (orange and purple leads) Ford 500 - 1500 (white and green leads) General Motors Pickup Coil Resistance (Ohms) Manufacturer
During cranking, most pickup coils should produce a minimum of 0.25 volt AC.
Figure 11-4 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.
CURRENT RAMPING IGNITION COILS
Ignition coil operation begins with the module completing the primary circuit through the ignition coil winding. This allows primary current to ramp upward (primary charging time) to a preset limit determined by the control module.
This angle of current flow, as seen on a scope display, is called a current ramp and the process of testing for the proper current ramp is called current ramping.
Using the digital storage oscilloscope and a current probe, a quick check can be made of the overall primary condition of the two most important parameters of the ignition circuit, the module current limits and the charging rise time of the circuit.
To perform current probe testing on the system, first locate the feed wire and make it current probe accessible.
Set up the scopes to read approximately 100 mV per division and 2 ms per division. This may be adjusted to suit the waveform, but will give a starting reference point.
Figure 11-5 A waveform showing the primary current flow through the primary windings of an ignition coil.
Figure 11-6 Schematic of a typical distributorless ignition system showing the location for the power feed and grounds. (Courtesy of Fluke Corporation)
Figure 11-7 Connect the scopes current clamp around the feed wire for the primary side of the coil(s). Start the engine and view the current flow waveform. (Courtesy of Fluke Corporation)
Observed Current Ramp Times (Continued)
Figure 11-8 An example of a good coil current flow waveform pattern. Note the regular shape of the rise time and slope. Duration of the waveform may change as the module adjusts the dwell. The dwell is usually increased as the engine speed is increased. (Courtesy of Fluke Corporation)
Figure 11-9 (a) A waveform pattern showing an open in the coil primary. (b) A shorted coil pattern waveform. (Courtesy of Fluke Corporation) A
Figure 11-9 (a) A waveform pattern showing an open in the coil primary. (b) A shorted coil pattern waveform. (Courtesy of Fluke Corporation) B
CHECKING PICKUP COILS
The waveform is created by the strengthening and weakening of the magnetic field as the points of time core rotates past the points of the pole pieces.
The changing magnetic field is sent to the ignition control module where it turns off the current through the primary winding of the ignition coil.
Figure 11-10 A typical pickup coil showing how the waveform is created as the timer core rotates inside the pole piece.
Figure 11-11 (a) A voltage waveform of a pickup coil at low engine speed. (b) A current waveform of the current through the primary windings of the ignition coil at low engine speed. (c) A voltage waveform of a pickup coil at high speed. (d) A current waveform through the primary winding of the ignition coil at high engine speed.
TESTING MAGNETIC SENSORS
If the sensor is removed from the engine, hold a metal (steel) object against the end of the sensor.
The sensor can be tested using a digital meter set to read AC volts.
Figure 11-12 An AC voltage is produced by a magnetic sensor. Most sensors should produce at least 0.1 volt AC while the engine is cranking if the pickup wheel has many teeth. If the pickup wheel has only a few teeth, you may need to switch the meter to read DC volts and watch the display for a jump in voltage as the teeth pass the magnetic sensor. (Courtesy of Fluke Corporation)
TESTING HALL-EFFECT SENSORS
Using a digital voltmeter, check for the presence of a changing DC (digital hi-low) voltage when the engine is being cranked. The best test is to use an oscilloscope and observe the waveform.
Figure 11-13 (a) The connection required to test a Hall-effect sensor. (b) A typical waveform from a Hall-effect sensor. (Courtesy of Fluke Corporation) A
Figure 11-13 (a) The connection required to test a Hall-effect sensor. (b) A typical waveform from a Hall-effect sensor. (Courtesy of Fluke Corporation) B
TESTING OPTICAL SENSORS
Perform a thorough visual inspection to look for an oil leak that could cause dirty oil to get on the LED or phototransistor.
An optical sensor can also be checked using an oscilloscope.
When performing engine testing (such as a compression test), always ground the coil wire.
Figure 11-14 (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. (Courtesy of Fluke Corporation)
CHECKING FOR SPARK
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.
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.
Figure 11-15 A track inside an ignition coil is not a short, but rather a low-resistance path or hole that has been burned through from the secondary wiring to the steel core.
Some engines use a positive distributor position notch or clamp that enables the distributor to be placed in only one position, while others use a method of indexing to verify the distributor position.
A mis-indexed distributor may exhibit surging, light bucking, or intermittent engine misfiring, which can cause a random misfire DTC (P0300).
Many OBD-II vehicles with distributors such as, Jeep, late model DaimlerChrysler V-6 and V-8 engines, and some GM trucks, require indexing.
Figure 11-16 The relationship between the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor is affected by wear in the timing gear and/or chain.
Figure 11-17 A scan tool displays the cam retard on a Chevrolet V-6. The cam retard value should be ± 2 degrees .
Figure 11-18 A worn distributor drive gear can be the cause of an out-of-specification camshaft position (CMP) signal.
IGNITION SYSTEM DIAGNOSIS USING VISUAL INSPECTION
One of the first steps in the diagnosis process is to perform a thorough visual inspection of the ignition system, including the following items:
Check all spark plug wires for proper routing.
Check that all spark plug wires are securely attached to the spark plugs and to the distributor cap or ignition coil(s).
Check that all spark plug wires are clean and free from excessive dirt or oil.
Remove the distributor cap and carefully check the cap and distributor rotor for faults.
Remove the spark plugs and check for excessive wear or other visible faults.
Check the Ignition Keys
Some ignition-related faults are caused by a loose or worn lock cylinder and/or ignition switch assembly that can be caused by having an excessive amount of weight hanging from the key chain.
Figure 11-19 Keys used in a vehicle that had an ignition switch intermittent problem.
TESTING FOR POOR PERFORMANCE
A simple method of testing distributorless (waste-spark systems) ignition with the engine off involves removing the spark plug wires (or connectors) from the spark plugs (or coils or distributor cap) and installing short lengths (2 in.) of rubber vacuum hose in series.
Start the engine and ground out each cylinder one at a time by touching the tip of a grounded test light to the rubber vacuum hose.
Figure 11-22 Using a vacuum hose and a grounded test light to ground one cylinder at a time on a DIS. This works on all types of ignition systems and provides a method for grounding out one cylinder at a time without fear of damaging any component.
Check all cylinders by grounding them out one at a time. If one weak cylinder is found, check the other cylinder using the same ignition coil (except on engines that use an individual coil for each cylinder).
To help eliminate other possible problems and determine exactly what is wrong, switch the suspected ignition coil to another position (if possible).
If the problem now affects the other cylinders, the ignition coil is defective and must be replaced.
If the problem does not "change positions," the control module affecting the suspected coil or either cylinder's spark plug or spark plug wire could be defective.
Figure 11-20 A length of vacuum hose being used for a coil wire. The vacuum hose is conductive because of the carbon content of the rubber in the hose. This hose measures only 1,000 ohms and was 1 foot long, which is lower resistance than most spark plug wires. Notice the spark from the hose’s surface to the tip of a grounded screwdriver.
Figure 11-21 A distributorless ignition system (DIS) can be checked by unplugging both spark plug wires from one ignition coil and starting the engine. The spark should be able to jump the 1-in. (25-mm) distance between the terminals of the coil. No damage to the coil (or module) results because a spark occurs and does not find ground elsewhere.
TESTING FOR A NO START CONDITION
To determine exactly what is wrong, follow these steps:
Step 1: Test the output signal from the crankshaft sensor.
Step 2: If the sensor tests okay in step 1, check for a changing AC or DC voltage signal, depending on the system at the ignition module.
Step 3: If the ignition control module is receiving a changing signal from the crankshaft position sensor, it must be capable of switching the power to the ignition coils on and off.
Firing order means the order that the spark is distributed to the correct spark plug at the right time. The firing order of an engine is determined by crankshaft and camshaft design.
The firing order determines the location of the spark plug wires in the distributor cap of an engine equipped with a distributor. The firing order is often cast into the intake manifold for easy reference
Figure 11-23 The firing order is cast or stamped on the intake manifold on most engines that have a distributor ignition.
DISTRIBUTOR CAP AND ROTOR INSPECTION
Inspect a distributor cap for a worn or cracked center carbon insert, excessive side insert wear or corrosion, cracks, or carbon tracks, and check the towers for burning or corrosion by removing spark plug wires from the distributor cap one at a time.
The rotor should be replaced every time the spark plugs are replaced, because all ignition current flows through the rotor. Generally, distributor caps should only need replacement after every 3 or 4 years of normal service.
Look Before You Pry
Some distributor rotors are secured to the distributor shaft with a retaining screw.
Figure 11-24 Note where the high-voltage spark jumped through the plastic rotor to arc into the distributor shaft. Always check for a defective spark plug(s) when a defective distributor cap or rotor is discovered. If a spark cannot jump to a spark plug, it tries to find a ground path wherever it can.
Figure 11-25 This distributor cap should be replaced because of the worn inserts and excessive dusting inside the cap.
Figure 11-26 This rotor had arced through to the distributor shaft. The engine would not run above an idle speed and the spark from the coil could easily fire a spark tester.
Figure 11-27 Carbon track in a distributor cap. These faults are sometimes difficult to spot and can cause intermittent engine missing. The usual cause of a tracked distributor cap (or coil, if it is a distributorless ignition) is a defective (open) spark plug wire.
Figure 11-28 Some rotors are retained by a screw, so look before you pry.
SPARK PLUG WIRE INSPECTION
Spark plug wires should be visually inspected for cuts or defective insulation and checked for resistance with an ohmmeter. Good spark plug wires should measure less than 10,000 ohms per foot of length.
Figure 11-29 With careful visual inspection, the technician discovered this defective spark plug wire.
Figure 11-30 Measuring the resistance of a spark plug wire with a multimeter set to the ohms position. The reading of 16.03 kΩ (16,030 ohms) is okay because the wire is about 2-ft. long. Maximum allowable resistance for a spark plug wire this long would be 20kΩ (20,000 ohms).
Spark Plug Wire Pliers Are a Good Investment
Spark plug wires are often difficult to remove. Using good-quality spark plug wire pliers, saves time and reduces the chance of harming the wire during removal.
Route the Wires Right!
To prevent any problems associated with high-voltage spark plug wires, be sure to route them the same as the original plug wires, using all the factory holding brackets and wiring combs.
Figure 11-31 Spark plug wire boot pliers are a handy addition to any tool box.
Figure 11-32 Always take the time to install spark plug wires back into the original holding brackets (wiring combs).
SPARK PLUG SERVICE
Spark plugs should be inspected when an engine performance problem occurs and should be replaced regularly to ensure proper ignition system performance.
Be certain that the engine is cool before removing spark plugs, especially on engines with aluminum cylinder heads. To help prevent dirt from getting into the cylinder of an engine while removing a spark plug, use compressed air or a brush to remove dirt from around the spark plug before removal.
Spark Plug Inspection
Two indications and their possible root causes include the following:
Carbon fouling. If the spark plug(s) has dry black carbon (soot), the usual causes include:
Slow-speed driving under light loads that keeps the spark plug temperatures too low to burn off the deposits
Overrich air-fuel mixture
Weak ignition system output
Oil fouling. If the spark plug has wet, oily deposits with little electrode wear, oil may be getting into the combustion chamber from the following:
Worn or broken piston rings
Defective or missing valve stem seals
All spark plugs should be in the same condition, and the color of the center insulator should be light tan or gray.
When installing spark plugs, always use the correct tightening torque to ensure proper heat transfer from the spark plug shell to the cylinder head.
NOTE: General Motors does not recommend the use of antiseize compound on the threads of spark plugs being installed in an aluminum cylinder head, because the spark plug will be overtightened.
This excessive tightening torque places the threaded portion of the spark plug too far into the combustion chamber where carbon can accumulate and result in the spark plugs being difficult to remove. If antiseize compound is used on spark plug threads, reduce the tightening torque by 40%. Always follow the vehicle manufacturer’s recommendations.
Figure 11-33 When removing spark plugs, it is wise to arrange them so that they can be compared and any problem can be identified with a particular cylinder.
Figure 11-34 A spark plug thread chaser is a low-cost tool that hopefully will not be used often, but is necessary to clean the threads before new spark plugs are installed.
Figure 11-35 Since 1991, General Motors engines have been equipped with slightly (1/8 in. or 3 mm) longer spark plugs. This requires that a longer spark plug socket should be used to prevent the possibility of cracking a spark plug during installation. The longer socket is shown next to a normal 5/8-in. spark plug socket.
Figure 11-36 An extended-reach spark plug that shows normal wear. The color and condition indicate that the cylinder is operating correctly.
Figure 11-37 Spark plug removed from an engine after a 500-mile race. Note the clipped side (ground) electrode. The electrode design and narrow (0.025 in.) gap are used to ensure that a spark occurs during extremely high engine speed operation. The color and condition of the spark plug indicate that near-perfect combustion has been occurring.
Figure 11-38 Typical worn spark plug. Notice the rounded center electrode. The deposits indicate a possible oil usage problem.
Figure 11-39 A new spark plug that was fouled by a too-rich air-fuel mixture. The engine from which this spark plug came had a defective (stuck partially open) injector on this one cylinder only.
QUICK AND EASY SECONDARY IGNITION TESTS
Following are some quick and easy secondary ignition tests.
Test 1 If there is a crack in a distributor cap, coil, or spark plug, or a defective spark plug wire, a spark may be visible at night.
Test 2 For intermittent problems, use a spray bottle to apply a water mist to the spark plugs, distributor cap, and spark plug wires.
Test 3 To determine if the rough engine operation is due to secondary ignition problems, connect a 6- to 12-volt test light to the negative side (sometimes labeled “tach”) of the coil. Connect the other lead of the test light to the positive lead of the coil.
With the engine running, the test light should be dim and steady in brightness. If the test light varies noticeably, this indicates that the secondary voltage cannot find ground easily and is feeding back through the primary windings of the coil. This feedback causes the test light to become brighter.
Figure 11-40 A water spray bottle is an excellent diagnostic tool to help find an intermittent engine miss caused by a break in a secondary ignition circuit component.
Ignition timing should be checked and adjusted according to the manufacturer's specifications and procedures for best fuel economy and performance, and lowest exhaust emissions.
To be assured of the proper ignition timing, follow exactly the timing procedure indicated on the underhood emission decal.
Before the ignition timing is checked or adjusted, the following items should be checked to ensure accurate timing results:
The engine should be at normal operating temperature
The engine should be at the correct timing RPM
Check the timing procedure specified by the manufacturer. This may include disconnecting a "set timing" connector wire, grounding a diagnostic terminal, disconnecting a four-wire connector, or similar procedure.
Timing Light Connections
For checking or adjusting ignition timing, make the timing light connections as follows:
Connect the timing light battery leads to the vehicle battery: the red to the positive terminal and the black to the negative terminal.
Connect the timing light high-tension lead to spark plug cable 1.
Follow this rule of thumb: If cylinder 1 is unknown for a given type of engine, it is the most forward cylinder as viewed from above (except in Pontiac V-8 engines).
Checking or Adjusting Ignition Timing
Use the following steps for checking or adjusting ignition timing:
Start the engine and adjust the speed to that specified for ignition timing.
With the timing light aimed at the stationary timing pointer, observe the position of the timing mark on the harmonic balancer or flywheel with the light flashing. Refer to the manufacturer's specifications on underhood decal for the correct setting.
To adjust timing, loosen the distributor locking bolt or nut and turn the distributor housing until the timing mark is in correct alignment. Turn the distributor housing in the direction of rotor rotation to retard the timing and against rotor rotation to advance the timing.
After adjusting the timing to specifications, carefully tighten the distributor locking bolt. It is sometimes necessary to readjust the timing after the initial setting because the distributor may rotate slightly when the hold-down bolt is tightened.
Two Marks Are the Key to Success
When a distributor is removed from an engine, always mark the direction the rotor is pointing to ensure that the distributor is reinstalled in the correct position. Because of the helical cut on the distributor drive gear, the rotor rotates as the distributor is being removed from the engine.
To help reinstall a distributor without any problems, simply make another mark where the rotor is pointing just as the distributor is lifted out of the engine. Then to reinstall, simply line up the rotor to the second mark and lower the distributor into the engine. The rotor should then line up with the original mark as a double-check.
Figure 11-41 Typical timing marks. The degree numbers are on the stationary plate and the notch is on the harmonic balancer.
Figure 11-42 Cylinder 1 and timing mark location guide.
Figure 11-43 (a) Typical SPOUT connector as used on many Ford engines equipped with distributor ignition (DI). (b) The connector must be opened (disconnected) to check and/or adjust the ignition timing. On DIS/EDIS systems, the connector is called SPOUT/SAW (spark output/spark angle word). A
Figure 11-43 (continued) (a) Typical SPOUT connector as used on many Ford engines equipped with distributor ignition (DI). (b) The connector must be opened (disconnected) to check and/or adjust the ignition timing. On DIS/EDIS systems, the connector is called SPOUT/SAW (spark output/spark angle word). B
Figure 11-44 The first mark indicates the direction the rotor is pointing when the distributor is in the engine. The second mark indicates where the rotor is pointing just as it is pulled from the engine.
SCOPE-TESTING THE IGNITION SYSTEM
Any automotive scope will show an ignition system pattern.
The height of the scope pattern indicates voltage. The length (from left to right) of the scope pattern indicates time.
The leftmost vertical (upward) line is called the firing line. The height of the firing line should be between 5000 and 15,000 volts (5 and 15 kV) with not more than a 3-kV difference between the highest and the lowest cylinder's firing line.
A higher-than-normal height (or height higher than that of other cylinders) can be caused by one or more of the following:
Spark plug gapped too wide
Lean fuel mixture
Defective spark plug wire
The spark line is a short horizontal line connected to the firing line. The height of the spark line represents the voltage required to maintain the spark across the spark plug after the spark has started. The height of the spark line should be one-fourth of the height of the firing line (between 1.5 and 2.5 kV).
The spark duration should be between 0.8 and 2.2 milliseconds (usually between 1.0 and 2.0 ms).
This remaining energy dissipates in the coil windings and the entire secondary circuit. The intermediate oscillations are also called the "ringing" of the coil as it is pulsed.
A correctly operating ignition system should display five or more "bumps" (oscillations) (three or more for a GM HEI system).
When the transistor turns on an electronic system, the coil is being charged. Note that the charging of the coil occurs slowly (coil-charging oscillations) because of the inductive reactance of the coil.
Dwell is the amount of time that the current is charging the coil from the transistor-on point to the transistor-off point. The end of the dwell section marks the beginning of the next firing line. This point is called “transistor off,” and indicates that the primary current of the coil is stopped, resulting in a high-voltage spark out of the coil.
Reading the Scope on Display (Parade)
Firing lines are visible only on the display (parade) position. The firing lines should all be 5 to 15 kV in height and be within 3 kV of each other.
A lean mixture (not enough fuel) requires a higher voltage to ignite because there are fewer droplets of fuel in the cylinder for the spark to use as "stepping stones" for the voltage to jump across. Therefore, a lean mixture is less conductive than a rich mixture.
Reading the Spark Lines
Spark lines can easily be seen on either the superimposed or raster (stacked) position. On the raster position, each individual spark line can be viewed.
The spark lines should be level and one-fourth as high as the firing lines (1.5 to 2.5 kV, but usually less than 2 kV). The spark line voltage is called the burn kV.
Many scopes are equipped with a millisecond (ms) sweep. This means that the scope will sweep only that portion of the pattern that can be shown during a 5- or 25-ms setting. Following are guidelines for spark line length:
0.8 ms - too short
1.5 ms - average
2.2 ms - too long
If the spark line is too short, possible causes include the following:
Spark plug(s) gapped too widely
Rotor tip to distributor cap insert distance gapped too widely (worn cap or rotor)
High-resistance spark plug wire
Air-fuel mixture too lean (vacuum leak, broken valve spring, etc.)
If the spark line is too long, possible causes include the following:
Fouled spark plug(s)
Spark plug(s) gapped too closely
Shorted spark plug or spark plug wire
Spark Line Slope
Downward-sloping spark lines indicate that the voltage required to maintain the spark duration is decreasing during the firing of the spark plug. This downward slope usually indicates that the spark energy is finding ground through spark plug deposits (the plug is fouled) or other ignition problems.
An upward-sloping spark line usually indicates a mechanical engine problem. A defective piston ring or valve would tend to seal better in the increasing pressures of combustion.
An upward-sloping spark line can also indicate a lean air-fuel mixture. Typical causes include:
Sticking intake valve
Figure 11-45 Typical engine analyzer hookup that includes a scope display. (1) Coil wire on top of the distributor cap if integral type of coil; (2) number 1 spark plug connection; (3) negative side of the ignition coil; (4) ground (negative) connection of the battery.
Figure 11-46 Clip-on adapters are used with an ignition system that uses an integral ignition coil. (Courtesy of Fluke Corporation)
Figure 11-48 A single cylinder is shown at the top and a 4-cylinder engine at the bottom. (Courtesy of Fluke Corporation)
Figure 11-49 Drawing shows what is occurring electrically at each part of the scope pattern.
Figure 11-50 Typical secondary ignition pattern. Note the lack of firing lines on the superimposed pattern.
Figure 11-51 Raster is the best scope position to view the spark lines of all the cylinders to check for differences. Most scopes display cylinder 1 at the bottom. The other cylinders are positioned by firing order above cylinder 1.
Figure 11-52 Display is the only position to view the firing lines of all cylinders. Cylinder 1 is displayed on the left (except for its firing line, which is shown on the right). The cylinders are displayed from left to right by firing order.
Figure 11-53 A downward-sloping spark line usually indicates high secondary ignition system resistance or an excessively rich air-fuel mixture.
Figure 11-54 An upward-sloping spark line usually indicates a mechanical engine problem or a lean air-fuel mixture.
Figure 11-55 The relationship between the height of the firing line and length of the spark line can be illustrated using a rope. Because energy cannot be destroyed, the stored energy in an ignition coil must dissipate totally, regardless of engine conditions.
SCOPE-TESTING A WASTE-SPARK IGNITION SYSTEM
A handheld digital storage oscilloscope can be used to check the pattern of each individual cylinder. Some larger scopes can be connected to all spark plug wires and therefore are able to display both power and waste-spark waveforms.
Figure 11-56 A dual-trace scope pattern showing both the power and the waste spark from the same coil (cylinders 1 and 6). Note that the firing line is higher on the cylinder that is under compression (power); otherwise, both patterns are almost identical.
IGNITION SYSTEM TROUBLESHOOTING GUIDE
The following list will assist technicians in troubleshooting ignition system problems.